Structure of a symmetric photosynthetic reaction center–photosystem ($)

Plants, green algae and cyanobacteria carry out oxygenic photosynthesis through the coordination of two photosystems, PSI and PSII. Many other photosynthetic prokaryotes use a single reaction center to carry out anoxygenic photosynthesis. Gisriel et al. describe the structure of a photosynthetic reaction center from Heliobactrium modesticaldum, “a thermophilic anaerobe isolated from volcanic soil in Iceland.” The novelty of the new structure is that it is composed of a homodimeric reaction center, whereas others are heterodimeric, and is thought to more closely resemble the ancestral state. The authors point out that these bacteria branched early in bacterial evolution, meaning that the reaction center has had a long time to diverge from that of other species, but that its anoxic niche with its similarities to early Earth may have helped it maintain some of the ancestral traits. Science 10.1126/science.aan5611