EFFECTS OF THE AMPLIFICATION OF THE GENES-CODING FOR THE L-THREONINE BIOSYNTHETIC-ENZYMES ON THE L-THREONINE PRODUCTION FROM METHANOL BY A GRAM-NEGATIVE OBLIGATE METHYLOTROPH, METHYLOBACILLUS GLYCOGENES

We constructed recombinant plasmids carrying the genes coding for the L-threonine biosynthetic enzymes, the horn gene, the hom-thrC genes, a nd the thrB genes, of a gram-negative obligate methylotroph, Methyloba cillus glycogenes, and examined the effects of them on the production of L-threonine from methanol. The hom gene, which encodes the homoseri ne dehydrogenase, and the hom-thrC genes, containing the gene coding f or threonine synthase together with the hom gene, were cloned from a w ild-type strain, and the thrB gene encoding the desensitized homoserin e kinase was cloned from an L-threonine-producing mutant, ATR80. The r ecombinant plasmids were transferred into ATR80 and its L-isoleucine a uxotroph, A513, by conjugation. Amplification of the genes coding for the L-threonine biosynthetic enzymes elevated the activities of the L- threonine biosynthetic enzymes of the transconjugants 10- to 30-fold o ver those of the strains containing only vectors. The L-threonine prod uction from methanol in test-tube cultivation was increased about 30% and 40% by the amplification of the horn gene and the hom-thrC gene re spectively, and it was slightly increased by that of the thrB gene. Th e effects of gene amplification were confirmed by the cultivation in 5 -l jar fermenters. The best producer, an A513 transconjugant containin g the plasmid carrying the hom-thr C genes, produced 16.3 g/l L-threon ine for 72 h.