Presentation Information
[1ASPR-09]Cyanobacteria-specific rhodopsins mediate environmental adaptation and diversification
○Masumi Hasegawa-Takano1 (1. WPI-AIMEC, JAMSTEC (Japan))
Keywords:
Cyanobacteria,Microbial rhodopsins
Most biological processes on Earth are driven by the energy supplied from sunlight. In the aquatic environment, sunlight energy is converted into chemical energy by the chlorophyll-based photosystems of cyanobacteria and phytoplankton. Cyanobacteria have developed various chlorophyll pigments with different absorption wavelengths to capture sunlight efficiently in their diverse habitats. The accumulation of genomic sequence information has revealed the widespread prevalence of microbial rhodopsins, which are also commonly found in cyanobacteria. It has been suggested that this dual light-harvesting system enables efficient capture of sunlight energy by using complementary absorption wavelengths. However, 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.
In this study, we employed a metagenomic mining approach, which led to the identification of a novel rhodopsin clade unique to cyanobacteria: cyanorhodopsin (CyR) and cyanorhodopsin-II (CyR-II). Our genomic and phylogenetic studies have shown multiple evolutionary gains and 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. On the other hand, rhodopsin possession in cyanobacteria varies by morphology, being lower in unicellular forms than in multicellular or filamentous ones. This pattern suggests a role for rhodopsin in coping with light competition caused by self-shading. Heterologous expression analysis in Escherichia coli revealed that both CyRs and CyR-IIs function as light-driven outward proton pumps. The CyRs absorb green light (λmax = 550 nm) that chlorophyll pigments cannot directly absorb. The CyR-IIs are further divided into two subclades based on their specific absorption wavelength: yellow (λmax = 570 nm, YCyR-II) and green (λmax = 550 nm, GCyR-II). These two subclades have characteristic environmental distribution: YCyR-II mainly from sediment and soil of terrestrial environments, whereas GCyR-II from bacterial mats and biofilms of marine environments. 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. The evolutionary trajectory of cyanobacterial rhodopsins suggests that both their function and light-absorbing range have been adapted to diverse habitats with variable light and environmental conditions. Collectively, these findings illuminate the significance of rhodopsins in the evolution and environmental adaptation of cyanobacteria.
In this study, we employed a metagenomic mining approach, which led to the identification of a novel rhodopsin clade unique to cyanobacteria: cyanorhodopsin (CyR) and cyanorhodopsin-II (CyR-II). Our genomic and phylogenetic studies have shown multiple evolutionary gains and 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. On the other hand, rhodopsin possession in cyanobacteria varies by morphology, being lower in unicellular forms than in multicellular or filamentous ones. This pattern suggests a role for rhodopsin in coping with light competition caused by self-shading. Heterologous expression analysis in Escherichia coli revealed that both CyRs and CyR-IIs function as light-driven outward proton pumps. The CyRs absorb green light (λmax = 550 nm) that chlorophyll pigments cannot directly absorb. The CyR-IIs are further divided into two subclades based on their specific absorption wavelength: yellow (λmax = 570 nm, YCyR-II) and green (λmax = 550 nm, GCyR-II). These two subclades have characteristic environmental distribution: YCyR-II mainly from sediment and soil of terrestrial environments, whereas GCyR-II from bacterial mats and biofilms of marine environments. 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. The evolutionary trajectory of cyanobacterial rhodopsins suggests that both their function and light-absorbing range have been adapted to diverse habitats with variable light and environmental conditions. Collectively, these findings illuminate the significance of rhodopsins in the evolution and environmental adaptation of cyanobacteria.
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