Cytochrome c2 (Mr 12,840) of the purple photosynthetic bacterium Rhodospirillum rubrum functions as a mobile electron carrier in the cyclic photosynthetic electron-transport system of this organism. It acts as the electron donor to photochemically oxidized reaction centres and is reduced in turn by electrons from the cytochrome bc1 complex. By using synthetic oligonucleotides based on the known amino acid sequence of the protein, the structural gene (cycA) has been identified and isolated. DNA sequence analysis indicates the presence of a typical prokaryotic 23-residue signal sequence, suggesting that the protein is synthesized as a precursor which is processed during its secretion into the periplasm. Evidence is presented for the production of assembled cytochrome c2 in Escherichia coli, but recombinants grow poorly and are unstable, suggesting toxicity of the gene product in this organism.