Evidence that the repellent receptor form of sensory rhodopsin I is an attractant signaling state.

The lifetime of the Halobacterium halobium sensory rhodopsin I (SR-I) photocycle intermediate S373 was modulated by incorporating retinal analogs into SR-I apoprotein in vitro and in vivo. Photocycles by SR-I analog pigments exhibit the same reaction scheme and similar formation rates, but different decay rates, of their S373-like species as monitored by flash spectroscopy in membrane vesicle suspensions. The attractant receptor signaling efficiencies determined by physiological measurements are proportional to the lifetimes of the S373-like intermediates, indicating that S373 is a physiological active conformation (signaling state) of the receptor. A model incorporating this finding into the SR-I photocycle is presented.