SENSORY RHODOPSIN-I PHOTOCYCLE INTERMEDIATE SRI(380) CONTAINS 13-CIS RETINAL BOUND VIA AN UNPROTONATED SCHIFF-BASE

Sensory rhodopsin I (SRI), the mutated derivative SRI-D76N and the com plex of SRI with its transducer HtrI were overexpressed in Halobacteri um salinarium and analyzed by resonance Raman spectroscopy. In the ini tial state SRI contains all-trans retinal bound via a protonated Schif f base as confirmed by retinal extraction which yields 95 +/- 3% all-t rans retinal. The photocycle intermediate absorbing maximally at 380 n m (SRI(380)) contains a Schiff base linkage between the protein and 13 -cis retinal. Extraction of illuminated SRI yields up to 93% 13-cis re tinal. Neither the mutation D76N nor HtrI changed the vibrational patt ern of the chromophore.