Proteorhodopsin functions throughout the Earth's oceans as a light-driven H+ pump, by a mechanism similar to that of bacteriorhodopsin. As in bacteriorhodopsin, the retinal chromophore of proteorhodopsin is covalently bound to the apoprotein via a protonated Schiff base at Lys231. The configuration of the retinal chromophore in unphotolyzed proteorhodopsin is predominantly all-trans
, and isomerizes to 13-cis upon illumination with light. Several models of the complete proteorhodopsin photocycle have been proposed, based on FTIR and UV–visible spectroscopy; they resemble established photocycle models for bacteriorhodopsin. Complete proteorhodopsin based photosystems have been discovered and expressed in E. coli, giving them additional light mediated energy gradient capability for ATP generation without external need for retinal or precursors; with the PR, gene five other proteins code for the photopigment biosynthetic pathway.