Regulation of cyclic AMP of the ilvB-encoded biosynthetic acetohydroxy acid synthase in Escherichia coli K-12.

The biosynthetic acetohydroxy acid synthase activities of E. coli K 12 are encoded by three genetic loci namely, ilvB (acetohydroxy acid synthase I), ilvG (acetohydroxy acid synthase II) and ilvHI (acetohydroxy acid synthase III). The previously reported involvement of cyclic AMP in the regulation of the biosynthetic acetohydroxy acid synthase isozymes in E. coli K-12 was found to be due to the effect of this nucleotide on the expression of ilvB. Cyclic AMP had no effect on acetohydroxy acid synthase activity in strains lacking wild-type ilvB activity but containing the remaining isozymes. Very little activity of acetohydroxy acid synthase coded for by ilvV was found when ppGpp and cyclic AMP were severely limited. Addition of cyclic AMP under these conditions increased ilvB expression 24-fold. The data suggest that in addition to multivalent repression and ppGpp, cyclic AMP plays a major role in the regulation of the ilvB biosynthetic operon.