Oxygenic photosynthesis is an important bioenergetic process that maintains the Earth's atmosphere and allows carbon fixation. A critical enzyme in this process, the cytochrome b 6 f complex, differs from other protein complexes of the same family by an unusual covalently attached cofactor chemically defined as a c′ heme. We have identified a set of pioneer proteins that carry the biogenesis of this c′ heme and started their characterization. They are encoded by the genomes of all organisms performing oxygenic photosynthesis, whatever their phylogenetic distances. These proteins are thus among the few that distinguish photosynthetic cells evolving oxygen from other types of living cells.