The abundant retinal protein of the Chlamydomonas eye is not the photoreceptor for phototaxis and photophobic responses

The chlamyopsin gene (cop) encodes the most abundant eyespot protein in the unicellular green alga Chlamydomonas reinhardtii. This opsin-related prote in (COP) binds retinal and was thought to be the photoreceptor controlling photomovement responses via a set of photoreceptor currents. Unfortunately, opsin-deficient mutants are not available and targeted disruption of non-s electable nuclear genes is not yet possible in any green alga. Here we show that intron-containing gene fragments directly linked to their intron-less antisense counterpart provide efficient post-transcriptional gene silencin g (PTGS) in C. reinhardtii, thus allowing an efficient reduction of a speci fic gene product in a green alga. In opsin-deprived transformants, Hash-ind uced photoreceptor currents (PC) are left unchanged. Moreover, photophobic responses as studied by motion analysis and phototaxis tested in a light-sc attering assay were indistinguishable from the responses of untransformed w ild-type cells. We conclude that phototaxis and photophobic responses in C. reinhardtii are triggered by an as yet unidentified rhodopsin species.