Salmonella typhimurium mutants with alternate requirements for vitamin B 6 or isoleucine

Several mutants of Salmonella typhimurium LT-2, isolated as auxotrophs for vitamin B(6), grew without the added vitamin when supplied with either isoleucine, alpha-ketobutyrate, or alpha-keto-beta-methylvalerate, but not with threonine or with other alpha-keto acids. When grown on minimal medium supplemented with isoleucine, these mutants synthesized vitamin B(6) in amounts comparable to wild-type cells; they thus appeared to contain a modified l-threonine dehydratase and to belong to genotype ilvA (threonine dehydratase) instead of pdx (pyridoxine). Direct assays confirmed this hypothesis. Wild-type cells (toluene-treated) showed approximately the same threonine dehydratase activity whether grown in the presence or absence of added pyridoxal-P; mutant cells approached the activity of wild-type cells only when they were grown with added vitamin B(6) and were assayed in the presence of pyridoxal-P. In cell-free extracts, the threonine dehydratase from mutant cells was cold labile and more labile to oxidative inactivation than the wild-type enzyme; furthermore, activation of the mutant apoenzyme required a 10- to 20-fold higher concentration of pyridoxal-P than was required for the wild-type apoenzyme. These results show that cultures which appear auxotrophic for a given vitamin may synthesize that vitamin in normal amounts, the exogenous requirement arising from impaired binding of the vitamin-derived coenzyme to a genetically altered apoenzyme dependent on that coenzyme. Inadequate nutritional data to support the genetic findings can lead to erroneous genotype classification for such mutants.