Nitrogen regulatory locus "glnR" of enteric bacteria is composed of cistrons ntrB and ntrC: identification of their protein products.

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Abstract
The N regulatory locus glnR of Escherichia coli and Salmonella typhimurium is composed of 2 cistrons, tentatively named ntrB and ntrC (N regulation B and C). Frameshift mutations in ntrB and ntrC were isolated on a .lambda. phage that carries the E. coli ntrB and ntrC genes and the closely linked glnA gene, the structural gene encoding glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2]; mutations were selected as suppressors of glnF (renamed ntrA), a selection used previously to isolate glnR mutations. Phage DNA from 1 mutant (ntrB) failed to direct synthesis of a 36-kilodalton (kDal) protein whose synthesis was directed by DNA from the parent phage (ntrB+) in a coupled in vitro transcription/translation system. DNA from 3 other mutants (ntrC) failed to direct synthesis of a 54-kDal protein; DNA from 2 of these mutants instead directed synthesis of smaller proteins, 53 and 50 kDal, respectively. In all 4 cases, DNA from frameshift revertants directed synthesis of both the 36-kDal and 54-kDal proteins. ntrB and ntrC may be separate genes which encoded 36-kDal and 54-kDal protein products, respectively. Frameshift mutations in ntrB and ntrC complemented each other with regard to regulation of glnA expression in vivo and growth on arginine as N source, another N-controlled phenotype; this confirmed that ntrB and ntrC are separate cistrons that encode diffusible products. The ntrB and ntrC genes were also defined in S. typhimurium. Studies of mutant strains provided information on the roles of the ntrB and ntrC products in activation and repression of glnA expression and raised the possibility that these products function as a protein complex in regulating expression of N-controlled genes.