Organization and regulation of genes encoding the molybdenum nitrogenase and the alternative nitrogenase in Rhodobacter capsulatus

Abstract

The phototrophic non-sulfur purple bacterium Rhodobacter capsulatus is able to fix atmospheric dinitrogen either via a conventional molybdenum nitrogenase or via an alternative iron-only nitrogenase. At least 53 genes are involved in the synthesis and regulation of these two nitrogenase systems, most of which are clustered in four regions widely spread in the genome. Expression of both nitrogenase systems is regulated at the transcriptional level by NifR1 and NifR2, homologues of NtrC and NtrB, respectively. However, this ntr system is only involved in the regulation of the two nitrogenase systems and the high-affinity molybdenum transport system and is not required for utilization of other N sources such as proline and arginine. In contrast to enteric bacteria, the R. capsulatus NtrC homologue does not act in concert with the alternative sigma factor RpoN (σ⁵⁴). Nitrogen fixation in R. capsulatus is regulated at the transcriptional level and also at the post-translational level. The draTG gene products are responsible for covalent modification of the dinitrogenase reductases of both nitrogenase systems. In addition, mutations in hvrA, a gene previously described as being responsible for low-light activation of the photosynthetic apparatus, also affect regulation of nitrogen fixation.