The Hfq-like protein NrfA of the phototrophic purple bacterium Rhodobacter capsulatus controls nitrogen fixation via regulation of nifA and anfA expression.

The Rhodobacter capsulatus nrfA gene product exhibits extensive similarity to the nif (nitrogen fixation) regulatory factor NrfA of Azorhizobium caulinodans and the nucleoid-associated protein Hfq of Escherichia coli. Mutational analysis revealed that, in contrast to the situation in A. caulinodans, NrfA is not essential for diazotrophic growth of R. capsulatus, but it is required for maximal growth rates with N(2) as sole nitrogen source via either molybdenum nitrogenase or the alternative nitrogenase. NrfA was shown to control N(2) fixation in R. capsulatus at the level of expression of the regulatory genes nifA1, nifA2 and anfA, encoding the transcriptional activators of all the other nitrogen fixation genes.