title: Multivalent repression and genetic depression of isoleucine-valine biosynthetic enzymes in Serratia marcescens.
abstract: The regulation of the formation of isoleucine-valine biosynthetic enzymes was examined to elucidate the mechanism of isoleucine-valine accumulation by alpha-aminobutyric acid-resistant (abu-r) mutants of Serratia marcescens. In the isoleucine-valine auxotroph, l-threonine dehydratase, acetohydroxy acid synthetase, and transaminase B were repressed when isoleucine, valine, and leucine were simultaneously added to minimal medium. These enzymes were derepressed at the limitation of any single branched-chain amino acid. Pantothenate, which stimulated growth of this auxotroph, had no effect on the enzyme levels. It became evident from these results that in S. marcescens isoleucine-valine biosynthetic enzymes are subject to multivalent repression by three branched-chain amino acids. The abu-r mutants had high enzyme levels in minimal medium, with or without three branched-chain amino acids. Therefore, in abu-r mutants, isoleucine-valine biosynthetic enzymes are genetically derepressed. This derepression was considered to be the primary cause for valine accumulation and increased isoleucine accumulation.
mesh_terms: D002474:Cell-Free System; D003470:Culture Media; D004789:Enzyme Activation; D004794:Enzyme Repression; D005827:Genetics, Microbial; D006836:Hydro-Lyases / Q000378:metabolism*; D007532:Isoleucine / Q000096:biosynthesis*; D007930:Leucine / Q000378:metabolism; D008190:Lyases / Q000378:metabolism*; D009154:Mutation; D010205:Pantothenic Acid / Q000494:pharmacology; D011773:Pyruvates; D012706:Serratia marcescens / Q000201:enzymology* / Q000254:growth & development / Q000378:metabolism; D013912:Threonine; D000637:Transaminases / Q000378:metabolism*; D014633:Valine / Q000096:biosynthesis*
chemical_list: D003470:Culture Media; D011773:Pyruvates; D007532:Isoleucine; D010205:Pantothenic Acid; D013912:Threonine; D000637:Transaminases; D008190:Lyases; D006836:Hydro-Lyases; D007930:Leucine; D014633:Valine