Expression of regulatory nif genes in Rhodobacter capsulatus

Translational fusions of the Escherichia coli lacZ gene to Rhodobacter capsulatus nif genes were constructed in order to determine the regulatory circuit of nif gene expression in R. capsulatus, a free-living photosynthetic diazotroph. The expression of nifH, nifA (copies I and II), and nifR4 was measured in different regulatory mutant strains under different physiological conditions. The expression of nifH and nifR4 (the analog of ntrA in Klebsiella pneumoniae) depends on the NIFR1/R2 system (the analog of the ntr system in K. pneumoniae), on NIFA, and on NIFR4. The expression of both copies of nifA is regulated by the NIFR1/R2 system and is modulated by the N source of the medium under anaerobic photosynthetic growth conditions. In the presence of ammonia or oxygen, moderate expression of nifA was detectable, whereas nifH and nifR4 were not expressed under these conditions. The implications for the regulatory circuit of nif gene expression in R. capsulatus are discussed and compared with the situation in K. pneumoniae, another free-living diazotroph.