Photophobic responses and phototaxis in Chlamydomonas are triggered by a single rhodopsin photoreceptor.

The rhodopsin nature of the photoreceptor for the behavioural light responses in Chlamydomonas has originally been revealed by action spectroscopy. Meanwhile most physiological experiments and the identification of all-trans-retinal in cell extracts favour that this chlamyrhodopsin contains an all-trans-type retinal chromophore with strong similarity to the light sensors SR I and SRII from Halobacteria. Reconstitution of retinal-deficient cells with [3H]retinal identified a single retinal protein with a MW of 30,000. Chlamyrhodopsin triggers a photoreceptor current in the eyespot region resulting in direction changes or phototaxis. Furthermore, when the light stimulus oversteps a critical level, two flagellar currents appear, which are the basis for photophobic responses. The physiological, electrophysiological and biochemical experiments suggest that all behavioural responses are triggered by a single rhodopsin-type receptor.