A PROKARYOTIC ORIGIN FOR LIGHT-DEPENDENT CHLOROPHYLL BIOSYNTHESIS OF PLANTS

Flowering plants require light for chlorophyll synthesis, Early studie s indicated that the dependence on light for greening stemmed in part from the light-dependent reduction of the chlorophyll intermediate pro tochlorophyllide to the product chlorophyllide. Light-dependent reduct ion of protochlorophyllide by flowering plants is contrasted by the ab ility of nonflowering plants, algae, and photosynthetic bacteria to re duce protochlorophyllide and, hence, synthesize (bacterio)chlorophyll in the dark, In this report, we functionally complemented a light-inde pendent protochlorophyllide reductase mutant of the eubacterium Rhodob acter capsulatus with an expression library composed of genomic DNA fr om the cyanobacterium Synechocystis sp, PCC 6803. The complemented R. capsulatus strain is capable of synthesizing bacteriochlorophyll in th e light, thereby indicating that a chlorophyll biosynthesis enzyme can function in the bacteriochlorophyll biosynthetic pathway, However, un der dark growth conditions the complemented R, capsulatus strain fails to synthesize bacteriochlorophyll and instead accumulates protochloro phyllide. Sequence analysis demonstrates that the complementing Synech ocystis genomic DNA fragment exhibits a high degree of sequence identi ty (53-56%) with light-dependent protochlorophyllide reductase enzymes found in plants, The observation that a plant-type, light-dependent p rotochlorophyllide reductase enzyme exists in a cyanobacterium indicat es that light-dependent protochlorophyllide reductase evolved before t he advent of eukaryotic photosynthesis, As such, this enzyme did not a rise to fulfill a function necessitated either by the endosymbiotic ev olution of the chloroplast or by multicellularity; rather, it evolved to fulfill a fundamentally cell-autonomous role.