MICROBIAL SENSORY RHODOPSINS - PHOTOCHEMISTRY AND FUNCTION

The review covers recent progress on microbial sensory rhodopsins, vis ual pigment-like retinylidene photoreceptors that function in phototax is by archaeons, such as Halobacterium salinarium, and by unicellular eukaryotic algae, such as Chlamydomonas reinhnrdtii. Six demonstrably different sensory rhodopsins are known in halophilic archaea. The best characterized is sensory rhodopsin I (SR-I), a color-sensitive recept or that relays attractant and repellent photosignals to a tightly boun d transducer protein HtrI (halobacterial transducer for sensory rhodop sin I). New advances in the mechanism of signal transduction by the SR -L/HtrI complex from molecular-biological and biophysical approaches a re summarized. Effects of HtrI on light-induced proton transfers in th e receptor are discussed for their possible role in signaling. Current knowledge concerning the growing family of related archaeal sensory r hodopsins is presented. The evidence for a sensory rhodopsin in photot axis by C. reinhardtii and other unicellular eukaryotic algae is revie wed. The molecular information is more limited than for the archaeal o rganisms, but the physiological information is rich and complex. Compe lling data exist for a single retinal-containing receptor mediating bo th phototaxis and photophobic responses in C. reinhardtii. From retina l analog studies, the isomeric configuration and ring/chain conformati on of the retinal in the receptor appear to be identical to those of t he archaeal sensory rhodopsins. Also, photoisomerization from all-tran s- to 13-cis-retinal appears to be the trigger for signaling, as in th e archaeal pigments. Conflicting early studies suggesting an 11-cis-re tinal chromophore and signaling without photoisomerization are analyze d and possible explanations for those reports are suggested. As a gene ral conclusion, the microbial sensory rhodopsins provide an opportunit y to explore photochemistry and protein/protein interaction in photose nsory transduction in genetically tractable organisms.