Use of homologous amdS genes as selectable markers

Abstract

The present invention discloses novel amdS genes from fungi previously not known to contain and amdS gene, such as Aspergillus niger and Penicillium chrysogenum. The novel amdS genes can be used as homologous selectable marker genes in the transformation of these fungi. Alternatively, the cloned amdS genes can be used to inactivate the endogenous copy of the gene in order to reduce the background in transformation experiments.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of molecular biology, in particular the invention is concerned with selectable marker genes to be used in transformation of organisms.

BACKGROUND OF THE INVENTION

[0002] The Aspergillus nidulans amdS gene is probably the most frequently used selectable marker for the transformation of filamentous fungi and has been applied in most of the industrially important filamentous fungi such as e.g. Aspergillus niger (Kelly and Hynes 1985, EMBO J. 4: 475-479), Penicillium chrysogenum (Beri and Turner 1987, Curr. Genet. 11: 639-641), Trichoderma reesei (Pentillä et al. 1987, Gene 61: 155-164), Aspergillus oryzae (Christensen et al. 1988, Bio/technology 6: 1419-1422) and Trichoderma harzianum (Pe'er et al. 1991, Soil Biol. Biochem. 23: 1043-1046).

[0003] The popularity of the amdS gene as a selectable marker is most likely a result of the fact that it is the only available non-antibiotic marker gene which can be used as a dominant selectable marker in the transformation of fungi. Dominant selectable markers provide the advantage that they can be used directly in any strain without the requirement for mutant recipient strains. The antibiotic-resistance genes are, however, not preferred for use in industrial strains because the regulatory authorities in most countries object to the use of antibiotic markers in view of the potential risks of spread of antibiotic-resistance genes in the biosphere upon large-scale use of production strains carrying such genes.

[0004] The amdS gene has been used as a dominant marker even in fungi known to contain an endogenous amdS gene, i.e. A. nidulans (Tilburn et al. 1983, Gene 26: 205-221) and A. oryzae (Gomi et al. 1991, Gene 108: 91-98). In these cases the background of non-transformants can be suppressed by the inclusion of CsCl in the selection medium. In addition, high-copynumber transformants are provided with a growth advantage over the non-transformants (when acetamide is the sole nitrogen-source) because of the higher gene dosage.

[0005] Apart from the A. nidulans and A. oryzae amdS genes, by coincidence a sequence was found in the genome of the yeast Saccharomyces cerevisiae, which shows homology to the A. nidulans amdS gene (Chang and Abelson 1990, Nucleic Acids Res. 18:7180). The yeast amdS-like sequence was shown not to be essential in yeast. It is, however, not known whether the yeast amdS-like gene actually encodes a protein with amidase activity which might allow to use the gene as selectable marker. amdS genes have not been found in other fungi, despite attempts to detect such genes with heterologous hybridization using the A. nidulans amdS gene as probe (see e.g. Kelly and Hynes 1985 EMBO J. 4: 475-479). This is also in line with the observation that, in contrast to A. nidulans and A. oryzae, most fungi grow very poor, if at all, on acetamide (see e.g. Beri and Turner 1987, Curr. Genet. 11: 639-641; Pentillä et al. 1987, Gene 61: 155-164). The cloning and sequencing of two bacterial acetamidase genes has been reported, i.e. those of Pseudomonas aeruginosa (Brammar et al. 1987, FEBS Lett. 215: 291-294) and of Mycobacterium smegatis (Mahenthiralingam et al. 1993, J. Gen. Microbiol. 139: 575-583). However, these bacterial acetamidases appear to be unrelated to the above mentioned fungal acetamidases since no sequence similarities can be detected and the bacterial acetamidases are also much smaller than their fungal counterparts. No reports of the use of these bacterial acetamidases as selectable markers have appeared.

[0006] In addition to its dominant character, the amdS selectable marker provides the advantage of being a bidirectional marker. This means that, apart from the positive selection for the presence of the amdS gene using acetamide as sole carbon- or nitrogen-source, a counterselection can be applied using fluoracetamide to select against the presence of the amdS gene (Hynes and Pateman 1970, Mol. Gen. Genet. 108, 107-106). The fluoracetamide counterselection has been applied to cure genetically engineered strains from recombinant constructs carrying the amdS gene (e.g. Ward et al. 1993, Appl. Microbiol. Biotechnol. 39, 738-743).

[0007] A disadvantage of the amdS marker is the fact that the A. nidulans amdS gene is a heterologous gene in industrial fungi such as A. niger, A. oryzae, T. reesei and P. chrysogenum. Even though this may seem trivial to most molecular biologists, regulatory authorities often object that production strains containing the heterologous A. nidulans amdS gene posses a new (the gene being heterologous) and unnecessary (the marker gene not being necessary once the transformant strain is obtained) property, the risks of which cannot be foreseen. Unfortunately, the only industrial filamentous fungus for which an homologous amdS gene is available is A. oryzae.

[0008] We have previously addressed this problem by developing a method to obtain recombinant fungal production strains that are free of selectable markers (EP-A-0 635 574). In this method the bidirectionality of the amdS marker is used to remove the marker from specially constructed expression cassettes once they have been introduced in the fungal genome. The method is, however, less compatible with the high copy numbers which are often necessary in industrial production strains. For these situations, a homologous and dominant selectable marker would still be required.

SUMMARY OF THE INVENTION

[0009] The present invention discloses novel DNA sequences encoding acetamidase genes from fungi other than Aspergillus nidulans, Aspergillus oryzae and Saccharomyces cerevisiae.

[0010] Preferably, these DNA sequences encode acetamidases which comprise an internal consensus fragment, the amino acid positional identity of which is less than 100% when compared with each of the amino acid sequences of SEQ ID NO: 1, SEQ ID NO: 2, and SEQ ID NO: 3, whereas this amino acid positional identity is more than 30% when compared with one of the amino acid sequences of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, and SEQ ID NO: 5.

[0011] The invention also discloses recombinant DNA constructs comprising the DNA sequences encoding the acetamidases of the invention, as well as recombinant cells containing these constructs.

[0012] The invention further discloses recombinant cells in which an endogenous copy of the gene encoding the acetamidase of the invention has been inactivated.

[0013] In a further embodiment, the invention discloses a process in which the recombinant cells of the invention are cultured in order to obtain a product of interest.

[0014] Finally, the invention discloses methods for obtaining the acetamidase genes of the invention, as well as methods for the inactivation of endogenous copies of these acetamidase genes.

BRIEF DESCRIPTION OF THE FIGURES

[0015] FIG. 1A: Amino acid comparison of amdS internal consensus fragments of the amdS genes from A. nidulans, A. oryzae, S. cerevisiae, A. niger and P. chrysogenum.

[0016] FIG. 1B: Amino acid positional identities between each of the amdS internal consensus fragments of FIG. 1A.

[0017] FIG. 2: Partial restriction map of the phage clone &lgr;AMD-1 and the subclones pGBAMD-2, pGBAMD-3 and pGBAMD-4.

[0018] Abbreviations used for the restriction enzymes:

[0019] B=BamHI, X=XbaI, P=PstI, S=SmaI, Sp=SpeI, K=KpnI, E=EcoRI.

[0020] FIG. 3: BamHI digests of two pGBAMD-4 transformants (lanes 1 and 2), two pGBAMD-3 transformants (lanes 3 and 4) and the parental strain A. niger CBS 513.88 (lane 5) probed with a 32P labelled EcoRI/BamHI fragment isolated from pGBAMD-1.

[0021] FIG. 4: shows schematically the amdS gene replacement vector pGBAMD-11.

[0022] Abbreviations used for the restriction enzymes:

[0023] B=BamHI, Sp=SpeI, K=KpnI, E=EcoRI, S=SmaI, N=NotI, H=HindIII.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Several terms used in the present description and claims are defined as follows.

[0025] The term gene is herein defined as a DNA sequence encoding a polypeptide, irrespective of whether the DNA sequence is a cDNA or a genomic DNA sequence which may contain one or more introns.

[0026] The term selection marker gene (or selectable marker gene) is herein defined as a gene which encodes a polypeptide that provides a phenotype to the cell containing the gene such that the phenotype allows either positive or negative, selection or screening of cells containing the selection marker gene. The selection marker gene may be used to distinguish between transformed and non-transformed cells or may be used to identify cells having undergone recombination or other kinds of genetic modifications.

[0027] An acetamidase is herein defined as an enzyme which is capable of catalysing the hydrolysis of acetamide into acetic acid and ammonium, and/or which is capable of catalysing the hydrolysis of related amide-compounds such as acrylamide or &ohgr;-amino acids.

[0028] An amdS gene is herein defined as a gene, which is preferably obtainable from a eukaryote, more preferably from a fungus, and which encodes a polypeptide that is an acetamidase as defined above. Preferably an amdS gene shows sequence similarity with one or more of the three amdS genes known in the art, i.e. the amdS genes from A. nidulans, A. oryzae or the amdS-like gene from S. cerevisiae. A more accurate description of the sequence similarity using the amino acid positional identity of an amdS internal consensus fragment is provided below. An amdS gene preferably encodes a protein of about 500 to 600 amino acids, more preferably of about 520 to 570 amino acids and most preferably of about 540 to 550 amino acids. An amdS gene is therefore usually contained within a DNA fragment of about 2.0 kb. Of course the presence of introns in a genomic amdS gene can increase the length to e.g about 2.5 kb or more.

[0029] The terms homologous gene is herein defined as a gene which is obtainable from a strain which belongs to the same species, including variants thereof, as does the strain actually containing the gene. Preferably, the donor and acceptor strain are the same. It is to be understood that the same applies to polypeptides encoded by homologous genes. Fragments and mutants of genes are also considered homologous when the gene from which the mutants or fragments are derived is a homologous gene. Also non-native combinations of regulatory sequences and coding sequences are considered homologous as long as the coding sequence is homologous. It follows that the term heterologous herein refers to genes or polypeptides for which donor and acceptor strains do not belong to the same species or variants thereof.

[0030] The term endogenous gene is herein defined as a naturally occurring copy of a gene in the genome of the organism in question.

[0031] The term fungus herein refers to all members of the division Eumycota of the kingdom Fungi and thus includes all filamentous fungi and yeasts.

[0032] In view of recent changes in the nomenclature of black Aspergilli, the term Aspergillus niger is herein defined as including all (black) Aspergilli that can be found in the Aspergillus niger Group as defined by Raper and Fennell (1965, In: The Genus Aspergillus, The Williams & Wilkins Company, Baltimore, pp 293-344). Similarly, also for the other Aspergillus species we will refer to the Aspergillus groups as defined by Raper and Fennell supra, thereby including all species and variants included in a particular group by these authors.

[0033] The present application describes the cloning of amdS genes from fungi not previously known to contain an amdS gene. A comparison of the three available amdS sequences was used to identify conserved regions in the amdS amino acid sequences. The conserved regions are herein defeined as short peptide fragments, e.g. 3-12 or more amino acids, which show a high degree of conservation, i.e. more than 80% identity, in the amino acid sequences of acetamidases from different organisms and which can be used to identify novel acetamidase genes, thereby relying on the fact that in the novel acetamidase these peptide fragments will also be conserved. On the basis of these conserved regions degenerate oligonucleotides were designed which were used as primers in experiments using Polymerase Chain Reactions (PCR) on genomic DNA isolated from A. niger and P. chrysogenum. Under certain PCR conditions amplified fragments were obtained which were subcloned and sequenced. The sequence analysis clearly identified the amplified PCR fragment as derived from the amdS genes of A. niger and P. chrysogenum by virtue of the homology of the encoded amino acid sequences to the known (translated) amdS amino acid sequences (see FIG. 1A).

[0034] The A. niger and P. chrysogenum amdS-PCR fragments, which only contained a small part of the amdS genes (approximately 500 and 400 bp, respectively), were used as hybridization probes to screen genomic libraries of A. niger and P. chrysogenum in order to obtain cloned genomic DNA fragments containing the entire amdS gene for these fungi (see e.g. FIG. 2 for A. niger). Restriction fragments in the genomic clones that hybridized to the PCR probes were subcloned into plasmids and subjected to sequence analysis. The resulting nucleotide sequences of the genomic amdS genes of A. niger and P. chrysogenum are presented in SEQ ID NO: 18 and SEQ ID NO: 19, respectively. In the absence of the corresponding cDNA sequences we cannot determine the exact positions of the introns in these genomic sequences. We have therefore not deduced the predicted amino acid sequences. Nevertheless, translation of all three reading frames of the genomic sequences allows to identify several areas (in addition to those corresponding to the above mentioned PCR fragments which encode the internal consensus fragments), which have significant amino acid positional identity with the known amdS amino acid sequences.

[0035] The present disclosure of the presence of amdS genes in A. niger and P. chrysogenum provides an incentive for the identification of amdS genes in other organisms, preferably fungi, which at present are not known to contain an amdS gene. The preferred candidates in this respect are the industrially important fungi such as the filamentous fungi belonging to the Aspergillus niger group, the Aspergillus glaucus group, the Aspergillus terreus group, the Aspergillus restrictus group, the Aspergillus fumigatus group, the Aspergillus cervinus group, the Aspergillus ornatus group, the Aspergillus clavatus group, the Aspergillus versicolor group, the Aspergillus ustus group, the Aspergillus wentii group, the Aspergillus ochraceus group, the Aspergillus candidus group, the Aspergillus cremeus group, the Aspergillus sparsus group, Trichoderma species such as T. reesei and T. harzianum, Mucor species such as M. miehei, Rhizopus species, Phanerochaete species, Neurospora species, Humicola species, Claviceps species, Sordaria species, Ustilago species, Fusarium species, Schizophyllum species, Penicillium species such as P. chrysogenum, Cephalosporium species, Acremonium species and edible fungi such as Agaricus bisporus, and yeasts such as Kluyveromyces species, Yarrowia species, Candida species, Hansenula and Pichia species. As many of the above fungi grow in their natural habitat by decomposing plant material, it is not unlikely that also plants will express genes involved in the metabolism of compounds like acetamide. Hence, plants may also contain an acetamidase gene.

[0036] The amdS genes of the invention show sequence similarity with other amdS (-like) genes. This sequence similarity is best defined by the amino acid positional identity of an internal consensus fragment within proteins encoded by amdS genes. The internal consensus fragment is the DNA (or protein) fragment which corresponds to a fragment in the A. nidulans amdS gene which encodes amino acids 125 to 226 (or the corresponding protein fragment), the amino acid sequence of which is provided in SEQ ID NO: 1. For the determination of the amino acid positional identity, the (encoded) amino acid sequences of the internal consensus fragments are lined up, introducing gaps if necessary for maximal identity, as is shown in FIG. 1A. The amino acid positional identity of two amdS sequences is subsequently expressed as the percentage of identical amino acids in the sequence of the complete internal consensus fragment of the shortest of the two amdS sequences (FIG. 1B). Using the amino acid positional identity, the amdS genes of the invention are defined as DNA sequences encoding proteins which comprise an amino acid sequence of which the amino acid positional identity with each of the amino acid sequences of SEQ ID NO: 1, SEQ ID NO: 2, and SEQ ID NO: 3 is less than 80%, preferably less than 90%, more preferably less than 95, and most preferably less than 100%, and of which the amino acid positional identity with one of the amino acid sequences of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, and SEQ ID NO: 5 is more than 30%, preferably more than 35, more preferably more than 40 and most preferably more than 45%.

[0037] The novel amdS sequences of the present invention can be used in conjunction with the already available amdS sequences to more accurately define the conserved regions in the amdS amino acid sequences and type of substitutions occurring therein. This will facilitate the design of improved degenerate oligonucleotides which will increase the chance of obtaining new amdS genes in PCRs or hybridization experiments.

[0038] Even though the preferred method for cloning new amdS genes is the method of the present invention, i.e. the use of degenerate oligonucleotides in a PCR on genomic DNA (or cDNA) and subsequent hybridization-screening of DNA (genomic- or cDNA) libraries to obtain the full length amdS gene, other methods can also be used for the cloning of new amdS genes. Such methods may include inverse PCR, heterologous hybridization, hybridization with (degenerate) oligonucleotides, (heterologous) complementation of amdS-negative mutants, or even screening of expression-libraries with suitable antibodies.

[0039] The novel amdS genes of the invention, e.g. those from A. niger, P. chrysogenum, or one of the other fungi mentioned above, can be used as a homologous selectable marker gene, which is herein understood to mean that the amdS gene is used to select transformants of the same species as the species from which the amdS gene was originally derived. This offers the advantage that the transformants obtained do not contain a foreign selectable marker gene. In principle this allows to construct recombinant strains which contain no foreign DNA other than absolutely necessary, i.e. the (heterologous) gene of interest to be expressed.

[0040] In a further embodiment of the invention, the native promoter of the homologous amdS gene is replaced by a different promoter. This replacement promoter, which is referred to as foreign promoter herein, can either be stronger than the native amdS promoter or it can be regulated in a different manner. Either way, the replacement of the native amdS promoter is intended to facilitate the selection of transformants, e.g. by increasing the growth advantage of transformants over non-transformants when grown on acetamide or related amide-compounds as sole N- or C-source. Preferably the foreign promoters are also homologous to the host in which they are used. Suitable foreign promoters can be derived from genes encoding glycolytic enzymes or enzymes involved in alcohol metabolism, such as the promoters from genes encoding phosphoglycerate kinases, glyceraldehyde-phosphate dehydrogenases, triose-phosphate kinases, pyruvate kinase or alcohol dehydrogenases.

[0041] In yet a further embodiment of the invention, the sequences of the novel amdS gene are used to inactivate the endogenous copy (or copies) of the amdS gene in the genome of the organism from which the novel amdS gene is derived. To this extent an inactivation vector can be constructed using the sequences of the novel amdS gene to target the vector to an endogenous copy of the gene by homologous recombination. The inactivation can then be caused either by replacement of, or by insertion into the endogenous amdS gene. Inactivation of the endogenous amdS gene provides the advantage of reducing the background of non-transformed cells in transformations using an amdS gene as selectable marker for the introduction of a gene of interest. Alternatively, the endogenous amdS locus can serve as a defined site of integration for genes of interest to be expressed.

[0042] The homologous amdS genes of the invention can be used in many different transformation procedures available to the skilled person, including inter alia direct transformation of integrating as well as autonomously replicating vectors, cotransformations in which the DNA to be transformed and the selectable marker are not physically linked, and transformation and subsequent curing of transformants in order to obtain MARKER GENE FREE™ recombinant strains as outlined in EP-A1-O 635 574, which is herein incorporated by reference.

[0043] The invention also discloses a method of culturing cells, at least a proportion of which consists of cells according to the invention, in a culture medium, wherein the culture medium comprises acetamide as sole carbon and/or nitrogen source, as well as a method wherein said culturing results in the enrichment of the proportion of cells according to invention.

[0044] The invention further discloses living cells according to the invention, preferably fungal cells, with the ability to grow well on a culture medium containing acetamide as sole carbon and/or nitrogen source and wherein said ability is not caused by the expression of a heterologous acetamidase gene but is rather caused by the expression, preferably overexpression, of a homologous acetamidase gene. The ability of a cell to grow well on a culture medium containing acetamide as sole carbon and/or nitrogen source is herein defined as the ability to grow faster than the corresponding wild-type cell, wherein wild-type is understood to mean wild-type with respect to its acetamidase genotype.

[0045] The present invention allows the preparation of recombinant cells which contain a recombinant homologous amdS gene, and/or which do not contain an active copy of an endogenous amdS gene. Usually these recombinant cells will further comprise genes of interest to be expressed and/or endogenous genes of interest which have been inactivated. Any one of these recombinant cells can be used in processes for the production of a product of interest. Such a process will usually include the steps of culturing the recombinant cells in a medium conducive to the production of the product of interest and recovery of the product of interest from the culture medium. The products of interest can be proteins, such as an enzyme, and/or primary metabolites, such as CO2, alcohol or organic acids, and/or secondary metabolites, such as antibiotics or carotenoids. The product of interest can also be the recombinant cells themselves, i.e. the biomass obtained in the process.

[0046] The following examples are given to illustrate the present invention.

EXAMPLES

Experimental

[0047] General Molecular Cloning Techniques

[0048] In the examples described herein, standard molecular cloning techniques such as isolation and purification of nucleic acids, electrophoresis of nucleic acids, enzymatic modification, cleavage and/or amplification of nucleic acids, transformation of E. coli, etc., were performed as described in the literature (Sambrook et al. (1989) “Molecular Cloning: a laboratory manual”, Cold Spring Harbour Laboratories, Cold Spring Harbour, N.Y.; Innis et al. (eds.) (1990) “PCR protocols, a guide to methods and applications” Academic Press, San Diego). Synthesis of oligo-deoxynucleotides and DNA sequence analysis were performed on an Applied Biosystems 380B DNA synthesizer and 373A DNA sequencer, respectively, according to the user manuals supplied by the manufacturer.

[0049] Transformation of A. niger

[0050] Transformation of A. niger was performed according to the method described by Tilburn, J. et. al. (1983) Gene 26, 205-221 and Kelly, J. & Hynes, M. (1985) EMBO J., 4, 475-479 with the following modifications:

[0051] spores were grown for 16 hours at 30° C. in a rotary shaker at 300 rpm in Aspergillus minimal medium. Aspergillus minimal medium consists of the following components: Per liter: 6 g NaNO3; 0.52 g KCl; 1.52 g KH2PO4; 1.12 ml 4M KOH; 0.52 g MgSO4.7H2O; 10 g glucose; 1 g casaminoacids; 22 mg ZnSO4.7H2O; 11 mg H3BO3; 5 mg FeSO4.7H2O; 1.7 mg CoCl2.6H2O; 1.6 mg CuSO4.5H2O; 5 mg MnCl2.4H2O; 1.5 mg Na2MoO4.2H2O; 50 mg EDTA; 2 mg riboflavin; 2 mg thiamine.HCl; 2 mg nicotinamide; 1 mg pyridoxine.HCl; 0.2 mg panthotenic acid; 4 &mgr;g biotin; 10 ml Penicillin (5000 IU/ml)/Streptomycin (5000 UG/ml) solution (Gibco).

[0052] only Novozym 234 (Novo Industri), and no helicase, was used for formation of protoplasts;

[0053] after protoplast formation (60-90 minutes), KC buffer (0.8 M KCl, 9.5 mM citric acid, pH6.2) was added to a volume of 45 ml. and the protoplast suspension was centrifuged at 2500 g at 4° C. for 10 minutes in a swinging-bucket rotor. The protoplasts were resuspended in 20 ml. KC buffer. Then, 25 ml of STC buffer (1.2 M sorbitol, 10 mM Tris-HCl pH7.5, 50 mM CaCl2) was added and subsequently the protoplast suspension was centrifuged at 2500 g at 4° C. for 10 minutes in a swinging-bucket rotor, washed in STC-buffer and resuspended in STC-buffer at a concentration of 108 protoplasts/ml;

[0054] to 200 &mgr;l of the protoplast suspension the DNA fragment, in a volume of 10 &mgr;l in TE buffer (10 mM Tris-HCl pH7.5, 0.1 mM EDTA), was added and subsequently 100 &mgr;l of a PEG solution (20% PEG 4000 (Merck), 0.8 M sorbitol, 10 mM Tris-HCl pH7.5, 50 mM CaCl2);

[0055] after incubation of the DNA-protoplast suspension at room temperature for 10 minutes, 1.5 ml PEG solution (60% PEG 4000 (Merck), 10 mM Tris-HCl pH7.5, 50 mM CaCl2) was added slowly, with repeated mixing of the tubes. After incubation at room temperature for 20 minutes, the suspensions were diluted with 5 ml STC buffer, mixed by inversion and centrifuged at 2000 g at room temperature for 10 minutes. The protoplasts were resuspended gently in 1 ml 1.2 M sorbitol and plated onto selective regeneration medium consisting of Aspergillus minimal medium without riboflavin, thiamine.HCL, nicotinamide, pyridoxine.HCl, panthotenic acid, biotin, casaminoacids and glucose but with 10 mM acetamide as the sole nitrogen source, 1 M sucrose, solidified with 2% bacteriological agar #1 (Oxoid, England).

[0056] Following growth for 6-10 days at 30° C., the plates were replica plated onto selective acetamide plates consisting of Aspergillus selective regeneration medium with 2% glucose instead of sucrose and 1.5% agarose instead of agar. Single transformants were isolated after 5-10 days of growth at 30° C.

[0057] Isolation of Chromosomal DNA from Aspergillus.

[0058] The isolation of DNA from Aspergillus was performed according to the procedure as described by Yelton, et al. (1984), Proc. Natl. Acad. Sci. 81, 1470-1474.

[0059] Construction of a Genomic Library of Asoergillus niger CBS 513.88

[0060] Chromosomal DNA isolated from A. niger CBS 513.88 was partially digested with Sau3Al, ligated to the BamHI sites of the &lgr;EMBL3 arms (e.g. Promega), packaged and transfected to E. coli according to Sambrook et al. (1989) “Molecular Cloning: a laboratory manual”, Cold Spring Harbour Laboratories, Cold Spring Harbour.

[0061] Construction of a Genomic Library of Penicillium chrysogenum

[0062] Chromosomal DNA isolated from P. chrysogenum Wisconsin 54-1255 was partially digested with Sau3AI. Fragments with length varying between 7-12 Kb were ligated to the artificially created BamHI sites of the &lgr;ZAPII arms (e.g. Stratagene), packaged and transfected to E. coli according to Sambrook et al. (1989) “Molecular Cloning: a laboratory manual”, Cold Spring Harbour Laboratories, Cold Spring Harbour.

[0063] Counter-Selection on Fluoracetamide

[0064] Removal of the A. nidulans amdS selection marker is achieved by internal recombination between the 3′-A. niger amdS non coding repeats that flank the A. nidulans amdS selection marker. Selection of cells that have lost the amdS selection marker is achieved by growth on plates containing fluoracetamide. Cells harbouring the amdS gene metabolize fluoracetamide to ammonium and fluoracetate which is toxic to the cell so only cells that have lost the amdS gene are able to grow on plates containing fluoracetamide.

[0065] In case of removal of the amdS marker from Aspergillus transformants, spores from these transformants were plated onto selective regeneration medium (described above) containing 32 mM fluoracetamide and 5 mM ureum instead of 10 mM acetamide, 1.1% glucose instead of 1 M sucrose and 1.1% instead of 2% bacteriological agar #1 (Oxoid, England). After 7-10 days of growth at 35° C. single colonies were harvested and plated onto 0.4% potato dextrose agar (Oxoid, England).

Example 1

[0066] Cloning of the amdS of Aspergillus niger CBS 513.88

Example 1.1

[0067] Synthesis of an amdS Specific PCR Fragment

[0068] Oligonucleotide mixes corresponding to the coding and the non-coding DNA strands were designed in well conserved amino acid sequences of the amdS genes from Aspergillus nidulans (Corrick M. C., Twomey A. P., Hynes M. J. (1987) Gene 53 63-71), Aspergillus oryzae (Gomi K., Kitamoto K., Kumagai C. (1991) Gene 108 91-98) and the amdY gene from Saccharomyces cerevisiae (Chang T. H., Abelson J. (1990) Nucleic Acids Res. 18 7180. These oligonucleotide mixes have the following sequences: 1 4078: 5′ CGG GAT CCG CNT TTT GTA ANA GNG CNG C 3′                     C   CC   C 4079: 5′ CGG GAT CCN ATT AGN CTN AAG GAT CA 3′                  C TC  T     A   C                  A 4080: 5′ GGA ATT CCC TCN CCN CCN CTN CTN CC 3′              T             GA  GA 4081: 5′ GGA ATT CTA ATN CTN CCN CC 3′             GT      GA              G 4082: 5′ GGA ATT CCN CCA ATA TCN GTN CC 3′                  G   G                  T

[0069] The oligonucleotide mixes were used in PCR with chromosomal DNA from A. niger CBS 513.88 as template. The combinations of two oligonucleotide mixes 4078/4082; 4079/4080 and 4079/4082 (100 pmole each) respectively, were used in reactions with a 50 &mgr;l reaction volume also containing 0.5 &mgr;g chromosomal DNA from A. niger CBS 513.88, 100 nmole dNTP's, Amplitaq reaction buffer (Perkin Elmer) and 1 U Amplitaq (Perkin-Elmer). The conditions for PCR were as follows: After denaturation for 1 min at 94° C., at 72° C. the Amplitaq is added. Next, 30 cycles each 2 min. 94° C.; 2 min. x° C. (x=65°-50° C.; every two cycles x decreases with 1° C.) and 3 min. 72° C. were carried out finally followed by 7 min. 72° C.

[0070] The reaction products were analyzed by electrophoresis using an 1% TBE-agarose gel. Only the combination of oligonucleotide mixture 4078 and 4082 resulted in a reaction product, which was approximately 500 bp in lenght. This PCR fragment was digested with BamHI and EcoRI, purified by agarose electrophoresis and ethanol precipitation and cloned into the BamHI and EcoRI sites of pTZ18R (United States Biochemicals). The resulting plasmid was designated pGBAMD-1.

Example 1.2

[0071] Screening of the Asperguillus niger CBS 513.88 Genomic Library for the amdS Gene

[0072] An A. niger CBS 513.88 genomic library, constructed in &lgr;-EMBL3 as described in the experimental section, was screened using the 32P-labelled EcoRI/BamHI fragment isolated from pGBAMD-1. Hybridization with the 32P labelled EcoRI/BamHI fragment isolated from pGBAMD-1 took place overnight at 65° C. in hybridization buffer containing 4×SSC, 5× Denhardt's solution, 0.1% SDS and 100 &mgr;g/ml heat denatured calf thymus DNA. After hybridization, the filters were washed in 4×SSC/0.1% SDS, 2×SSC/0.1% SDS and 1×SSC/0.1% SDS at 65° C.

[0073] Four plaques, hybridizing with the PCR fragment were identified and isolated and purified. These phage clones were designated &lgr;AMD1-&lgr;AMD4.

Example 1.3

[0074] Restriction Analysis of amdS Containing Phase Clones &lgr;AMD1-&lgr;AMD4

[0075] A partial restriction map was constructed for one of the four phage clones, i.e. &lgr;AMD1. The isolated phage DNA was digested with several restriction enzymes, run on a 0.7% agarose gel, blotted onto nitrocellulose (0.2 &mgr;m; Schleicher & Schull) and hybridized with the 32P-labelled EcoRI/BamHI fragment isolated from pGBAMD-1. From the results obtained, a partial restriction map was constructed (see FIG. 2).

Example 1.4

[0076] Subcloning Fragments of Phage Clone &lgr;AMD-1

[0077] Phage clone &lgr;AMD-1 contained an insert that was supposed large enough to comprise the entire amdS gene. Several fragments from this phage clone &lgr;AMD-1 were subcloned into either pTZ18R or pTZ19R (United States Biochemicals). First, an approximately 2.3 kb EcoRI fragment was isolated from &lgr;AMD-1 by digestion of the phage DNA by EcoRI, followed by agarose electrophoresis. The fragment was cloned into the EcoRI site of pTZ18R. The resulting plasmid was designated pGBAMD-2.

[0078] Next, the approximately 5 kb SpeI/KpnI fragment was isolated by digesting the phage DNA with SpeI and KpnI followed by agarose electrophoresis. The approximately 5 kb SpeI/KpnI fragment was cloned into the XbaI and KpnI sites of pTZ19R. In this cloning step both the SpeI and the XbaI sites are destroyed. The resulting plasmid was designated pGBAMD-3.

[0079] Finally, the approximately 8 kb KpnI fragment was isolated by digesting the phage DNA with KpnI followed by agarose electrophoresis. The isolated fragment was cloned into the Kpni site of pTZ19R. The resulting plasmid was designated pGBAMD-4. A schematic overview of the different subclones is given in FIG. 2.

Example 1.5

[0080] Sequence Analysis of the A. niger amdS Gene

[0081] In order to determine whether the isolated PCR fragment was a part of the A. niger amdS gene, the sequence of this fragment was determined (presented in SEQ ID NO: 16), translated to an amino acid sequence and compared to the amino acid sequences of the A. nidulans, A. oryzae amdS genes and the S. cerevisiae amdY gene (see FIG. 1A). A considerable homology was found between the PCR fragment and part of the amdS and amdY genes. Therefore it was concluded that the PCR fragment is a part of the A. niger homologue of the amdS gene. To obtain the entire genomic nucleotide sequence of the A. niger amdS locus, the sequence of part (about 2.8 kb) of the SpeI/KpnI DNA fragment of pGBAMD-3 was determined as well. This sequence is presented in SEQ ID NO: 18.

Example 2

[0082] Use of the A. niger amdS Gene as Selection Marker Gene in Transformation of A. niger CBS 513.88

[0083] In order to determine whether the A. niger homologue of the A. nidulans amdS gene could be used as a selection marker gene in transformations of A. niger, DNA from subclones pGBAMD-2, pGBAMD-3 and pGBAMD-4 containing probably the entire coding region of the A. niger amdS gene with more or less of the regulatory sequences (promoter and terminator sequences) was used to transform A. niger CBS 513.88 according to the method described in the experimental section.

Example 2.1

[0084] Transformation of A. niger CBS 513.88 with Subclones pGBAMD-2, pGBAMD-3 and pGBAMD-4

[0085] From the plasmids pGBAMD-2, pGBAMD-3 and pGBAMD-4 10 &mgr;g, 20 &mgr;g and 50 &mgr;g plasmid DNA was transformed to A. niger CBS 513.88 according to the method described in the experimental section. Only with plasmids pGBAMD-3 and pGBAMD-4 transformants could be generated that were able to grow on acetamide as sole nitrogen source.

Example 2.2

[0086] Genetic Analysis of A. niger pGBAMD-3 resp. pGBAMD-4 Transformants

[0087] To verify that the generated transformants were genuine transformants that had taken up the plasmid DNA two A. niger/pGBAMD-3 transformants and two A. niger/pGBAMD-4 transformants were analysed using Southern analysis. From these transformants and from the untransformed A. niger host strain, high molecular weight DNA was isolated, digested with BamHI, separated by agarose gelelectrophoresis and blotted onto nitrocellulose. The blotted DNA was hybridized with the 32P labelled EcoRI/BamHI fragment isolated from pGBAMD-1. The results are presented in FIG. 3.

[0088] Characteristic for the endogenous amdS gene is an approximately 9 kb hybridizing fragment (see FIG. 3, lane 5). Characteristic for the presence of the pGBAMD-3 plasmid is an approximately 5 kb hybridizing fragment and characteristic for the presence of the pGBAMD-4 plasmid is an approximately 7.5 kb hybridizing fragment. As can be seen in FIG. 3, lanes 3, 4, and lanes 1, 2, hybridizing fragments characteristic for the presence of pGBAMD-3 and pGBAMD-4, respectively, are detected in the transformants. Therefore it can be concluded that the A. niger amdS gene can be used as selection marker gene in transformations of A. niger.

Example 3

[0089] Marker Gene Free™ Deletion of the A. niger amdS Gene

[0090] This example describes the deletion of the A. niger amdS coding region and a (proximal) part of the amdS promoter with a replacement vector which integrates into the A. niger genome via a double cross-over homologous recombination. The replacement vector comprises a DNA region homologous to the target locus interrupted by a selectable marker gene flanked by DNA repeats.

[0091] The replacement vector comprises a part of the A. niger amdS genomic locus, wherein the amdS coding sequences as well as a part of the amdS promoter sequences are replaced by the A. nidulans amdS gene under control of the A. nidulans gpdA promoter as selection marker flanked by 3′-untranslated A. niger amdS sequences as direct repeats. Transformation of A. niger with this vector directs the replacement of the A. niger amdS gene by the A. nidulans amdS gene. By performing the fluoracetamide counter-selection on these transformants as described in the experimental procedures, the A. nidulans amdS gene is properly deleted by an internal recombination event between the 3′-A. niger amdS repeats, resulting in a MARKER GENE FREE™ &Dgr;amdS recombinant strain, containing no foreign DNA sequences at all.

Example 3.1

[0092] Construction Pathway of the amdS Gene Replacement Vector

[0093] The first steps in the construction pathway of the A. niger amdS gene replacement vector is the construction of a plasmid with a suitable multiple cloning site. To achieve this, the plasmid pTZ18R (United States Biochemicals) was digested with EcoRI and HindIII and the approximately 2.8 kb fragment was purified by agarose electrophoresis and ethanol precipitation and in this fragment two different synthetic fragments of two oligonucleotides were cloned. One synthetic fragment comprises the recognition sites for the restriction enzymes NotI, EcoRI, KpnI, Bg/II, SmaI and HindIII and has the following sequence: 2 5′ AATTG GCGGCCGC GAATTC GGTACC AGATCT ATAG GGGCCC        ¦ ¦¦¦¦¦¦¦¦ ¦¦¦¦¦¦ ¦¦¦¦¦¦ ¦¦¦¦¦¦ ¦¦¦¦ ¦¦¦¦¦¦ 3′     C CGCCGGCG CTTAAG CCATGG TCTAGA TATC CCCGGG A 3′     ¦        TTCGA 5′

[0094] The resulting plasmid was designated pGBAMD-5.

[0095] The other synthetic fragment comprises the recognition sites for the restriction sites NotI, HindIII, KpnI, SpeI, BamHI, PstI, and NotI and has the following sequence: 3 5′ AATTG GCGGCCGC AAGCTT GGTACC ACTAGT GGATCC GCAA        ¦ ¦¦¦¦¦¦¦¦ ¦¦¦¦¦¦ ¦¦¦¦¦¦ ¦¦¦¦¦¦ ¦¦¦¦¦¦ ¦¦¦¦     3′ C CGCCGGCG TTCGAA CCATGG TGATCA CCTAGG CGTT CTGCAG- GCGGCCGC T 3′ ¦¦¦¦¦¦  ¦¦¦¦¦¦¦¦ ¦ GACGTC- CGCCGGCG ATCGA 5′

[0096] The resulting plasmid was designated pGBAMD-6.

[0097] Next, the approximately 2.5 kb in size BamHI/SmaI fragment from pGBAMD-4, comprising the supposed 3′non-coding region of the A. niger amdS gene was cloned into the Bg/II and SmaI sites of pGBAMD-5. In this cloning step both the Bg/II and the BamHI sites were destroyed. The new plasmid was designated pGBAMD-7. This plasmid was digested with EcoRI and KpnI and in these sites the approximately 3.1 kb fragment comprising the A. nidulans amdS gene under control of the A. nidulans gpdA promoter, isolated from pGBGLA25 (EP O 635 574 A1), was ligated. The new plasmid was named pGBAMD-8.

[0098] Plasmid pGBAMD-6 was digested with BamHI and PstI and in these sites was ligated the approximately 2 kb BamHI/PstI fragment isolated from pGBAMD-4 and comprising part of the supposed 3′non-coding region of the amdS gene. The resulting plasmid was named pGBAMD-9.

[0099] Next, pGBAMD-9 was digested with KpnI and SpeI and in these sites was ligated the approximately 2.7 kb KpnI/SpeI fragment isolated from pGBAMD-4 amd comprising part of the 5′ promoter region of the amdS gene. The resulting plasmid was named pGBAMD-10.

[0100] Finally, plasmid pGBAMD-8 was digested with NotI and in this site was ligated the approximately 4.7 kb fragment isolated from pGBAMD-10 and comprising a 540 part of the promoter region and part of the 3′ non-coding sequence both of the amdS gene. The resulting plasmid with the cloned fragment in the correct orientation is named pGBAMD-11 and is the replacement vector that is used to delete the A. niger amdS gene using the MARKER GENE FREE™ approach.

Example 3.2

[0101] Inactivation of the Endogenous A. niger amdS Gene

[0102] Prior to transformation of A. niger with pGBAMD-11, the E. coli sequences were removed by HindIII digestion and agarose gel electrophoresis. The A. niger strain CBS 513.88 (deposited Oct. 10, 1988) was transformed with either 2.5, 5 or 10 &mgr;g DNA fragment by procedures as described in experimental procedures using acetamide as sole N-source in selective plates. Single A. niger transformants were purified several times onto selective acetamide containing minimal plates. Spores of individual transformants were collected by growing for about 5 days at 30° C. on 0.4% potato-dextrose (Oxoid, England) agar plates. Southern analyses were performed to verify the presence of the truncated amdS locus.

Example 3.3

[0103] Removal of the A. nidulans amdS Selection Marker Gene by Counter-Selection on Fluoracetamide Containing Plates.

[0104] The A. nidulans amdS gene in the generated transformants was removed again as described in the Experimental section. Correct removal of the A. nidulans amdS selection marker gene was verified by Southern analyses of chromosomal DNA of several fluoracetamide resistant strains.

Example 4

[0105] Cloning of the Penicillium Chrysogenum amdS Gene

Example 4.1

[0106] Amplification of an Internal Fragment of the Penicillium Chrysogenum amdS Gene using Degenerate Oligonucleotides in a PCR on Genomic DNA

[0107] At first, the same oligonucleotide combinations were used as described in example 1.1. The oligonucleotide mixtures were used in PCR with chromosomal DNA from P. chrysogenum Wisconsin 54-1255 as template. The conditions for PCR were exactly the same as described in example 1.1.

[0108] It was found that oligonucleotide mixtures with oligo AB4082 gave several reaction products. However, oligo AB 4082 itself also could generate a PCR product of the expected molecular size. Therefore it was decided to make a slightly different degenerate oligonucleotide i.e. oligo AB 5224 (SEQ ID NO: 15): 4 Oligo AB5224: 5′GGAATTCCAATNCTNCCNCCAATATC 3′          TG   GA      G  G           T           T

[0109] The new combination of oligonucleotides AB4079 and AB5424 gave a distinct PCR product; the other combinations of oligonucleotides were not successful.

[0110] The PCR product obtained was cloned using the ‘InVitroGen’ TA cloning kit. Cloned PCR fragments were further characterized by DNA sequence analysis (see SEQ ID NO: 17). These DNA sequences were analyzed for ORF's. The amino acid sequence as presented in SEQ ID NO: 5 was the result of this analysis. It was concluded that the cloned PCR fragment was part of the P. chrysogenum amdS gene. The plasmid with the cloned PCR fragment was called pPENAMDS2.

[0111] This PCR fragment was used to clone the entire P. chrysogenum amdS gene, which can subsequently be used as homologous selectable marker and/or to inactivate the endogenous P. chrysogenum amdS gene as we have outlined above for the A. niger amdS gene.

Example 4.2

[0112] Screening of the Penicillium Chrysogenum Genomic Library for the amdS Gene

[0113] A P. chrysogenum Wisconsin 54-1255 genomic library, constructed in &lgr;-ZAPII (Stratagene, San Diego) as described in the experimental section, was screened using the 32P-labelled EcoRI fragment isolated from pPENAMDS2. Hybridization with the 32P-labelled EcoRI fragment took place overnight at 65° C. in hybridization buffer containing 4×SSC, 5× Denhardt's solution, 0.1% SDS and 100 &mgr;g/ml heat denatured calf thymus DNA. After hybridization, the filters were washed in 4×SSC/0.1% SDS, 2×SSC/0.1% SDS and 1×SSC/0.1% SDS at 65° C.

[0114] One plaque, hybridizing with this probe was identified, isolated and purified. This phage clone was designated &lgr;PENAMD1.

Example 4.3

[0115] Sub-Cloning and Restriction Analysis of amdS Containing Phage Clone &lgr;PENAMD1

[0116] Sub-cloning was done according the protocol of the &lgr;-ZAPII system (Stratagene, San Diego). The result of the sub-cloning experiment is a plasmid that exists of the pBluescript SK vector and an insert of P. chrysogenum chromosomal DNA (pPENAMDS101). A partial restriction map was constructed for clone pPENAMDS101. The isolated plasmid DNA was digested with several restriction enzymes, run on a 0.7% agarose gel, blotted onto nitrocellulose (0.2 &mgr;m; Schleicher & Schull) and hybridized with the 32P-O labelled EcoRI fragment isolated from pPENAMDS2. From the results obtained, a partial restriction map was constructed.

Example 4.4

[0117] Subcloning and Sequencing of the Pc amdS Containing Fragment of Plasmid pPENAMDS101

[0118] Plasmid clone pPENAMDS101 contained an NruI-Sa/I insert of 3.3 Kb that was supposed large enough to comprise the entire amdS gene. The NruI/Sa/I fragment of pPENAMDS101 was isolated by digesting the plasmid DNA with NruI and Sa/I followed by agarose electrophoresis. The approximately 3 kb NruI/Sa/I fragment was ligated in the pBluescript IIKS vector that was already digested with Sa/I and SmaI. After ligation and transformation in E. coli Inv&agr;F, transformants were screened using restriction analysis. The resulting plasmid was designated pPENAMDSFL. The result of the DNA sequence analysis of clone pPENAMDSFL is given in SEQ ID NO: 19.

Claims

1. A DNA sequence encoding an acentamidase and characterized in that the DNA sequence is derived from an organism other than Aspergillus nidulans, Aspergillus oryzae and Saccharomyces cerevisiae.

2. A DNA sequence encoding an acetamidase which comprises an a amino acid sequence which is characterized in that:

a) its amino acid positional identity with one of the amino acid sequences of SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, and SEQ ID NO: 5 is more than 30%, and

b) its amino acid positional identity with each of the amino acid sequences of SEQ ID NO: 1, SEQ ID NO: 2, and SEQ ID NO: 3 is less than 100%.

3. A DNA sequences according to claims 1 or 2, and characterized in that the DNA sequence is derived from an organism selected from the group consisting of:

fungi from the Aspergillus niger group, the Aspergillus glaucus group, the Aspergillus terreus group, the Aspergillus restrictus group, the Aspergillus fumigatus group, the Aspergillus cervinus group, the Aspergillus ornatus group, the Aspergillus clavatus group, the Aspergillus versicolor group, the Aspergillus ustus group, the Aspergillus wentii group, the Aspergillus ochraceus group, the Aspergillus candidus group, the Aspergillus cremeus group, the Aspergillus sparsus group, Penicillius species, Trichoderma species, Mucor species, Rhizopus species, Phanerochaete species, Neurospora species, Humicola species, Claviceps species, Sordaria species, Ustilago species, Fusarium species, Schizophyllum species, Cephalosporium species, Acremonium species, edible fungi of which preferably Agaricus bisporus, Kluyveromyces species, Yarrowia species, Candia species, Hansenula species, Pichia species and Phaffia species.

4. A DNA sequence according to claim 3 and characterized in that the DNA sequence is derived from Aspergillus niger or Penicillium chrysogenum.

5. A recombinant DNA construct which comprises a DNA sequence as defined in any one of claims 1-4.

6. A recombinant DNA construct according to claim 5 and characterized in that the DNA sequence is operably linked to a promoter which is native to the DNA sequence.

7. A recombinant DNA construct according to claim 5 and characterized in that the DNA sequence is operably linked to a promoter which is foreign to the DNA sequence.

8. A recombinant DNA construct according to claim 7, wherein the promoter foreign to the DNA sequence is a promoter derived from a gene which is selected from the group consisting of the genes encoding glycolytic enzymes and enzymes involved in alcohol metabolism.

9. A recombinant DNA construct according to any one of claims 7-8, wherein the DNA sequence and the promoter are derived from the same species.

10. A recombinant DNA construct according to any one of claims 5-9, further comprising a gene of interest.

11. A cell which comprises a recombinant DNA construct as defined in any one of claims 5-10.

12. A cell characterized in that the cell does not contain an active endogenous copy of an acetamidase gene comprising a DNA sequence as defined in any one of claims 1-4.

13. A cell according to claim 12, characterized in that the endogenous copy of the acetamidase gene has been inactivated by a homologous recombination event.

14. A cell according to claim 13 and characterized in that the homologous recombination event is a gene-replacement.

15. A cell according to any one of claims 11-14, further comprising a recombinant gene of interest.

16. A cell according to any one of claims 11-15, further characterized in that an endogenous gene of interest has been inactivated.

17. A process for the production of a desired product, and characterized in that the process comprises the steps of:

a) culturing a recombinant cell as defined in claims 15 or 16, under conditions conducive to the production of the product, and

b) recovering the product.

18. A method for obtaining an DNA sequence as defined in any one of claims 1-4, wherein the method comprises the steps of:

a) identification of conserved regions in the amino acid sequences of known acetamidase genes,

b) amplification of a DNA fragment in a PCR, using as primers a set of degenerate oligonucleotides corresponding to the conserved regions identified in a), on a template nucleic acid containing the DNA sequence,

c) isolation of the DNA sequence using the DNA fragment amplified in b) as hybridization probe to screen a DNA library containing the DNA sequence.

19. A DNA sequence as defined in any one of claims 1-4, wherein the DNA sequence is obtainable by the method of claim 18.

20. A method for inactivation of an endogenous copy of an acetamidase gene comprising a DNA sequence as defined in any one of claims 1-4, wherein the method comprises the steps of:

a) construction of an inactivation vector comprising a disrupted copy of the acetamidase gene,

b) transformation of a host cell with the inactivation vector,

c) selection of transformants with an inactivated copy of the acetamidase gene.

21. A method according to claim 20, wherein the inactivation vector is capable of replacing the endogenous copy of the acetamidase gene.

22. A method of culturing cells at least a proportion of which consists of cells according to claim 15 or 16 in a culture medium, wherein the culture medium comprises acetamide as sole carbon and/or nitrogen source.

23. The method according to claim 22, wherein said culturing results in the enrichment of the proportion of cells according to claim 15 or 16.

24. A living cell selected from the group consisting of fungi from the Aspergillus niger group, the Aspergillus glaucus group, the Aspergillus terreus group, the Aspergillus restrictus group, the Aspergillus fumigatus group, the Aspergillus cervinus group, the Aspergillus ornatus group, the Aspergillus clavatus group, the Aspergillus versicolor group, the Aspergillus ustus group, the Aspergillus wentii group, the Aspergillus ochraceus group, the Aspergillus candidus group, the Aspergillus cremeus group, the Aspergillus sparsus group, Penicillium species, Trichoderma species, Mucor species, Rhizopus species, Phanerochaete species, Neurospora species, Humicola species, Claviceps species, Sordaria species, Ustilago species, Fusarium species, Schizophyllum species, Cephalosporium species, Acremonium species, edible fungi of which preferably Agaricus bisporus, Kluyveromyces species, Yarrowia species, Candida species, Hansenula species, Pichia species and Phaffia species, characterized by their ability to grow well on a culture medium containing acetamide as sole carbon and/or nitrogen source and wherein said ability is not caused by the expression of a heterologous acetamidase gene.