The sopI gene, which encodes the phototaxis receptor sensory rhodopsin I (SR-I), was expressed in Halobacterium salinarium strains with chromosomal deletions of (i) sopI only or of (ii) the region containing sopI and htrI. The htrI gene encodes a transducer protein for SR-I signals. Transformation of the sopI deletion mutant containing the htrI gene by a multicopy expression plasmid for sopI results in normal physiological and photochemical properties of SR-I. Transformation by the same plasmid of the mutant lacking the htrI gene as well as sopI results in production of pigment with a normal absorption spectrum but altered photochemical properties, and no phototaxis by the transformants. Analysis of flash-induced absorbance changes shows that the transducer protein increases light-induced production of the photocycle intermediate S373, the SR-I signaling conformation, and modulates the rate of S373 return to the prestimulus state, rendering this return pH-independent. These effects are interpreted in terms of receptor/transducer interactions that influence proton transfer reactions occurring in the photoactive site.