Overlapping promoters for two different RNA polymerase holoenzymes control Bradyrhizobium japonicum nifA expression.

The Bradyrhizobium japonicum NifA protein, the central regulator for nitrogen fixation gene expression, is encoded in the fixRnifA operon. This operon is activated during free-living anaerobic growth and in the symbiotic root nodule bacteroid state. In addition, it is expressed in aerobic conditions, albeit at a low level. Here, we report that this pattern of expression is due to the presence of two overlapping promoters: fixRp1, which is of the -24/-12 class recognized by the RNA polymerase sigma 54, and fixRp2, which shares homology with the -35 and -10 regions found in other putative B. japonicum housekeeping promoters. Primer extension analyses showed that fixRp1 directed the synthesis of a transcript, P1, that starts 12 nucleotides downstream of the -12 region. In addition to sigma 54, P1 was dependent on NifA and low oxygen tension. Transcripts originating from fixRp2 started at two sites: one coincided with P1, while the most abundant, P2 initiated just two nucleotides further downstream of P1. Expression from fixRp2 was dependent on the upstream -68 promoter region, a region known to bind a putative activator protein, but it was independent of sigma 54 and NifA. This promoter was expressed in aerobic and anaerobic conditions but was not expressed in 30-day-old bacteroids. Mutations in the conserved 12 region for the sigma 54 promoter did not show any transcript, because these mutations also disrupted the overlapping -10 region of the fixRp2 promoter. Conversely, mutations at the -24 region only affected the sigma 54-dependent P1 transcript, having no effect on the expression of P2. In the absence of omega(54), anaerobic expression from the fixRp(2) promoter was enhanced threefold, suggesting that in the wild-type strain, the two RNA polymerase holoenzymes must compete for binding to the same promoter region.