Abstract: The present invention relates to a microbial expression system for the production of polypeptides based on the use of extrachromosomal DNA, whereby no antibiotic marker genes for the selection of the host cell but DNA sequences that encode glycerine-3-phosphate dehydrogenase are used, and, thus, the production of the desired polypeptide, e.g., xylanase, does not need the addition of antibiotics. The expression system is free from antibiotic-resistance genes. The invention further relates to a DNA sequence that encodes a polypeptide with glycerine-3-phosphate dehydrogenase activity as well as a polypeptide with glycerine-3-phosphate dehydrogenase activity.

Abstract: The invention relates to an expression system for the production of one or more target polypeptide/target polypeptides, comprising a host cell in whose genome the DNA sequence that codes glycerine-3-phosphate dehydrogenase is inactivated or partially or completely deleted and which is transformed by an extrachromosomal element that comprises a DNA sequence that codes the target polypeptide(s) and glycerine-3-phosphate dehydrogenase, whereby not only the host cell genome but also the extrachromosomal element do not carry an antibiotic-resistance gene, as well as a DNA sequence that codes for a polypeptide with glycerine-3-phosphate dehydrogenase activity characterized in that the DNA sequence is selected from a) DNA sequences that comprise a nucleotide sequence according to SEQ ID NO: 1, b) DNA sequences that comprise a nucleotide sequence represented by the nucleotides 1338 to 2375 of SEQ ID NO: 1, c) DNA sequences that are coded by the plasmid pTP01 with the plasmid map according to FIG.