The glnB gene of Rhizobium leguminosarum biovar viceae

Abstract An open-reading frame (ORF111) upstream of the glutamine synthetase I structural gene ( glnA ) in Rhizobium leguminosarum biovar viceae encodes a protein which is highly homologous to the P_II protein (encoded by glnB ) of enteric bacteria. ORF111 was cloned in a number of different plasmid vectors and shown to complement a K. pneumoniae glnB mutant. We propose that ORF111 encodes the P_II protein of R. leguminosarum and that it should be designated glnB .     

Expression of the nif and ntr genes of Klebsiella pneumoniae in Rhodobacter sphaeroides cells

The genes glnA, ntr, nif or their promoters from Klebsiella pneumoniae cloned on the vectors, based on the plasmid RSF1010, were introduced into Rhodobacter sphaeroides cells. It was found that K. pneumoniae genes glnA, nifB, nifE, nifL and nifH are not expressed in R. sphaeroides. Neither was the glnA gene from cyanobacterium Anabaena 7120 expressed in R. sphaeroides. No functional activity of K. pneumoniae product of ntrA gene which is expressed from its own promoter, and the product of the gene nifA which is expressed from the constitutive promoter of the kanamycin resistance gene of the transposon Tn903, was detected. The implications of these findings are discussed.     

The Rhodobacter capsulatus glnB gene is regulated by NtrC at tandem rpoN-independent promoters.

The protein encoded by glnB of Rhodobacter capsulatus is part of a nitrogen-sensing cascade which regulates the expression of nitrogen fixation genes (nif). The expression of glnB was studied by using lacZ fusions, primer extension analysis, and in vitro DNase I footprinting. Our results suggest that glnB is transcribed from two promoters, one of which requires the R. capsulatus ntrC gene but is rpoN independent. Another promoter upstream of glnB is repressed by NtrC; purified R. capsulatus NtrC binds to sites that overlap this distal promoter region.     

Bradyrhizobium japonicum glnB, a putative nitrogen-regulatory gene, is regulated by NtrC at tandem promoters.

The glnB gene from Bradyrhizobium japonicum, the endosymbiont of soybeans (Glycine max), was isolated and sequenced, and its expression was examined under various culture conditions and in soybean nodules. The B. japonicum glnB gene encodes a 12,237-dalton polypeptide that is highly homologous to the glnB gene products from Klebsiella pneumoniae and Escherichia coli. The gene is located directly upstream from glnA (encoding glutamine synthetase), a linkage not observed in enteric bacteria. The glnB gene from B. japonicum is expressed from tandem promoters, which are differentially regulated in response to the nitrogen status of the medium. Expression from the downstream promoter involves the B. japonicum ntrC gene product (NtrC) in both free-living and symbiotic cells. Thus, glnB, a putative nitrogen-regulatory gene in B. japonicum, is itself Ntr regulated, and NtrC is active in B. japonicum cells in their symbiotic state.     

The Rhizobium leguminosarum glnB gene is down-regulated during symbiosis

Symbiotic nitrogen fixation involves the development, on the legume plant root, of specialised organs called nodules, within which plant photosynthates are exchanged for combined nitrogen of bacterial origin. The glnB gene encodes a signal transduction protein (P(II)) which is a component of the bacterial nitrogen regulation (Ntr) system and an essential regulator of ammonium assimilation. We demonstrate that in Rhizobium leguminosarum the glnB promoter is strongly regulated by nitrogen and NtrC, but still shows a significant level of activity in conditions of nitrogen excess. Expression of genes involved in nitrogen assimilation has been shown to be absent in nitrogen-fixing bacteroids, and, in agreement with this, we find that the glnB promoter is down-regulated during bacteroid differentiation at a time coincident with the arrest of bacterial division in the nodule. This pattern is common to other bacterial genes involved in nitrogen assimilation and it is noteworthy that the zone where the glnB promoter is active is coincident with the region in which NtrC is expressed.