title: Salmonella typhimurium mutants with alternate requirements for vitamin B 6  or isoleucine.
abstract: 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.
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D002087:Butyrates / Q000378:metabolism; D002474:Cell-Free System; D002874:Chromosome Mapping; D003470:Culture Media; D004789:Enzyme Activation; D005827:Genetics, Microbial; D005838:Genotype; D006836:Hydro-Lyases / Q000378:metabolism; D007532:Isoleucine / Q000378:metabolism*; D007651:Keto Acids / Q000378:metabolism; D009153:Mutagens; D009154:Mutation; D011732:Pyridoxal Phosphate / Q000378:metabolism; D011736:Pyridoxine / Q000378:metabolism*; D012486:Salmonella typhimurium / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism*; D012997:Solvents; D013431:Sulfates; D013912:Threonine; D014050:Toluene; D014631:Valerates / Q000378:metabolism
chemical_list: D000596:Amino Acids; D002087:Butyrates; D003470:Culture Media; D007651:Keto Acids; D009153:Mutagens; D012997:Solvents; D013431:Sulfates; D014631:Valerates; D007532:Isoleucine; D013912:Threonine; D014050:Toluene; D011732:Pyridoxal Phosphate; D006836:Hydro-Lyases; D011736:Pyridoxine