PHOTOPHOBIC RESPONSES AND PHOTOTAXIS IN CHLAMYDOMONAS ARE TRIGGERED BY A SINGLE RHODOPSIN PHOTORECEPTOR

The rhodopsin nature of the photoreceptor for the behavioural light re sponses in Chlamydomonas has originally been revealed by action spectr oscopy. Meanwhile most physiological experiments and the identificatio n of all-trans-retinal in cell extracts favour that this chlamyrhodops in contains an all-trans-type retinal chromophore with strong similari ty to the light sensors SR I and SRII from Halobacteria. Reconstitutio n of retinal-deficient cells with [H-3]retinal identified a single ret inal protein with a MW of 30,000. Chlamyrhodopsin triggers a photorece ptor current in the eyespot region resulting in direction changes or p hototaxis. Furthermore, when the light stimulus oversteps a critical l evel, two flagellar currents appear, which are the basis for photophob ic responses. The physiological, electrophysiological and biochemical experiments suggest that all behavioural responses are triggered by a single rhodopsin-type receptor.