Rhodobacter capsulatus AnfA is essential for production of Fe-nitrogenase proteins but dispensable for cofactor biosynthesis and electron supply.

The photosynthetic α-proteobacterium Rhodobacter capsulatus reduces and thereby fixes atmospheric dinitrogen (N2 ) by a molybdenum (Mo)-nitrogenase and an iron-only (Fe)-nitrogenase. Differential expression of the structural genes of Mo-nitrogenase (nifHDK) and Fe-nitrogenase (anfHDGK) is strictly controlled and activated by NifA and AnfA, respectively. In contrast to NifA-binding sites, AnfA-binding sites are poorly defined. Here, we identified two highly similar AnfA-binding sites in the R. capsulatus anfH promoter by studying the effects of promoter mutations on in vivo anfH expression and in vitro promoter binding by AnfA. Comparison of the experimentally determined R. capsulatus AnfA-binding sites and presumed AnfA-binding sites from other α-proteobacteria revealed a consensus sequence of dyad symmetry, TAC-N6 -GTA, suggesting that AnfA proteins bind their target promoters as dimers. Chromosomal replacement of the anfH promoter by the nifH promoter restored anfHDGK expression and Fe-nitrogenase activity in an R. capsulatus strain lacking AnfA suggesting that AnfA is required for AnfHDGK production, but dispensable for biosynthesis of the iron-only cofactor and electron delivery to Fe-nitrogenase, pathways activated by NifA. These observations strengthen our model, in which the Fe-nitrogenase system in R. capsulatus is largely integrated into the Mo-nitrogenase system.