Glutamine synthetase (GS) is an unusual multitasking protein that functions as an enzyme, a transcription coregulator, and a chaperone in ammonium assimilation and in the regulation of genes involved in nitrogen metabolism (PubMed:25691471). It catalyzes the ATP-dependent biosynthesis of glutamine from glutamate and ammonia (PubMed:24158439). Feedback-inhibited GlnA interacts with and regulates the activity of the transcriptional regulator TnrA (PubMed:11719184, PubMed:12139611). During nitrogen limitation, TnrA is in its DNA-binding active state and turns on the transcription of genes required for nitrogen assimilation (PubMed:11719184, PubMed:12139611, PubMed:25691471). Under conditions of nitrogen excess, feedback-inhibited GlnA forms a stable complex with TnrA, which inhibits its DNA-binding activity (PubMed:11719184, PubMed:12139611, PubMed:25691471). In contrast, feedback-inhibited GlnA acts as a chaperone to stabilize the DNA-binding activity of GlnR, which represses the transcription of nitrogen assimilation genes (PubMed:25691471). ATP + L-glutamate + NH4(+) = ADP + H(+) + L-glutamine + phosphate Binds 2 Mg(2+) ions per subunit. Completely inhibited by glutamine and partially inhibited by glycine, alanine and AMP (PubMed:4149044, PubMed:24158439). Also inhibited by L-methionine-SR-sulphoximine (Met-Sox) (PubMed:8093698, PubMed:24158439). Oligomer of 12 subunits arranged in the form of two hexagons. In its feedback-inhibited form, interacts with TnrA in order to block its DNA-binding activity. This inhibitory effect is the highest when both glutamine and AMP are present. Repressed by GlnR under conditions of nitrogen excess (PubMed:2573733). Repressed by TnrA under conditions of nitrogen limitation. In cells lacking this gene, expression of glnR, tnrA, nasB, nrgAB, gabP and ure genes is derepressed. Belongs to the glutamine synthetase family.