For phototactic migration, Chlamydomonas scans the surrounding light environment by rotating the cell body with an eyespot located on the equator. The intensity of the light signal received by the eyespot should therefore change cyclically at the frequency of the cell body rotation. In this study, the response of the photoreceptor to cyclically changing light stimuli was analyzed using immotile mutant cells. To simulate the light intensity change perceived by a rotating cell, light stimuli were applied that consisted of a light phase with the intensity changing similar to a half cycle of a sine wave and a dark phase of the same length. The fluence rate at the peak of the sine wave was of the order of 1019 photons m-2 s-1, i.e. high intensity at which phototaxis is saturated. A photoreceptor current (PRC) was produced at the onset of each light phase. Interestingly, its amplitude varied depending on the frequency and was largest at 1-5 Hz, a frequency range similar to the frequency of cell body rotation. Experiments on the kinetics of the PRC indicate that the response was small at low frequency because of the inactivation of the PRC before full activation. In contrast, at high frequency the PRC was suppressed by adaptation to the repetitive stimuli. These characteristic kinetics of the PRC should be important for Chlamydomonas cells to extract information from the signals generated by the cell body rotation.
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