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Effects of branched chain amino acids, isoleucine, valine, and leucine, in growth media on the biosynthesis of threonine dehydratase (TD) and acetolactate synthase (ALS) in Serratia marcescens were examined. Both TD and ALS activities were decreased to 40% or slightly less in the presence of 2-20 mM of leucine. TD activity was decreased approximately by 20% in the presence of low concentrations of valine (2-4 mM), but ALS activity was increased by 40%, while 20 mM of valine concentration did not affect both activities. In the presence of high concentration of isoleucine (15-30 mM), both TD and ALS activities were decreased to about 25%. However, TD and ALS activities were increased up to 240 and 140% with low concentrations of isoleucine (1-4 mM), respectively. TD and ALS activities were decreased by 25% at each 2 mM of three branched chain amino acids, and decreased by 58 and 70% at each 10 mM of those amino acids, respectively. These data suggest that TD and ALS of S. marcescens are multivalently repressed by branched chain amino acids, and TD of S. marcescens is positively regulated by a low concentration of isoleucine alone, while ALS of S. marcescens is positively regulated by a low concentration of the isoleucine and valine combination, unlike TD and ALS activities in E. coli K-12.
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