Diversion of the sign of phototaxis in a Chlamydomonas reinhardtii mutant incorporated with retinal and its analogs.

The blind mutant FN68 of the unicellular flagellate green alga Chlamydomonas reinhardtii is negatively phototactic in the presence of the native chromophore all-trans retinal. In contrast, analog chromophores such as a ring-acyclic retinal and those in which trans/cis isomerization about the C11 = C12 double bond was blocked induced predominantly positive phototaxis in the same strain under the same experimental conditions. These observations can be interpreted by assuming that the negative and the positive phototaxis is mediated distinctively by two rhodopsin species which differ in their affinities with the exogenous chromophores. However, a more reasonable explanation, which requires fewer assumptions, is that the sign of phototaxis depends on a delay in intracellular photosignal transduction. This novel view was deduced directly from the widely accepted hypothesis [1980, Microbiol. Rev. 44, 572-630] on phototaxis mechanisms.