Metabolic basis for the isoleucine, pantothenate or methionine requirement of ilvG strains of Salmonella typhimurium.

Salmonella typhimurium strain DU501, which was found to be deficient in acetohydroxy acid synthase II (AHAS II) and to possess elevated levels of transaminase B and biosynthetic threonine deaminase, required isoleucine, methionine, or pantothenate for growth. This strain accumulated alpha-ketobutyrate and, to a lesser extent, alpha-aminobutyrate. We found that alpha-ketobutyrate was a competitive substrate for ketopantoate hydroxymethyltransferase, the first enzyme in pantothenate biosynthesis. This competition with the normal substrate, alpha-ketoisovalerate, limited the supply of pantothenate, which resulted in a requirement for methionine. Evidence is presented to support the conclusion that the ambivalent requirement for either pantothenate or methionine is related to a decrease in succinyl coenzyme A, which is produced from pantothenate and which is an obligatory precursor of methionine biosynthesis. The autointoxification by endogenously produced alpha-ketobutyrate could be mimicked in wild-type S. typhimurium by exogenously supplied alpha-ketobutyrate or salicylate, a known inhibitor of pantothenate biosynthesis. The accumulation of alpha-ketobutyrate was initiated by the inability of the residual AHAS activity provided by AHAS I to efficiently remove the alpha-ketobutyrate produced by biosynthetic threonine deaminase. The accumulation of alpha-ketobutyrate was amplified by the action of transaminase B, which decreased the isoleucine pool by catalyzing the formation of alpha-keto-beta-methylvalerate and aminobutyrate from isoleucine and alpha-ketobutyrate; this resulted in release of threonine deaminase from end product inhibition and unbridled production of alpha-ketobutyrate. Isoleucine satisfied the auxotrophic requirement of the AHAS II-deficient strain by curtailing the activity of threonine deaminase. Additional lines of evidence based on genetic and physiological experiments are presented to support the basis for the autointoxification of strain DU501 as well as other nonpolarigenic ilvG mutant strains.