Mutations in the ginA region of the Escherichia coli chromosome due to Mu prophage insertion result in two phenotypic classes. One class is G1n and does not synthesize glutamine synthetase IL-glutamate:ammonia ligase (ADPforming), EC 6.3

The glnG gene product is both a positive regulator and a negative regulator of the expression of glnA, the structural gene for glutamine synthetase, as well as a positive regulator of the expression of a number of genes whose products are involved in the uptake and degradation of nitrogen-containing compounds. The regulation of P-galactosidase in various strains containing a Mu dl (lac bla) insertion within glnG leads to the following conclusions regarding the expression of this gene: (i) like the synthesis of glutamine synthetase, the synthesis of the glnG product is regulated in response to the nitrogen source; (ii) high-level expression of glnG under nitrogen-limiting growth conditions depends on transcription initiated at the glnA promoter; and (iii) there is a second, ginA-distal promoter for ginG, whose activity is negatively controlled by the glnG product. Thus, the glnG product regulates the synthesis of the glnG product at two distinct promoters (positively and negatively at the ginA promoter and negatively at the glnA-distal promoter). In addition, a high level of glnG product, corresponding to the level produced by initiation of transcription at the gInA promoter under nitrogen-limiting conditions, is necessary for activation of histidase synthesis. The lower level of gInG product originating from transcription initiated at the ginAdistal promoter is not sufficient to activate histidase synthesis, but is sufficient to activate fully and to repress glnA expression. 202

We have isolated insertion and deletion mutants in glnB, the structural gene of PI,, a member of the adenylylation system for glutamine synthetase of Escherichia coli, to study the role of PI, in the regulation of the synthesis of glutamine synithetase and of histidase in response to nitrogen deprivation or excess. We have studied the effects of this mutation alone and combined with nul mutations resulting from the insertion of transposons or from a deletion in the other genes affecting this regulation, ginD, glnF (ntrA), ginG (ntrC), and glnL (ntrB). Our rqsults confirm that only the products of glnF and ginG are essential for this regulation. In cells of the wild type, the response is mediated by the products of ginD and glnB via the product of glnL, In the condition of nitrogen excess, PII, the product of glnB, appears to convert the product of glnL to a form that prevents the activation of transcription of the structural gekes for glutamine synthetase and for histidase by the products of glnF and ginG. During nitrogen deprivation, uridylyltransferase, the product of ginD, is activated

A mutant (gltB) of Escherichia coli lacking glutamate synthase (GOGAT) was unable to utilize a wide variety of compounds as sole nitrogen source (e.g., arginine, proline, gamma-aminobutyrate, and glycine). Among revertants of these Asm- strains selected on one of these compounds (e.g., arginine, proline, or gamma-aminobutyrate) were those that produce glutamine synthetase (GS) constitutively (GlnC phenotype). These revertants had a pleiotropically restored ability to grow on compounds that are metabolized to glutamate. This suggested that the expression of the genes responsible for the metabolism of these nitrogen sources was regulated by GS. An examination of the regulation of proline oxidase confirmed this hypothesis. The differential sensitivities of GlnC and wild-type strains to low concentrations (0.1 mM) of the glutamine analog L-methionine-DL-sulfoximine supported the conclusion that the synthesis of a glutamine permease was also positively controlled by GS. During the course of this study we found that the reported position of the locus (gltB) for glutamate synthase is incorrect. We have relocated this gene to be 44% linked to the argG locus by P1 transduction. Further mapping has shown that the locus previously called aspB is in reality the gltB locus and that the "suppressor" of the aspB mutation (A. M. Reiner, J. Bacteriol. 97:1431-1436, 1969) is the locus for glutamate dehydrogenase (gdhA).

One of the suspected regulators of glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] in enteric bacteria is glutamine synthetase itself. We isolated Escherichia coli strains carrying fusions of the beta-galactosidase structural gene to the promoter of the glutamine synthetase gene, with the aid of the Casadaban Mud1 (ApR, lac, cts62) phage. Some aspects of regulation were retained in haploid fusion strains despite the absence of glutamine synthetase, whereas other aspects required glutamine synthetase catalytic or regulatory activity or both. The direction of transcription of the glutamine synthetase gene was also determined.