New acetohydroxy acid synthase activity from mutational activation of a cryptic gene in Escherichia coli K-12.

A mutation in an allele identified as ilvJ662 causes the expression of acetohydroxy acid synthase activity that is resistant to feedback inhibition by L-valine. the ilvJ662 allele was transduced as an unselected marker into a strain, CU1126 (ilvB, ilvHI), deficient in acetohydroxy acid synthase activity. The ilvJ662 allele appears to code for a new acetohydroxy acid synthase activity (acetohydroxy acid synthase IV), with physical, kinetic, and physiological properties distinct from the other three isozymes. The catalytic function of acetohydroxy acid synthase IV is highly stable at 37 degrees C in the presence or absence of ethylene glycol. However, sensitivity to feedback inhibition by valine is rapidly lost at 37 degrees C, but this property is somewhat stabilized by ethylene glycol. The rate of synthesis of acetohydroxy acid synthase IV is uniquely repressed by either leucine or isoleucine. These results suggest that the ilvJ+ allele is cryptic for acetohydroxy acid synthase IV, an isozyme distinct from the other acetohydroxy acid synthases.