The physiological function of Rhodobacter capsulatus FdVI, a [2Fe-2S] ferredoxin, was investigated by the cloning, sequence analysis, and mutagenesis of its structural gene, called fdxE. The DNA region surrounding fdxE was mapped, and the nucleotide sequence of a 4.2-kb fragment was determined. fdxE is preceded by a sequence that is very similar to a sigma54 recognition site and is followed by a putative transcription stop signal, suggesting that fdxE forms a separate cistron. Two open reading frames were identified upstream and downstream of fdxE and were named ORFE0 and ORFE1, respectively. The former may encode a polypeptide having 34% similarity with HtrA, a serine protease found in enteric bacteria. ORFE1 is homologous to purU, a gene involved in purine biosynthesis. Interposon mutagenesis of fdxE was unsuccessful when attempted on the wild-type strain B10. Disruption of fdxE could be achieved only in strains harboring an additional copy of fdxE on a plasmid. Mutants obtained in this way and carrying a plasmid-borne copy of fdxE under the control of the nifH promoter grew only in N-free medium, thus demonstrating that fdxE expression is required for growth. Nevertheless, such mutants were found to spontaneously revert at a frequency of 5 x 10(-6) to an apparent wild-type phenotype, although they contained no detectable amount of FdVI. Taken together, the results indicate that FdVI is required for an essential metabolic function in R. capsulatus and that this FdVI dependence could be relieved by a single-mutation event. In accordance, FdVI biosynthesis was found to be constitutive in R. capsulatus.