Autoregulation of rhodopsin synthesis in Chlamydomonas reinhardtii.

Abstract

A sensitive assay for the induction of carotenoid and rhodopsin synthesis, based on the phototactic response, has been developed in a mutant of the unicellular alga Chlamydomonas reinhardtii. In the dark, the mutant fails to synthesize carotene and retinal, but it contains the apoprotein opsin. When retinal synthesis is induced by light treatment, the retinal combines with opsin to form rhodopsin, and the cells swim away from a source of light. Since the amount of light required to trigger a phototactic response is inversely proportional to the concentration of rhodopsin, the decrease in amount of light necessary to generate that response can serve as a measure of the amount of retinal synthesized in cells after induction. Using this assay, we found that (i) light induction of retinal depends linearly on light exposure and rhodopsin concentration during the exposure; (ii) the action spectrum of light induction is identical with that for phototaxis for which the receptor pigment is rhodopsin; and (iii) incubation with alltrans-7,8-dihydroretinal before light exposure shifts the actionspectrum peak for light induction 0.41 eV (-71 nm). We conclude that the photopigment for induction of retinal synthesis is a rhodopsin. The time lag required for induction of retinal synthesis and preliminary experiments with transcription or translation inhibitors suggest that alterations in gene expression could be involved in the induction process. Its control could be similar to other processes in which membrane receptors for hormones, neurotransmitters, or growth factors regulate gene expression.Rhodopsin plays multiple roles in animals. It is the photoreceptor for vision and a regulator of circadian rhythm and melatonin production. In vertebrates, it acts as a regulator both in the pineal gland (1) and, to a lesser extent, in the retinal photoreceptor cells (2). In Chlamydomonas, a flagellated eukaryotic alga, we found that a bovine-like rhodopsin serves as the photoreceptor pigment for phototaxis (3). Rhodopsin in eukaryotes is thus of ancient origin. Analysis of 5S RNAs (4) has shown that the Chlamydomonas evolutionary line branched from the plant line soon after the plant and animal lines diverged. The question naturally arises whether the regulatory function of rhodopsin is also ancient.The identity of the light receptors that regulate carotene synthesis in bacteria, fungi, algae, and angiosperms has been uncertain. Flavoproteins, porphyrins, and carotenoid proteins have all been suggested on the basis of action spectra in various organisms (5).The measurement of phototaxis threshold (3) has given us a sensitive assay for the presence of rhodopsin. We noticed that carotenoid mutants exposed to light recovered their phototaxis. Here we report that rhodopsin excitation induces synthesis of carotene and retinal. We do not know the nature of the induced products or how they modulate the synthetic pathway. Nevertheless, since retinal, the chromophore for rhodopsin, is synthesized as a result of rhodopsin excitation, we ...

Blue Light Regulation of Cell Division in Chlamydomonas reinhardtii

“…Several blue light effects have been described for C. reinhardtii by different authors, such as induction of carbonic anhydrase (3, 4), nitrogen metabolism (1, 14), survival of dark-lethal mutants (24), and phototaxis (6,7,9,11). On the other hand, Ruyters et al (19) reported evidence for the presence of phytochrome in C reinhardtii.…”

“…There is evidence for the presence of phytochrome (19) and at least one blue light receptor (1,2,4,6,7,9,11,14,24) in Chlamydomonas. Our data clearly demonstrate that the light/dark control of cell division is not mediated by phytochrome but that the light-induced delay of cell division is a blue light effect.…”

“…7B). The peak at 500 nm might correspond to rhodopsin, which has been reported to be involved in the blue light effects on phototaxis (2,6,7,9). The second peak at 400 nm might be an indication that there is a second blue light receptor that is also involved in light/ dark control of cell division in Chlamydomonas (Fig.…”

Evolution of the Channelrhodopsin Photocycle Model

“…Among these are behavioral responses to light (phototaxis and phobic responses), [1] developmental processes, [2] retinal biosynthesis [3] to mention a few. For the best studied model alga Chlamydomonas reinhardtii, so far seven opsin-related genes have been found in genomic or EST data bases.…”

Photoreceptor for Curling Behavior in Peranema trichophorum and Evolution of Eukaryotic Rhodopsins

“…An additional ancestral function of rhodopsin may be its control of gene expression. This was observed for a rhodopsin in the green alga Chlamydomonas (17) and in rats (43). In the nonmotile fungus Neurospora, rhodopsin probably (3) controls gene expression.…”