The genes of the glutamine synthetase adenylylation cascade are not regulated by nitrogen in Escherichia coli

Regulation of glutamine-synthetase (GS) activity in enteric bacteria involves a complex cascade of events. In response to nitrogen limitation, a transferase catalyses the uridylylation of the PII protein, which in turn stimulates deadenylylation of GS. Deadenylylated GS is the more active form of the enzyme. Here we characterize in detail the genes from Escherichia coli encoding uridylyl-transferase (glnD), the PII protein (glnB), and adenylyl-transferase (glnE). glnD is transcribed from its own promoter, glnE is contranscribed with another gene, orfXE, whereas glnB is partly contranscribed with a gene encoding a homologue of the transcription activator NtrC. All three gln regulatory genes were constitutively expressed at a low level, i.e. their expression was independent of the nitrogen status and the RNA polymerase sigma factor sigma 54. We conclude that the functioning of the GS adenylylation cascade is regulated by modulation of the activities of uridylyl-transferase and adenylyl-transferase, rather than by changes in the expression of their genes.