MOLECULAR ASPECTS OF LYSINE, THREONINE, AND ISOLEUCINE BIOSYNTHESIS IN CORYNEBACTERIUM-GLUTAMICUM

The Gram-positive bacterium Corynebacterium glutamicum is used for the industrial production of amino acids, e.g. of L-glutamate and L-lysin e. In the last ten years genetic engineering methods were developed fo r C. glutamicum and consequently, recombinant DNA technology was emplo yed to study the biosynthetic pathways and to improve the amino acid p roductivity by manipulation of enzymatic, transport and regulatory fun ctions of this bacterium. The present review summarizes the current kn owledge on the synthesis and overproduction of the aspartate derived a mino acids L-lysine, L-threonine and L-isoleucine in C. glutamicum. A special feature of C. glutamicum is its abilily to convert the lysine intermediate piperideine2,6-dicarboxylate to diaminopimelate by two di fferent routes, i.e. by reactions involving succinylated intermediates or by the single reaction of diaminopimelate dehydrogenase. The flux distribution over the two pathways is regulated by the ammonium availa bility. The overall carbon flux from aspartate to lysine, however, is governed by feedback-control of the aspartate kinase and by the level of dihydrodipicolinate synthase. Consequently, expression of lysC(FBR) encoding a deregulated aspartate kinase and/or the overexpression of dapA encoding dihydrodipicolinate synthase led to overproduction of ly sine. As a further specific feature C. glutamicum possesses a specific lysine export carrier which shows high activity in lysine overproduci ng mutants. Threonine biosynthesis is in addition to control by the as partate kinase tightly regulated at the level of homoserine dehydrogen ase which is subject to feedback-inhibition and to repression. C. glut amicum strains possessing a deregulated aspartate kinase and a deregul ated homoserine dehydrogenase produce lysine and threonine. Amplificat ion of deregulated homoserine dehydrogenase in such strains led to an almost complete redirection of the carbon flux to threonine. For a fur ther flux from threonine to isoleucine the allosteric control of threo nine dehydratase and of the acetohydroxy acid synthase are important. The expression of the genes encoding the latter enzyme is additionally regulated at the transcriptional level. By addition of 2-oxobutyrate as precursor and by bypassing the expression control of the acetohydro xy acid synthase genes high isoleucine overproduction can be obtained.