Rhodopsin is a visual pigment ubiquitous in multicellular animals. If visual pigments have a common ancient origin, as is believed, then some unicellular organisms might also use a rhodopsin photoreceptor. We show here that the unicellular alga Chlamydomonas does indeed use a rhodopsin photoreceptor. We incorporated analogues of its retinal chromophore into a blind mutant; normal photobehaviour was restored and the colour of maximum sensitivity was shifted in a manner consistent with the nature of the retinal analogue added. The data suggest that 11-cis-retinal is the natural chromophore and that the protein environment of this retinal is similar to that found in bovine rhodopsin, suggesting homology with the rhodopsins of higher organisms. This is the first demonstration of a rhodopsin photoreceptor in an alga or eukaryotic protist and also the first report of behavioural spectral shifts caused by exogenous synthetic retinals in a eukaryote. A survey of the morphology and action spectra of other protists suggests that rhodopsins may be common photoreceptors of chlorophycean, prasinophycean and dinophycean algae. Thus, Chlamydomonas represents a useful new model for studying photoreceptor cells.

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With an instrument that can record the motion of both cilia of the unicellular alga Chlamydomonas reinhardtii for many hours, the behavioral differences of its two cilia have been studied to determine their specific role in phototaxis. The organism was held on a fixed micropipette with the plane of ciliary beating rotated into the imaging plane of a quadrant photodetector. The responses to square-wave light patterns of a wide range of temporal frequencies were used to characterize the responses of each cilium. Eighty-one cells were examined showing an unexpectedly diverse range of responses. Plausible common signals for the linear and nonlinear signals from the cell body are suggested. Three independent ciliary measures--the beat frequency, stroke velocity, and phasing of the two cilia--have been identified. The cell body communicates to the cilia the direction of phototaxis the cell desires to go, the absolute light intensity, and the appropriate graded transient response for tracking the light source. The complexity revealed by each measure of the ciliary response indicates many independent variables are involved in the net phototactic response. In spite of their morphological similarity, the two cilia of Chlamydomonas respond uniquely. Probably the signals from the cell body fan out to independent pathways in the cilia. Each cilium modifies the input in its own way. The change in the pattern of the effective and recovery strokes of each cilium associated with negative phototaxis has been demonstrated and its involvement in phototactic turning is described.

The unicellular eukaryote Chlamydomonas reinhardtii is a phototactic alga that swims toward or away from light, using rhodopsin as the photopigment. The activity of retinal analogues was tested in the mutant FN68, which has high phototactic sensitivity only after incubation with retinal or analogues of retinal. Analogues prevented from isomerizing about the 7-ene, 9-ene, 11-ene, 13-ene, or 15-ene (C = N+H) bonds retained full activity. Also, bleaching, protonation of the N, and a stable geometrically altered chromophore are not required for full activity. An attractive hypothesis is that charge redistribution in the excited state of retinal directly triggers the activity of rhodopsin.