Control of Nitrogen Assimilation by the NRI-NRIITwo-Component System of Enteric Bacteria

Enteric bacteria such as Escherichia coli, Salmonella typhimurium, and their relatives regulate the expression of glutamine synthetase (GS) and other enzymes important in nitrogen assimilation in response to changes in the availability of nitrogen. In this review, the current state of knowledge about the mechanisms of signal transduction by NRI and NRII is summarized briefly. Escherichia coli and related bacteria precisely regulate the level of GS activity by three distinct mechanisms. First, the intracellular concentration of the enzyme is regulated in response to the intracellular nitrogen status. Second, the activity of the enzyme is regulated by reversible covalent modification. Finally, the activity of GS is allosterically controlled by cumulative feedback inhibition by eight small molecules: tryptophan, histidine, carbamyl phosphate, glucosamine-6-phosphate, CTP, AMP, alanine, and glycine. A hypothesis to explain the different phenotypes resulting from the suppressor mutations in glnL is as follows: the suppressors resulting in the constitutive expression of glnA are likely to have either eliminated the capacity of NRII to interact with PII or rendered this interaction unproductive in bringing about the regulated phosphatase activity. The authors examined the ability of partially modified PII to elicit the regulated phosphatase activity and found that it was partially active. They also examined the ability of immobilized NRII to retain PII, using a column chromatography method.