Modular organization of proteins has been postulated as a widely used strategy for protein evolution. The multidomain transmembrane protein DsbD catalyzes the transfer of electrons from the cytoplasm to the periplasm of Escherichia coli. Most bacterial species do not have DsbD, but instead their genomes encode a much smaller protein, CcdA, which resembles the central hydrophobic domain of DsbD. We used reciprocal heterologous complementation assays between E.coli and Rhodobacter capsulatus to show that, despite their differences in size and structure, DsbD and CcdA are functional homologs. While DsbD transfers reducing potential to periplasmic protein disulfide bond isomerases and to the cytochrome c thioreduction pathway, CcdA appears to be involved only in cytochrome c biogenesis. Our findings strongly suggest that, by the acquisition of additional thiol-redox active domains, DsbD expanded its substrate specificity.