Transcription of the genes that code for proteins involved in nitrogen fixation in free-living diazotrophs is typically repressed by high internal oxygen concentrations or exogenous fixed nitrogen. The DNA sequence of a regulatory locus required for repression of Rhodobacter capsulatus nitrogen fixation genes was determined. It was shown that this locus, defined by Tn5 insertions and by ethyl methanesulfonate-derived mutations, is homologous to the glnB gene of other organisms. The R. capsulatus glnB gene was upstream of glnA, the gene for glutamine synthetase, in a glnBA operon. beta-Galactosidase expression from an R. capsulatus glnBA-lacZ translational fusion was increased twofold in cells induced by nitrogen limitation relative to that in cells under nitrogen-sufficient conditions. R. capsulatus nifR1, a gene that was previously shown to be homologous to ntrC and that is required for transcription of nitrogen fixation genes, was responsible for approximately 50% of the transcriptional activation of this glnBA fusion in cells induced under nitrogen-limiting conditions. R. capsulatus GLNB, NIFR1, and NIFR2 (a protein homologous to NTRB) were proposed to transduce the nitrogen status in the cell into repression or activation of other R. capsulatus nif genes. Repression of nif genes in response to oxygen was still present in R. capsulatus glnB mutants and must have occurred at a different level of control in the regulatory circuit
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