Summary: Cells of K12 were sensitive to 100 m-methylammonium when cultured under nitrogen limitation, and resistant when grown with an excess of either NHC1 or glutamine. Glutamine synthetase activity was required for expression of the methylammonium-sensitive phenotype. Mutants were isolated which were resistant to 100 m-methylammonium, even when grown under nitrogen limitation. P1 bacteriophage transduction and F′ complementation analysis revealed that the resistance-conferring mutations mapped either inside the structural gene and/or elsewhere in the chromosome. Glutamine synthetase was purified from the wild-type and from some of the mutant strains. Strains carrying -linked mutations that were solely responsible for the methylammonium-resistant phenotype yielded an altered enzyme, which was less active biosynthetically with either ammonium or methylammonium as substrate. Sensitivity to methylammonium appeared to be due to synthesis of γ-glutamylmethyl-amide by glutamine synthetase, which was synthesized poorly, if at all, by mutants carrying an altered glutamine synthetase enzyme.


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