Functional characterization of flavobacteria rhodopsins reveals a unique class of light-driven chloride pump in bacteria

Ion-translocating, light-activated membrane proteins known as rhodopsins are found in all three domains of life. Proton-pumping rhodopsins, such as proteorhodopsin, are known to be broadly distributed in marine bacteria. The first known sodium-pumping rhodopsin was recently described in marine flavobacterium. We report the discovery and characterization of a unique type of light-activated ion-translocating rhodopsin that translocates chloride ions into the cell and is evolutionarily distinct from the other known rhodopsin chloride pump, halorhodopsin, found in haloarchaea. Our data show that rhodopsins with different ion specificities have evolved independently in marine bacteria, with individual strains containing as many as three functionally different rhodopsins.