In order to find optimal light conditions for photosynthetic growth, the green alga Chlamydomonas uses a visual system. An optical device, a rhodopsin photoreceptor and an electrical signal transduction chain that mediates between photoreceptor and flagella comprise this system. Here we present an improved strategy for the preparation of eyespot membranes. These membranes contain a retinal binding protein, which has been proposed to be the apoprotein of the phototaxis receptor. The retinal binding protein, which we named chlamyopsin, was purified and opsin‐specific antibodies were raised. Using these antibodies, the opsin was localized in the eyespot region of whole cells during growth and cell division. The opsin cDNA was purified and sequenced. The sequence reveals that chlamyopsin is not a typical seven helix receptor. It shows some homology to invertebrate opsins but not to opsins from halobacteria. It contains many polar and charged residues and might function as a light‐gated ion channel complex. It is likely that this lower plant rhodopsin diverged from animal opsins early in opsin evolution.

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-truns-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 SRI1 from Halobacteriu. Reconstitution of retinal-deficient cells with ['HIretinal 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.

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