Bacterial Rhodopsin: Evidence for a New Type of Phototrophy in the Sea

TLDR

Bacteriorhodopsins are retinal‑binding proton pumps found in halophilic archaea, and until now only archaea have been shown to generate a light‑driven chemiosmotic potential. The study reports a bacterial rhodopsin identified from marine bacterioplankton genomes. The protein, encoded by an uncultivated γ‑proteobacterium, was expressed in *E. coli*, bound retinal, and functioned as a light‑driven proton pump.

Abstract

Extremely halophilic archaea contain retinal-binding integral membrane proteins called bacteriorhodopsins that function as light-driven proton pumps. So far, bacteriorhodopsins capable of generating a chemiosmotic membrane potential in response to light have been demonstrated only in halophilic archaea. We describe here a type of rhodopsin derived from bacteria that was discovered through genomic analyses of naturally occuring marine bacterioplankton. The bacterial rhodopsin was encoded in the genome of an uncultivated γ-proteobacterium and shared highest amino acid sequence similarity with archaeal rhodopsins. The protein was functionally expressed in Escherichia coli and bound retinal to form an active, light-driven proton pump. The new rhodopsin exhibited a photochemical reaction cycle with intermediates and kinetics characteristic of archaeal proton-pumping rhodopsins. Our results demonstrate that archaeal-like rhodopsins are broadly distributed among different taxa, including members of the domain Bacteria . Our data also indicate that a previously unsuspected mode of bacterially mediated light-driven energy generation may commonly occur in oceanic surface waters worldwide.