The structural gene for cytochrome c(2) (cycA) of the photosynthetic bacterium Rhodopseudomonas capsulata has been cloned, and the nucleotide and deduced polypeptide sequences have been determined. Compared with the known amino acid sequence of the purified cytochrome c(2), the nucleotide sequence corresponding to the N-terminal part of the cycA gene product indicates the presence of a putative 21 amino acid signal sequence. Thus, cytochrome c(2) may be synthesized as a precursor which is processed during its secretion to the periplasm. Insertion and insertion-deletion mutations were constructed in vitro and the chromosomal cycA(+) allele of a wild-type strain was replaced with these mutations by homologous recombination to yield c(2) (-) mutants of R. capsulata. The c(2) (-) mutants are stable, and they can grow by photosynthesis and by respiration. Since cytochrome c(2) is the primary electron donor to the reaction center during photosynthesis, the ability of these mutants to grow photosynthetically indicates that an alternative way(s) of reducing the oxidized reaction center must exist in R. capsulata. One candidate for this role may be the membrane-bound cytochrome c(1).