Bacterial rhodopsin: Evidence for a new type of phototrophy in the sea

Extremely halophilic archaea contain retinal-binding integral membrane prot eins called bacteriorhodopsins that function as Light-driven proton pumps. So far, bacteriorhodopsins capable of generating a chemiosmotic membrane po tential in response to Light have been demonstrated only in halophilic arch aea. We describe here a type of rhodopsin derived from bacteria that was di scovered through genomic analyses of naturally occuring marine bacterioplan kton. The bacterial rhodopsin was encoded in the genome of an uncultivated gamma-proteobacterium and shared highest amino acid sequence similarity wit h archaeal rhodopsins. The protein was functionally expressed in Escherichi a coli and bound retinal to form an active, Light-driven proton pump. The n ew rhodopsin exhibited a photochemical reaction cycle with intermediates an d kinetics characteristic of archaeal proton-pumping rhodopsins. Our result s demonstrate that archaeal-like rhodopsins are broadly distributed among d ifferent taxa, including members of the domain Bacteria. Our data also indi cate that a previously unsuspected mode of bacterially mediated Light-drive n energy generation may commonly occur in oceanic surface waters worldwide.