Insensitivity to Valine of Streptomycin-Dependent Escherichia coli K12

A streptomycin dependent mutant was obtained from wild-type (valine-sensitive) Escherichia coli K12. This mutant ( E. coli DK 12) grew exponentially in minimal medium containing valine (plus dihydrostreptomycin). The inhibition by valine of the acetohydroxy acid synthetase from both dependent and parent cells was quantitatively similar. However, the activity of the acetohydroxy acid synthetase of DK 12 cells was higher than that of the parent cells. It was concluded that the insensitivity to valine of E. coli DK 12 was the result of derepression of (acetohydroxy) acid synthetase in this mutant, the role of the antibiotic in dependent cells being that of a "derepressor."