Expression of the Bacillus subtilis nrgAB operon is derepressed during nitrogen-limited growth. We have identified a gene, tnrA, that is required for the activation of nrgAB expression under these growth conditions. Analysis of the DNA sequence of the tirA gene revealed that it encodes a protein with sequence similarity to GlnR, the repressor of the B. subtilis glutamine synthetase operon. .The tnrA mutant has a pleiotropic phenotype. Compared with wild-type cells, the tnrA mutant is impaired in its ability to utilize allantoin, y-aminobutyrate, isoleucine, nitrate, urea, and valine as nitrogen sources. During nitrogen-limited growth, transcription of the nrgAB, nasB, gabP, and ure genes is significantly reduced in the tnrA mutant compared with the levels seen in wild-type cells. In contrast, the level of glnRA expression is 4-fold higher in the tnrA mutant than in wild-type cells during nitrogen restriction. The phenotype of the tnrA mutant indicates that a global nitrogen regulatory system is present in B. subtilis and that this system is distinct from the Ntr regulatory system found in enteric bacteria.Bacillus subtilis is a Gram-positive bacterium that can sporulate in response to nutrient limitation (1, 2). The expression of many enzymes involved in carbon (3), nitrogen (4, 5), and phosphorus (6) assimilation is also regulated in response to nutrient availability in this bacterium. Although the B. subtilis regulatory systems affecting carbon and phosphorus metabolism have been the subject of much study, relatively little is known about the regulation of nitrogen metabolism in B.subtilis (4,5).In Enterobacteriaceae, the Ntr system regulates the expression of glutamine synthetase (GS) and many degradative pathways in response to nitrogen availability (7). Two key regulatory factors in this system are NRI (NtrC) and NRI, (NtrB). These proteins are members of the two-component family of regulatory proteins. The phosphorylated form of the DNA binding protein NRI activates transcription of Ntrregulated genes transcribed by RNA polymerase containing the o-4 sigma factor. The level of NRI phosphorylation is determined by the kinase/phosphatase activities of NRII.

Bacillus subtilis TnrA, a global regulator of transcription, responds to nitrogen availability, but the specific signal to which it responds has been elusive. Genetic studies indicate that glutamine synthetase is required for the regulation of TnrA activity in vivo. We report here that the feedback-inhibited form of glutamine synthetase directly interacts with TnrA and blocks the DNA binding activity of TnrA. Mutations in the tnrA gene (tnrA(C)) that allow constitutive high level expression of tnrA-activated genes were isolated and characterized. Feedback-inhibited glutamine synthetase had a significantly reduced ability to block the in vitro DNA binding by three of the TnrA(C) proteins. Thus, glutamine synthetase, an enzyme of central metabolism, directly interacts with and regulates the DNA binding activity of TnrA.

The first enzymes of the histidine (hut) and proline degradative pathways, histidase

Microbial ureases are multisubunit metalloenzymes that hydrolyze urea to form carbonic acid and two molecules of ammonia (24). The carbonic acid then dissociates, and the ammonia molecules protonate to form ammonium, causing the pH to increase. Thus, the degradation of urea provides ammonium for incorporation into intracellular metabolites and facilitates survival in acidic environments (7,24). The structural genes encoding both the urease subunits, ureA, ureB, and ureC, and the accessory proteins required for assembly of the urease nickel metallocenter are typically clustered at a single locus (24).

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