The unicellular protozoan ciliate, Stentor coeruleus, is able to sense differences in light intensity and wavelength, being most sensitive to red light (610 nm). Stentor also senses the direction of light propagation, as evidenced by their light-avoiding and negative phototactic swimming behaviors. This aneural photosensory phenomenon is triggered by the photoreceptor, and the proposed work involves the elucidation of how the organism perceives the light signal and processes it for subsequent sensory transduction of the light energy into behavioral responses. In particular, we propose to study the possible involvement of a light-induced proton release from the photoreceptor as a primary mechanism of light-signal processing. The primary sensory signal, in the form of proton release, triggers subsequent transduction steps that include calcium ion influx from the extracellular medium. It is proposed that the calcium ion influx causes the Stentor cell to reverse its ciliary beating and subsequently turn away from the light source. The present project is aimed at elucidating the mechanisms of these various transduction steps and their functional network in the photoresponse of Stentor coeruleus. For this aim, the photoreceptor protein will be further characterized, and its primary photoprocess will be elucidated spectroscopically in both H20 and D20. The nature of calcium channel(s) and Ca2+-pump involved in the ciliary reversal will also be characterized.
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