The three tricarboxylate synthase activities of Corynebacterium glutamicum and increase of l-lysine synthesis
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Corynebacterium glutamicum owns a citrate synthase and two methylcitrate synthases. Characterization of the isolated enzymes showed that the two methylcitrate synthases have comparable catalytic efficiency, k cat/K m, as the citrate synthase with acetyl-CoA as substrate, although these enzymes are only synthesized during growth on propionate-containing media. Thus, the methylcitrate synthases have a relaxed substrate specifity, as also demonstrated by their activity with butyryl-CoA, whereas the citrate synthase does not accept acyl donors other than acetyl-CoA. A double mutant deleted of the citrate synthase gene gltA and one of the methylcitrate synthase genes, prpC1, was made unable to grow on glucose. From this mutant, a collection of suppressor mutants could be isolated which were demonstrated to have regained citrate synthase activity due to the relaxed specificity of the methylcitrate synthase PrpC2. Molecular characterization of these mutants showed that the regulator PrpR (Cg0800) located downstream of prpC1 is mutated with mutations likely to effect the secondary structure of the regulator, thus, resulting in expression of prpC2. This expression results in a citrate synthase activity, which is lower than that due to gltA in the original strain and results in increased l-lysine accumulation.
KeywordsCitrate synthase Methylcitrate synthase MerR-type regulator Regulator mutations l-lysine production
This paper is dedicated to Hermann Sahm on the occasion of his 65th birthday. Thanks are due to him for his untiring and extremely successful efforts at turning C. glutamicum into a research subject. I (LE) would like to thank him personally for his confidence in me over a period of several decades, permitting me the freedom to choose and implement research projects of the most varied nature. This work is part of BMBF project 0313704.
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