Control of glnA (glutamine synthetase) expression by urea in non-pathogenic and uropathogenic Escherichia coli

Previous transcriptomic studies have shown that glnA, which codes for glutamine synthetase (GS), is induced during growth of uropathogenic E. coli in urine. GS is at the intersection of carbon and nitrogen metabolism and is, not surprisingly, highly regulated. The glnALG operon specifies GS and two regulators of the nitrogen-regulated (Ntr) response, which scavenges nitrogenous compounds during nitrogen limitation. Transcription of glnA initiates from the cAMP receptor protein (Crp)-dependent glnAp1 and the GlnG-dependent glnAp2 promoters which respond to carbon and nitrogen limitation, respectively. High glnA expression is most often associated with nitrogen limitation, and its expression in urine could suggest nitrogen limitation, despite a high concentration of ammonia which suppresses glnA expression. To understand the basis for this expression, we compared growth of a non-pathogenic and a uropathogenic strain of E. coli in minimal media and a urine-like medium in wild-type, ΔglnG and Δcrp strains and assessed glnA expression with transcriptional and translational fusions, quantitative reverse transcription PCR, direct enzymatic assay, and Western blots. Our results showed that urea induced expression from the Crp-dependent glnAp1 promoter which produced a transcript that was not translated, inhibited expression from the Ntr glnAp2 promoter, and did not induce other Ntr genes, i.e., high glnA transcription can occur without induction of the Ntr response. We conclude that urea induces glnA expression in E. coli that is independent of the regulators of the Ntr response.