Amplification of three threonine biosynthesis genes inCorynebacterium glutamicum and its influence on carbon flux in different strains

Summary The hom-thrB operon (homoserine dehydrogenase/homoserine kinase) and the thrC gene (threonine synthase) of Corynebacterium glutamicum ATCC 13 032 and the hom _FBR (homoserine dehydrogenase resistant to feedback inhibition by threonine) alone as well as hom _FBR-thrB operon of C. glutamicum DM 368-3 were cloned separately and in combination in the Escherichia coli/C. glutamicum shuttle vector pEK0 and introduced into different corynebacterial strains. All recombinant strains showed 8- to 20-fold higher specific activities of homoserine dehydrogenase, homoserine kinase, and/or threonine synthase compared to the respective host. In wild-type C. glutamicum, amplification of the threonine genes did not result in secretion of threonine. In the lysine producer C. glutamicum DG 52-5 and in the lysine-plus-threonine producer C. glutamicum DM 368-3 overexpression of hom-thrB resulted in a notable shift of carbon flux from lysine to threonine whereas cloning of hom _FBR-thrB as well as of hom _FBR in C. glutamicum DM 368-3 led to a complete shift towards threonine or towards threonine and its precursor homoserine, respectively. Overexpression of thrC alone or in combination with that of hom _FBR and thrB had no effect on threonine or lysine formation in all recombinant strains tested. Offprint requests to: B. J. Eikmanns