The ilvG gene is expressed in Escherichia coli K-12

Three genes code for isozymes of acetohydroxy acid synthase (AHAS) in Escherichia coli K-12. To test the previously published supposition that one of them, ilvG, is silent in ilvO+ strains, we isolated mutants which had deletions of various lengths in the ilvGEDA operon. Some of these mutants have severely reduced levels of AHAS activity. We conclude that ilvG is expressed in ilvO+ strains but is deleted in these mutants. In addition, we find that AHAS II, the ilvG gene product, is sensitive to feedback inhibition by valine. We hypothesize that ilvO- mutations are ilvG frameshift mutations which render AHAS II valine resistant and enhance transcription of distal genes.