Role of the Escherichia coli glnALG operon in regulation of ammonium transport.

Escherichia coli expresses a specific ammonium (methylammonium) transport system (Amt) when cultured with glutamate or glutamine as the nitrogen source. Over 95% of this Amt activity is repressed by growth of wild-type cells on media containing ammonia. The control of Amt expression was studied with strains containing specific mutations in the glnALG operon. GlnA- (glutamine synthetase deficient) mutants, which contain polar mutations on glnL and glnG genes and therefore have the Reg- phenotype (fail to turn on nitrogen-regulated operons such as histidase), expressed less than 10% of the Amt activity observed for the parental strain. Similarly, low levels of Amt were found in GlnG mutants having the GlnA+ Reg- phenotype. However, GlnA- RegC mutants (a phenotype constitutive for histidase) contained over 70% of the parental Amt activity. At steady-state levels, GlnA- RegC mutants accumulated chemically unaltered [14C]methylammonium against a 60- to 80-fold concentration gradient, whereas the labeled substrate was trapped within parental cells as gamma-glutamylmethylamide. GlnL Reg- mutants (normal glutamine synthetase regulation) had less than 4% of the Amt activity observed for the parental strain. However, the Amt activity of GlnL RegC mutants was slightly higher than that of the parental strain and was not repressed during growth of cells in media containing ammonia. These findings demonstrate that glutamine synthetase is not required for Amt in E. coli. The loss of Amt in certain GlnA- strains is due to polar effects on glnL and glnG genes, whose products are involved in expression of nitrogen-regulated genes, including that for Amt.