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The implementation of multiconfigurational quantum chemistry methods into a quantum-mechanics/molecular-mechanics protocol has allowed the construction of a realistic computer model for the sensory rhodopsin of the cyanobacterium Anabaena PCC 7120. The model, which reproduces the absorption spectra of both the all-trans and 13-cis forms of the protein and their associated K and L intermediates, is employed to investigate the light-driven steps of the photochromic cycle exhibited by the protein. It is found that the photoisomerizations of the all-trans and 13-cis retinal chromophores occur through unidirectional, counterclockwise 180° rotations of the =C14-C15= moiety with respect to the Lys210-linked end of the chromophore axis. Thus, the sequential interconversions of the all-trans and 13-cis forms during a single photochromic cycle yield a complete (360°) unidirectional rotation of the =C14-C15= moiety. This finding implies that Anabaena sensory rhodopsin is a biological realization of a light-driven molecular rotor.