BRANCHED-CHAIN AMINO-ACID BIOSYNTHESIS IN SALMONELLA-TYPHIMURIUM - A QUANTITATIVE-ANALYSIS

We report here the first quantitative study of the branched-chain amin o acid biosynthetic pathway in Salmonella typhimurium LT2. The intrace llular levels of the enzymes of the pathway and of the 2-keto acid int ermediates were determined under various physiological conditions and used for estimation of several of the fluxes in the cells. The results led to a revision of previous ideas concerning the way in which multi ple acetohydroxy acid synthase (AHAS) isozymes contribute to the fitne ss of enterobacteria. In wild-type LT2, AHAS isozyme I provides most o f the flux to valine, leucine, and pantothenate, while isozyme II prov ides most of the flux to isoleucine. With acetate as a carbon source, a strain expressing AHAS II only is limited in growth because of the l ow enzyme activity in the presence of elevated levels of the inhibitor glyoxylate. A strain with AAAS I only is limited during growth on glu cose by the low tendency of this enzyme to utilize 2-ketobutyrate as a substrate; isoleucine limitation then leads to elevated threonine dea minase activity and an increased 2-ketobutyrate/2-ketoisovalerate rati o, which in turn interferes with the synthesis of coenzyme A and methi onine. The regulation of threonine deaminase is also crucial in this r egard. It is conceivable that, because of fundamental limitations on t he specificity of enzymes, no single AHAS could possibly be adequate f or the varied conditions that enterobacteria successfully encounter.