Evidence that nitrogen regulatory gene ntrC of Salmonella typhimurium is transcribed from the glnA promoter as well as from a separate ntr promoter.

Previous work has indicated that nitrogen regulatory genes ntrB and ntrC of Salmonella typhimurium are closely linked to glnA, the structural gene encoding glutamine synthetase; proceeding clockwise the order of genes in the 86 U region of the map is polA...ntrC ntrB glnA glnA promoter...rha. To study ntrC transcription we have constructed operon fusions of ntrC to lacZ using the Casadaban Mu d1 (Apr lac) phage so that we can measure beta-galactosidase activity as a reflection of ntrC transcription and we have introduced into fusion strains promoter constitutive mutations at glnA [glnAp(Con)]. The glnAp(Con) mutations, which elevate glnA expression in fusion strains, also elevate beta-galactosidase activity, indicating that ntrC is cotranscribed with glnA. Consistent with this interpretation, polar insertion mutations in glnA decrease beta-galactosidase activity of fusion strains carrying glnAp(Con) mutations. However, glnA insertions do not eliminate beta-galactosidase activity of glnAp(Con) ntrC::Mu d1 strains and they have little effect on beta-galactosidase activity of the original ntrC::Mu d1 fusion strains. The latter results confirm that ntrC can also be transcribed from an ntr promoter downstream of glnA. Polar insertion mutations in ntrB eliminate beta-galactosidase activity of both the original fusion strains and fusion strains carrying glnA(Con) mutations, indicating that the ntr promoter lies between glnA and ntrB.