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Regulation of expression of the ilvB operon in Salmonella typhimurium

The ilvB gene of Salmonella typhimurium encodes the valine-sensitive form of acetohydroxy acid synthase, acetohydroxy acid synthase I, which catalyzes the first step in the parallel biosynthesis of isoleucine and valine. Although nearly all of the other genes involved in this pathway are clustered at minute 83, ilvB was found to lie at minute 80.5. Expression of ilvB was shown to be nearly completely repressed by the end products leucine and valine. Studies in which we used strains with mutations in cya (adenylate cyclase) and crp (cAMP receptor protein) demonstrated that synthesis of acetohydroxy acid synthase I is enhanced by the cAMP-cAMP receptor protein complex. Although no stimulation was achieved by growth on poor carbon sources, introduction of crp on a multicopy plasmid led to markedly increased expression. Strains of S. typhimurium lacking valine-resistant acetohydroxy acid synthase II (ilvG) are like Escherichia coli K-12 in that they are not able to grow in the presence of L-valine owing to a conditional isoleucine auxotrophy. The valine toxicity of these ilvG mutants of S. typhimurium was overcome by increasing the level of acetohydroxy acid synthase I. Enzyme activity could be elevated either by maximally derepressing expression with severe leucine limitation, by introduction of either ilvB or crp on a multicopy plasmid, or by the presence of the ilv-513 mutation. This mutation, which is closely linked to genes encoding the phosphoenol pyruvate:sugar phosphotransferase system (pts), causes highly elevated expression of ilvB that is refractory to repression by leucine and valine, as is the major ilv operon. The response of ilvB to the cAMP-cAMP receptor protein complex was not affected by this lesion. Data obtained by using this mutant led us to propose that the two modes of regulation act independently. We also present some evidence which suggests that ilvB expression may be affected by the phosphoenol pyruvate:sugar phosphotransferase system.