Sensory rhodopsin. Associates with an unusual transducer lacking a methyl-accepting transducer domain found in all other photosensory transducers. The chromophore is all-trans-retinal in the dark. Interacts with HtrM. Expressed constitutively throughout the growth phases, both in presence and absence of white light. The covalent binding of retinal to the apoprotein, bacterioopsin, generates bacteriorhodopsin. Belongs to the archaeal/bacterial/fungal opsin family.