An ancient function of PGR5 in iron delivery?. (October 2022) - Leister, Dario
Abstract : In all phototrophic organisms, the photosynthetic apparatus must be protected from light-induced damage. One important mechanism that mitigates photodamage in plants is antimycin A (AA)-sensitive cyclic electron flow (CEF), the evolution of which remains largely obscure. Here we show that proton gradient regulation 5 (PGR5), a key protein involved in AA-sensitive CEF, displays intriguing commonalities – including sequence and structural features – with a group of ferritin-like proteins. We therefore propose that PGR5 may originally have been involved in prokaryotic iron mobilization and delivery, which facilitated a primordial type of CEF as a side effect. The abandonment of the bacterioferritin system during the transformation of cyanobacterial endosymbionts into chloroplasts might have allowed PGR5 to functionally specialize in CEF. Highlights: Cyclic electron flow (CEF) around photosystem I (PSI) has been known for decades. Although our understanding of the course of CEF is growing, the origins of the process are poorly understood. The key effector proton gradient regulation 5 (PGR5) appears to be the most ancient component of CEF, and it is remarkably well conserved. PGR5-like photosynthetic phenotype protein 1 (PGRL1) and the recently discovered PGRL2 safeguard modern plant CEF activity by regulating the abundance of PGR5, but they are evolutionarily relatively novel. Recently, attention has been redirected towards cytochrome b 6 f as the site and catalyst of PSI electron reinjection into the plastoquinone (PQ) pool, possibly in conjunction with PGR5. The evolution of PGR5 has not been assessed so far, probably owing to a lack of likely homologues. The detection of a physical interaction of PGR5 with bacterioferritin 1 in Synechocystis, and the implications thereof, offer a new understanding of the evolutionary origin of PGR5 and CEF in general.