Microbial Sensory Rhodopsins: Photochemistry and Function

The review covers recent progress on microbial sensory rhodopsins, visual pigment-like retinylidene photoreceptors that function in phototaxis by archaeons, such as Halobacterium salinarium, and by unicellular eukaryotic algae, such as Chlamydomonas reinhardtii. Six demonstrably different sensory rhodopsins are known in halophilic archaea. The best characterized is sensory rhodopsin I (SR-I), a color-sensitive receptor that relays attractant and repellent photosignals to a tightly bound transducer protein HtrI (halobacterial transducer for sensory rhodopsin I). New advances in the mechanism of signal transduction by the SR-I/HtrI complex from molecular-biological and biophysical approaches are summarized. Effects of HtrI on light-induced proton transfers in the receptor are discussed for their possible role in signaling. Current knowledge concerning the growing family of related archaeal sensory rhodopsins is presented. The evidence for a sensory rhodopsin in phototaxis by C. reinhardtii and other unicellular eukaryotic algae is reviewed. The molecular information is more limited than for the archaeal organisms, but the physiological information is rich and complex. Compelling data exist for a single retinal-containing receptor mediating both phototaxis and photophobic responses in C. reinhardtii. From retinal analog studies, the isomeric configuration and ring/chain conformation of the retinal in the receptor appear to be identical to those of the archaeal sensory rhodopsins. Also, photoisomerization from all-trans- to 13-cis-retinal appears to be the trigger for signaling, as in the archaeal pigments. Conflicting early studies suggesting an 11-cis-retinal chromophore and signaling without photoisomerization are analyzed and possible explanations for those reports are suggested. As a general conclusion, the microbial sensory rhodopsins provide an opportunity to explore photochemistry and protein/protein interaction in photosensory transduction in genetically tractable organisms.