[B-BG01] Earth and Planetary Science Frontiers for Life and Global Environment

[BBG01-07] Light-utilization strategies with microbial rhodopsins in cyanobacteria

★Invited Papers

*Masumi Hasegawa¹, Yosuke Nishimura¹, Susumu Yoshizawa^2,3 (1.Japan Agency for Marine-Earth Science and Technology, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Graduate School of Frontier Sciences, The University of Tokyo)

Keywords:Cyanobacteria, Microbial rhodopsin, Sunlight energy

Almost all biological processes on the Earth are driven by energy supplied from sunlight. In shallow-water environments, sunlight energy is converted into chemical energy by the chlorophyll-based photosystem of cyanobacteria and phytoplankton. Cyanobacteria have developed various chlorophyll pigments with different absorption wavelengths to capture sunlight efficiently in their diverse habitats. Recently, microbial rhodopsin (hereafter rhodopsin), a light-utilizing system distinct from the chlorophyll-based photosystem, was identified in cyanobacteria. Rhodopsin is a useful light-harvesting system for converting sunlight energy into chemical energy, especially for heterotrophic bacteria, and therefore the importance of rhodopsins in cyanobacteria has not been discussed. Our genomic and phylogenetic studies have shown multiple evolutionary gains or losses of cyanobacterial rhodopsins, with uneven distribution across the cyanobacterial lineage. For example, marine cyanobacteria with relatively small genomes (e.g., Prochlorococcus and Synechococcus) did not possess any rhodopsin. Phylogenetic analysis detected six rhodopsin clades including cyanobacterial rhodopsins: xanthorhodopsin-like rhodopsin, Na⁺ pumping rhodopsin, xenorhodopsin, cyanobacterial halorhodopsin, and two unnamed clades. We proposed that these clades composed exclusively of cyanobacteria-specific rhodopsins, cyanorhodopsin (CyR) and cyanorhodopsin-II (CyR-II). CyRs and CyR-IIs were experimentally confirmed to function as light-driven proton pumps in Escherichia coli cells. CyRs absorb green light (λ_max = 550 nm) that chlorophyll pigments cannot directly absorb. CyR-IIs were divided into two subclades absorbing green (λ_max = 550 nm) and yellow (λ_max = 570 nm) light, respectively. X-ray crystallography and mutation analysis demonstrated that their different absorption wavelengths were attributable to slight changes in the side chain structure near the retinal chromophore. Our results suggest that cyanobacterial rhodopsins have specialised to use green light in cyanobacterial cells and then differentiated to use yellow light in response to their habitat environment. In my talk, I would like to introduce the functional- and spectroscopic- characteristics, diversity, and evolution of cyanobacterial rhodopsins.