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. It catalyzes the ATP-dependent biosynthesis of glutamine from glutamate and ammonia. Feedback-inhibited GlnA also interacts with and regulates the activity of the transcriptional regulator TnrA. During nitrogen limitation, TnrA is in its DNA-binding active state and turns on the transcription of genes required for nitrogen assimilation. Under conditions of nitrogen excess, feedback-inhibited GlnA forms a stable complex with TnrA, which inhibits its DNA-binding activity. 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. ATP + L-glutamate + NH4(+) = ADP + H(+) + L-glutamine + phosphate Binds 2 Mg(2+) ions per subunit. Inhibited by glutamine. 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. Belongs to the glutamine synthetase family. Was originally thought to originate from C.acetobutylicum. Lacks the tyrosine conserved in bacteria and other archaea, involved in feedback inhibition.