The Evolution of Light-harvesting Antennas

Abstract

Light-harvesting antennas are central players in the conversion of solar energy to chemical energy. They have been evolving since the earliest anaerobic prokaryotes developed the first primitive photosystems. In the modern antennas, we see the results of many common processes of genetic change (duplication, divergence, acquisition and losses) as well as the primary and secondary endosymbiotic events that gave rise to the chloroplasts of photosynthetic eukaryotes. This chapter first reviews the assumptions and methods used in molecular evolution studies, then discusses the evolution of the enzymes involved in the synthesis of light-harvesting chromophores (chlorophylls, bacteriochlorophylls, phycobilins and carotenoids) and the evolution of the protein families that bind them, particular the core complex family, the LHC superfamily, and the phycobiliproteins. It is clear that the evolution of the proteins and the pigments were at least partly independent: `molecular opportunism' resulted in the proteins' binding whatever pigments were available, leading to the wide variety of chlorophyll-carotenoid pigment complexes that exist today. A model involving lateral gene transfer to account for the sharing of certain antennas by widely separated divisions of photosynthetic bacteria (including cyanobacteria) is proposed.