A new glnA-linked regulatory gene for glutamine synthetase in Escherichia coli

Mutations in the glnA region of the Escherichia coli chromosome due to Mu prophage insertion result in two phenotypic classes. One class is Gln- and does not synthesize glutamine synthetase[L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] under any growth condition. The other class produces a low level of glutamine synthetase under all growth conditions and is uncoupled from the regulatory effects of mutations in the glnF and glnD genes. Complementation analysis demonstrates that these two classes of insertions are in different cistrons. From these data we suggest that a regulatory gene, glnG, tightly linked to glnA, mediates both activation and repression of glutamine synthetase synthesis. An analysis of the evidence accumulated to date makes it unlikely that glnG is the only gene in the glnA region involved in the complex system of nitrogen regulation.