Zacks D N, Spudich J L
Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York.
Cell Motil Cytoskeleton. 1994;29(3):225-30. doi: 10.1002/cm.970290305.
The unicellular green alga (Chlamydomonas reinhardtii maintains sensitivity of its phototaxis response (alignment of swimming direction along the axis of a light beam) over several orders of magnitude of light intensities. It is widely accepted that the rotation of the swimming cell provides temporal comparisons of light intensities via periodic contrast generated by its asymmetrically positioned refractile eyespot organelle. The cells also exhibit a second behavioral response to light called the photophobic (or stop) response, which is a brief cessation of swimming caused by a temporal change in light intensity. The cells are desensitized to photophobic stimuli by light exposure. Through comparative measurements of both responses, we explain the behavioral basis of the large dynamic range of phototaxis in terms of precise desensitization of the photophobic response. The basis of the explanation is that the flagellar beat changes which cause phototactic orientation are the residual of the photophobic response after desensitization (i.e., "mini-photophobic" reactions which cause brief reorienting motions without a full stop). This interpretation predicts quantitatively the dependence of the extent of desensitization on light intensity and the dependence of onset and maintenance of phototaxis on extent of desensitization. These predictions are tested and confirmed in this report.
单细胞绿藻莱茵衣藻（Chlamydomonas reinhardtii）在几个数量级的光强范围内都能保持其趋光反应（沿光束轴排列游泳方向）的敏感性。人们普遍认为，游动细胞的旋转通过其不对称定位的折射性眼点细胞器产生的周期性对比度来提供光强的时间比较。这些细胞还表现出对光的第二种行为反应，称为光ophobic（或停止）反应，这是由光强的时间变化引起的短暂游泳停止。细胞通过光照对光ophobic刺激脱敏。通过对这两种反应的比较测量，我们用光ophobic反应的精确脱敏来解释趋光性大动态范围的行为基础。解释的基础是，导致趋光定向的鞭毛摆动变化是脱敏后光ophobic反应的残余（即“微小光ophobic”反应，导致短暂的重新定向运动而不完全停止）。这种解释定量地预测了脱敏程度对光强的依赖性以及趋光性的开始和维持对脱敏程度的依赖性。这些预测在本报告中得到了测试和证实。
