Molecular and biochemical insights from natural and engineered photosynthetic endosymbiotic systems

Mitochondria and chloroplasts evolved through the transformation of bacterial endosymbionts established within the host cells. Studies on these organelles have provided several phylogenetic and biochemical insights related to this remarkable evolutionary transformation. Additionally, comparative studies between naturally existing endosymbionts and present-day organelles have allowed us to identify important common features of endosymbiotic evolution. In this review, we discuss hallmarks of photosynthetic endosymbiotic systems, particularly focusing on some of the fascinating molecular changes that occur in the endosymbiont and the host as the endosymbiont/host chimera evolves and transforms endosymbionts into organelles; these include the following: (i) endosymbiont genome minimization and host/endosymbiont gene transfer, (ii) protein import/export systems, (iii) metabolic crosstalk between the endosymbiont, (iv) alterations to the endosymbiont peptidoglycan, and (v) host-controlled replication of endosymbionts/organelles. We discuss these hallmarks in the context of naturally existing photosynthetic endosymbiotic systems and present-day chloroplasts. Further, we also briefly discuss laboratory efforts to engineer endosymbiosis between photosynthetic bacteria and host cells, the lessons learned from these studies, future directions of these studies, and their implications on evolutionary biology and synthetic biology.