Abstract: The plasmid pSG5 which, because of its molecular length of about 12.7 kb and its four unique restriction sites, can be used for the construction of plasmid vectors for Streptomycetes, for example of shuttle vectors between Streptomycetes and other microorganisms, can be isolated from a culture of Streptomyces ghanaensis DSM 2932.

Abstract: Isolation and characterization of a gene which activated nitrogen fixation genes of Rhizobium meliloti when that bacterium is in a symbiotic relationship with a plant is disclosed. This newly discovered gene, designated fix D, can activate the nifHD promoter. A method of making this inducible gene constitutive is presented. This is useful for making nifHD constitutive. The combination of the fixD promoter with heterologous structural genes is taught. Such combinations are useful for limiting expression of an encoded protein to rhizobia involved in a symbiotic relationship with a plant. Plasmids and methods useful in performance of this invention are also disclosed.

Abstract: A Tn-Mob element is provided comprising a transposon with a Mob-site inserted therein. Vectors and bacterial strains are provided containing the Tn-Mob element, as well as methods for its use. Such methods include the production of plasmid free bacterial strains.

Abstract: Vectors capable of replication in a broad host range of gram-negative bacteria are provided. The vectors are composite plasmids comprising a segment derived from an E. coli vector plasmid and a segment derived from a broad host range plasmid. The broad host range plasmid segment includes Mob-functions and DNA coding for replication functions with regions non-essential for broad host range replication or mobilizablity having been deleted. Bacterial strains containing the above plasmids as well as methods of cloning DNA employing such plasmids are also provided.

Abstract: The plasmid p SVH 1, which is obtained from Streptomyces venezuelae DSM 40755 and which has a molecular weight of 8.4 megadaltons, a contour length of 4.1 .mu.m and a molecular size of 12.6 kilobases, and its use for the construction of a vector and for cloning foreign DNA in suitable host organisms.

Abstract: Modified E. coli vectors are disclosed that are mobilizable to a wide range of gram-negative bacteria, but are not self-transmissible. The modified E. coli plasmids replicate only in bacterial strains of the E. coli group. Mobilizer strains of E. coli are also provided, as well as methods for employing the modified E. coli vectors for transposon mutagenesis, site-specific gene transfer, and the construction of DNA libraries.

Abstract: The new streptomyces plasmid p SG 2, having a molecular weight of 9.2 megadaltons, a contour length of 4.58 .mu.m and a molecular length of about 13.8 kilobases, is described, together with its preparation from a culture of "Streptomyces ghanaensis" ATCC 14 672.