BIOSYNTHESIS OF VALINE IN STREPTOMYCIN-DEPENDENT ESCHERICHIA COLI

Acetolactate formation by streptomycin mutants of Escherichia coli

The activity of the enzyme system which forms acetolactate in Escherichia coli was found to be greater at both pH 6 and at pH 8 in streptomycin-dependent mutants than in the parent sensitive cultures. The activity of the acetolactate-forming system was observed to be lower in a streptomycin-sensitive revertant than in the dependent E. coli culture from which it was derived by back-mutation. The activity of the acetolactate-forming system from streptomycin-resistant (indifferent) mutants was within the range found for streptomycin-sensitive cultures and was not affected by growth in medium containing dihydrostreptomycin. It was concluded from these observations that excretion of L-valine by streptomycin-dependent E. coli was a consequence of the elevated acetolactate-forming activity of these mutants. To explain the (apparently specific) elevation of the acetolactate-forming system in streptomycin-dependent cells, it was postulated that the antibiotic functioned as a de-repressor while pyruvate functioned as the specific enzyme inducer.