UBC Theses and Dissertations
An investigation of modified metabolic regulation in streptomycin-dependent Escherichia coli Coukell, M.B.
The acetohydroxy acid synthetase levels in streptomycin-sensitive, -dependent and -resistant mutants have been studied in four different strains of Escherichia coli. The activity of the ∝-acetolactate-forming system was found to be greater both at pH 6.0 and at pH 8.0 in streptomycin-dependent mutants than in the corresponding streptomycin-sensitive cultures. In general, streptomycin-resistant mutants demonstrated enzyme activities within the range found for streptomycin-sensitive organisms regardless of whether they were grown in the presence or absence of antibiotic. The acetohydroxy acid synthetase activity of streptomycin-sensitive and -resistant revertants was observed to be lower than that of the dependent Escherichia coli culture from which they were derived by back-mutation. Mutation to streptomycin-resistance or -dependence had no effect on glucokinase and glutamic dehydrogenase activities. The addition of the coenzyme flavin adenine dinucleotide to the incubation mixtures markedly stimulated the activities of all the extracts. This enhancement of acetohydroxy acid synthetase activity had little or no effect on the ratio of activities of this enzyme in the dependent and sensitive Escherichia coli strains investigated. ∝-Acetohydroxybutyrate formation was found to be greater in extracts from the streptomycin-dependent organism than in extracts prepared from the same strain of sensitive and resistant Escherichia coli. The degree of elevation of ∝-acetohydroxybutyrate paralleled that of ∝-acetolactate formation in the dependent mutant. It was concluded from these observations that excretion of L-valine by streptomycin-dependent Escherichia coli was a consequence of the elevated acetohydroxy acid synthetase activity of these mutants. In the dependent organism, it was postulated that streptomycin functioned as a wde-repressorw of acetohydroxy acid synthetase thus permitting the biosynthetic pathway leading to L-valine to serve as an important route of pyruvate dissimilation.