Nitrogen-limitation independent control of glnA (glutamine synthetase) expression in Escherichia coli by urea, several amino acids, and post-transcriptional regulation

Abstract

The expression of glnA (ammonia-assimilating glutamine synthetase) is high for uropathogenic E. coli grown in urine. Because glnA is part of an operon that codes for regulators of the nitrogen-regulated (Ntr) response, high glnA expression has been interpreted to suggest nitrogen limitation, which is unexpected because of the high urinary ammonia concentration and the extremely rapid bacterial growth. We present evidence that glnA expression does not result from nitrogen limitation. First, in the presence of ammonia, urea induced expression of glnA from the cAMP receptor protein (Crp)- dependent glnAp1 promoter, which circumvents control from the nitrogen-regulated glnAp2 promoter. This urea effect on glnA expression has not been previously described. Second, the most abundant amino acids in urine inhibited GS activity, based on reversal of the inhibition by glutamate and glutamine, and increased glnA expression. The relevance of these inhibitory amino acids in natural environments has not been previously demonstrated. Third, neither urea nor the inhibitory amino acids induced other Ntr genes, i.e., high glnA expression can be independent of other Ntr genes. Finally, the urea-dependent induction did not result in GlnA synthesis because of a previously undescribed translational control. We conclude that glnA expression in urea-containing environments does not imply growth rate-limiting nitrogen restriction and is consistent with rapid growth of uropathogenic E. coli.