What We are Learning from the Diverse Structures of the Homodimeric Type I Reaction Center-Photosystems of Anoxygenic Phototropic Bacteria
Authors: Niederman Robert A. 
Published: December 2023

A single type I RC (Fe-S type, ferredoxin reducing) is found in several of the phyla of anoxygenic phototrophic bacteria. They include the heliobacteria (HB), the green sulfur bacteria (GSB) and the chloroacidobacteria (CAB), for which high-resolution homodimeric RC-PS structures have recently appeared [1-4]. The 2.2-Å Xray structure of the RC-PS of Heliomicrobium modesticaldum [1] revealed that the core PshA apoprotein (PshA1 and PshA2 homodimeric pair) exhibits a structurally conserved PSI arrangement comprised of five C-terminal transmembrane helices (TMHs) forming the RC domain and six N-terminal TMHs coordinating the light-harvesting (LH) pigments. The Hm. modesticaldum structure lacked quinones, indicating that electrons were transferred directly from the A0 (81-OH-chlorophyl (Chl) a) primary acceptor to the FX [4Fe4S] component, serving as the terminal RC acceptor. A pair of additional TMHs designated as Psh X were found, that function as a low-energy antennae. The 2.5Å resolution cryo-electron microscopy (cryo-EM) structure for the RC-PS of the GSB Chlorobaculum tepidum [2] included two Fenna-Matthews-Olson protein (FMO) antennae, which transfer excitations from the chlorosomes to the RC-PS (PscA1 and PscA2) core. Two cytochromes c (PscC) molecules were revealed, which act in series as electron donors to the photooxidized RC BChl a’ special pair, as well as PscB, housing the [4Fe-4S] cluster FA and FB, and the associated PscD protein [3]. Although their FMO components were missing from the 2.6-Å cyro-EM structure of the Zn-BChl a’ special pair containing RC-PS of Chloroacidobacterium thermophilum [4], a unique architecture was revealed that consisted of seven additional subunits including PscZ in place of PscD, the PscX and PscY cytochrome c serial electron donors and four low mol. wt. subunits of unknown function. Overall, these diverse structures have revealed that: (i) the HB RC-PS is the simplest light-energy transducing complex yet isolated, and represents the closest known homolog to a common homodimeric RC-PS ancestor; (ii) the symmetrically localized Ca2+binding sites found in each of the Type I homodimeric RC-PSs likely gave rise to the analogously positioned Mn4CaO5 cluster of the PSII RC and the TyrZ RC donor site; (iii) a close relationship between the GSB RC-PS and Chl protein (CP)43 and CP47 of PSII was demonstrated by the strongly conserved locations of their LH-(B)Chls; (iv) LH-BChls of the GSB-RC-PS are also localized in the conserved RC-associated positions of the PSII ChlZ-D1 and ChlZ-D2 sites; (v) glycosylated carotenoids of the GSB RC-PS are located in the homologous carotenoid-containing positions of PSII, reflecting an O2-tolerance mechanism capable of sustaining early stages in the evolution of oxygenic photosynthesis. In addition to the close relationships found between the homodimeric RC-PS and PSII, duplication of the gene encoding the ancestral Type I RC apoprotein, followed by genetic divergence my well account for the appearance of the heterodimeric Type I and Type II RC found in extant oxygenic phototrophs. Accordingly, the long-held view that PSII arose from the anoxygenic Type II RC is now supplanted by the new evidence provided by Type I RC-PS homodimer structures, indicating that the evolutionary origins of anoxygenic Type II RCs along with their distinct antenna rings are likely to have occured later than the events which gave rise to their oxygenic counterparts.