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

Photosynthetic organisms have a variety of accessory pigments, on which their classification has been based. Despite this variation, it is generally accepted that all chloroplasts are derived from a single cyanobacterial ancestor(1-3). How the pigment diversity has arisen is the key to revealing their evolutionary history. Prochlorophytes are prokaryotes which perform oxygenic photosynthesis using chlorophyll b, like land plants and green algae (Chlorophyta), and were proposed to be the ancestors of chlorophyte chloroplasts(4,5). However, three known prochlorophytes (Prochloron didemni, Prochlorothrix hollandica and Prochlorococcus marinus) have been shown to be not the specific ancestors of chloroplasts, but only diverged members of the cyanobacteria, which contain phycobilins but lack chlorophyll b(6,7). Consequently it has been proposed that the ability to synthesize chlorophyll b developed independently several times in prochlorophytes and in the ancestor of chlorophytes. Here we have isolated the chlorophyll b synthesis genes (chlorophyll a oxygenase)(8) from two prochlorophytes and from major groups of chlorophytes. Phylogenetic analyses show that these genes share a common evolutionary origin. This indicates that the progenitors of oxygenic photosynthetic bacteria, including the ancestor of chloroplasts, had both chlorophyll b and phycobilins.