Microbial rhodopsins are prevalent in many cyanobacterial groups as a light-energy-harvesting system in addition to the photosynthetic system. It has been suggested that this dual system allows efficient capture of sunlight energy using complementary ranges of absorption wavelengths. However, the diversity of cyanobacterial rhodopsins, particularly in accumulated metagenomic data, remains underexplored. Here, we used a metagenomic mining approach, which led to the identification of a novel rhodopsin clade unique to cyanobacteria, cyanorhodopsin-II (CyR-II). CyR-IIs function as light-driven outward H+ pumps. CyR-IIs, together with previously identified cyanorhodopsins (CyRs) and cyanobacterial halorhodopsins (CyHRs), constitute cyanobacterial ion-pumping rhodopsins (CyipRs), a phylogenetically distinct family of rhodopsins. The CyR-II clade is further divided into two subclades, YCyR-II and GCyR-II, based on their specific absorption wavelength. YCyR-II absorbed yellow light (λmax = 570 nm), whereas GCyR-II absorbed green light (λmax = 550 nm). X-ray crystallography and mutational analysis revealed that the difference in absorption wavelengths is attributable to slight changes in the side chain structure near the retinal chromophore. The evolutionary trajectory of cyanobacterial rhodopsins suggests that the function and light-absorbing range of these rhodopsins have been adapted to a wide range of habitats with variable light and environmental conditions. Collectively, these findings shed light on the importance of rhodopsins in the evolution and environmental adaptation of cyanobacteria.

中文翻译：

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Cyanorhodopsin-II 代表蓝细菌内一个吸收黄色的质子泵视紫红质分支


微生物视紫红质在许多蓝藻组中普遍存在，除了光合作用系统外，还是一种光能收集系统。有人提出，这种双系统允许使用互补的吸收波长范围有效地捕获太阳能量。然而，蓝藻视紫红质的多样性，特别是在积累的宏基因组数据中，仍未得到充分探索。在这里，我们使用了宏基因组采矿方法，这导致鉴定了蓝细菌特有的新型视紫红质分支，蓝视紫红质-II （CyR-II）。CyR-II 用作光驱动向外的 H+ 泵。CyR-IIs 与先前鉴定的蓝藻紫红质 （CyR） 和蓝藻盐视紫红质 （CyHRs） 一起构成蓝藻离子泵视紫红质 （CyipRs），这是一个系统发育不同的视紫红质家族。CyR-II 分支根据其特定的吸收波长进一步分为两个亚分支，YCyR-II 和 GCyR-II。YCyR-II 吸收黄光 （λmax = 570 nm），而 GCyR-II 吸收绿光 （λmax = 550 nm）。X 射线晶体学和突变分析表明，吸收波长的差异是由于视网膜发色团附近侧链结构的微小变化。蓝藻视紫红质的进化轨迹表明，这些视紫红质的功能和光吸收范围已经适应了具有不同光照和环境条件的广泛栖息地。总的来说，这些发现阐明了视紫红质在蓝细菌进化和环境适应中的重要性。