ENZYMATIC ACTIVITIES OF STREPTOMYCIN-DEPENDENT ESCHERICHIA COLI IN RELATION TO VALINE FORMATION

Repression by glucose of acetohydroxy acid synthetase in Escherichia coli B

Acetolactate formation in Escherichia coli B results from the activity of a single system, acetohydroxy acid synthetase, which has a pH optimum of 8.0 and is sensitive to end-product inhibition by l-valine. Acetohydroxy acid synthetase was found to be subject to catabolite repression, and the nature and concentration of the carbon source had a greater effect on the formation of the enzyme than had the known end products (valine, isoleucine, leucine and pantothenate) of the biosynthetic pathways of which this enzyme is a member. The results suggest that acetohydroxy acid synthetase may play an amphibolic role in E. coli B.

Regulation of acetohydroxy and synthetase in streptomycin-dependent Escherichia coli

The formation of acetolactate from pyruvate by extracts of streptomycin-dependent Escherichia coli was inhibited, competitively, by L-valine, thus establishing the potential in the dependent mutant for regulation of the activity of acetohydroxy acid synthetase through end-product inhibition. Furthermore, the level of acetohydroxy acid synthetase was decreased in streptomycin-dependent E. coli by growth in minimal medium to which had been added the end products of the biosynthetic pathways initiated by this enzyme, thus establishing the potential in dependent cells for regulation of acetohydroxy acid synthetase through repression. In contrast, the specific activity of acetohydroxy acid synthetase in extracts of streptomycin-dependent E. coli increased in proportion to the concentration of antibiotic (dihydrostreptomycin) present in the medium during growth, thereby indicating a positive regulatory effect of dihydrostreptomycin on the formation of this enzyme. The formation of two enzymes used as monitors, glucokinase, and glutamic dehydrogenase, was not affected by variation in antibiotic concentration.

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.