Chlorophyll b and phycobilins in the common ancestor of cyanobacteria and chloroplasts

Photosynthetic organisms have a variety of accessory pigments, on which the ir classification has been based. Despite this variation, it is generally a ccepted that all chloroplasts are derived from a single cyanobacterial ance stor(1-3). How the pigment diversity has arisen is the key to revealing the ir evolutionary history. Prochlorophytes are prokaryotes which perform oxyg enic photosynthesis using chlorophyll b, like land plants and green algae ( Chlorophyta), and were proposed to be the ancestors of chlorophyte chloropl asts(4,5). However, three known prochlorophytes (Prochloron didemni, Prochl orothrix hollandica and Prochlorococcus marinus) have been shown to be not the specific ancestors of chloroplasts, but only diverged members of the cy anobacteria, which contain phycobilins but lack chlorophyll b(6,7). Consequ ently it has been proposed that the ability to synthesize chlorophyll b dev eloped independently several times in prochlorophytes and in the ancestor o f chlorophytes. Here we have isolated the chlorophyll b synthesis genes (ch lorophyll a oxygenase)(8) from two prochlorophytes and from major groups of chlorophytes. Phylogenetic analyses show that these genes share a common e volutionary origin. This indicates that the progenitors of oxygenic photosy nthetic bacteria, including the ancestor of chloroplasts, had both chloroph yll b and phycobilins.