Improvement of histidine-producing strains of Serratia marcescens by cloning of a mutant allele of the histidine operon on a mini-F plasmid vector.

Two hybrid plasmids bearing a mutant allele of the histidine operon of Serratia marcescens on a mini-F plasmid vector were constructed: one was plasmid pSS503 harboring the 4.7-kb EcoRl fragment bearing the initial part of the histidine operon and the other was plasmid pSH368 harboring the 12-kb EcoRI-BamHI fragment bearing the whole histidine operon. Both plasmids increased the activities of ATP-phosphoribosyltransferase (hisG product), histidinol dehydrogenase (hisD product) and histidinol phosphate phosphatase (hisB product) about two-fold, as compared with those observed with the plasmid-free host strain, a histidine-producing strain, L120, of Serratia marcescens. Both plasmids were considerably unstable in the histidine-producing strain, although they were completely stable in the wild-type strain. Strain L120 carrying either pSS503 or pSH 368 produced 42mg/ml or L-histidine; this productivity was approximately 50% more than that of plasmid-free strain L120.