Photoisomerization of retinal at 13-ene is important for phototaxis of Chlamydomonas reinhardtii: Simultaneous measurements of phototactic and photophobic responses

Abstract

A real-time automated method was developed for simultaneous measurements of phototactic orientation (phototaxis) and step-up photophobic response of flagellated microorganisms. Addition of all- trans retinal restored both photoresponses in a carotenoid-deficient mutant strain of Chlamydomonas reinhardtii in a dose-dependent manner. The phototactic orientation was biphasic with respect to both the light intensity and the concentration of retinal. All- trans retinal was more effective than 11- cis retinal to regenerate both photobehavioral responses. Analogs having locked 11- cis configurations and a phenyl ring in the side chain also induced photoresponses, although at concentrations more than two orders of magnitude higher than all- trans retinal. According to the present assay method, the responses were hardly detectable in cells incubated with retinal analogs in which the 13-ene was locked in either its trans or cis configuration. The results strongly suggest that the isomerization of the 13–14 double bond is important for photobehavioral signal transduction and that a single retinal-dependent photoreceptor controls both phototactic and photophobic responses.