The phototactic archaeobacterium H. halobium contains the light-driven proton and chloride pumps bacteriorhodopsin and halorhodopsin. It has been found that mutants deficient in both rhodopsins are still phototactic, and that the light signal receptors are actually two retinal pigments, the sensory rhodopsins (sR's) I and II. They constitute a primitive color discrimination system based on pigments chemically similar to the light and color sensing pigments of animals. The goal of this proposal is to isolate, purify and characterize the recently identified sR-II receptor, and compare its function and properties with those of the sensory pigments of higher organisms. The results should yield new insights on the principles which underlie the design of membrane proteins for energy and sensory transduction and their evolutionary development.
