PRODUCTION OF ISOLEUCINE BY OVEREXPRESSION OF ILVA IN A CORYNEBACTERIUM-LACTOFERMENTUM THREONINE PRODUCER

Overproduction of isoleucine, an essential amino acid, was achieved by amplification of the gene encoding threonine dehydratase, the first e nzyme in the threonine to isoleucine pathway, in a Corynebacterium lac tofermentum threonine producer. Threonine over-production was previous ly achieved with C. lactofermentuun ATCC 21799, a lysine-hyperproducin g strain, by introduction of plasmid pGC42 containing the Corynebacter ium hom(dr) and thrB genes (encoding homoserine dehydrogenase and homo serine kinase respectively) under separate promoters. The pGC42 deriva tive, pGC77, also contains ilvA, which encodes threonine dehydratase. In a shake-flask fermentation, strain 21799(pGC77) produced 15 g/l iso leucine, along with small amounts of lysine and glycine. A molar carbo n balance indicates that most of the carbon previously converted to th reonine, lysine, glycine and isoleucine was incorporated into isoleuci ne by the new strain. Thus, in our system, simple overexpression of wi ld-type ilvA sufficed to overcome the effects of feedback inhibition o f threonine dehydratase by the end-product, isoleucine.