Analysis of the promoters and upstream sequences of nifA1 aud nifA2 in Rhodobacter capsulatus; activation requires ntrC but not rpoN

Transcription of Rhodobacter capsulatus genes encoding the nitrogenase polypeptides (nifHDK) is repressed by fixed nitrogen and oxygen. R capsulatus nifA1 and nifA2 encode identical NIFA proteins that activate transcription of nifHDK and other nif genes. In this study, we report that nifA1‐lacZ and nifA2‐lacZ fusions are repressed in the presence of NH₃ and activated to similar levels under nitrogen‐deficient conditions. This nitrogen‐controlled activation was dependent on R. capsulatus ntrC (which encodes a transcriptional activator) but not rpoN (which encodes an RNA polymerase sigma factor). We have used primer extension analyses of nifA1, nifA2 and nifH and deletion analyses of nifA1 and nifA2 upstream regions to define likely promoters and cis upstream activation sequences required for nitrogen control of these genes. Primer extension mapping confirmed that ntrC but not rpoN is required for nifA1 and nifA2 activation, and that nifA1 and nifA2 do not possess typical RPON‐activated promoters.