Volvoxrhodopsin, a light-regulated sensory photoreceptor of the spheroidalgreen alga Volvox carteri

Somatic cells of the multicellular alga Volvox carteri contain a visual rho dopsin that controls the, organism's phototactic behavior via two independe nt photoreceptor currents. Here, we report the identification of an opsinli ke gene, designated as volvoxopsin (vop). The encoded protein exhibits homo logies to the opsin of the unicellular alga Chlamydomonas reinhardtii (chla myopsin) and to the entire animal opsin family, thus providing new perspect ives on opsin evolution. Volvoxopsin accumulates within the eyes of somatic cells. However, the vop transcript is detectable only in the reproductive eyeless gonidia and embryos. vop mRNA levels increase 400-fold during embry ogenesis, when embryos develop in darkness, whereas the vop transcript does not accumulate when embryos develop in the light. An antisense transforman t, T3, was generated. This transformant produces 10 times less volvoxopsin than does the wild type. In T3, the vop transcript is virtually absent, whe reas the antisense transcript is predominant and light regulated. It follow s that vop expression is under light-dependent transcriptional control but that volvoxopsin itself is not the regulatory photoreceptor. Transformant T 3 is phototactic, but its phototactic sensitivity is reduced 10-fold relati ve to the parental wild-type strain HK10. Thus, we offer definitive genetic evidence that a rhodopsin serves as the photoreceptor for phototaxis in a green alga.