Evolutionary Integration of Chloroplast Metabolism with the Metabolic Networks of the Cells

Abstract

The merger of a prokaryotic cyanobacterium with a primitive eukaryotic mitochondriate cell was a crucial event in evolution, leading to the first photosynthetic eukaryote, the protoalga. A certainly essential step in this merger of two previously independent entities was the metabolic integration of cyanobacterial photosynthetic metabolism with that of the heterotrophic host. This required the insertion of specific metabolite transporters into the membrane system that separated the cyanobiont from the surrounding cytoplasm of the host cell. Based on phylogenetic and phylogenomic analyses of extant plastids, this metabolic connection between host and endosymbiont was predominantly established by routing of preexisting host transporters to the cyanobiont plasma membrane. In this chapter, we will review the current knowledge on connecting host with cyanobiont metabolism via metabolite transporters. We further discuss possible syntrophic associations between cyanobacteria and primitive eukaryotic cells that might have paved the way into endosymbiosis.