Negative phototaxis from blue light and the role of third rhodopsinlike pigment in halobacterium cutirubrum.

Wild-type cells of Halobacterium cutirubrum show phototaxis. In negative phototaxis the cells are repelled by blue-near ultraviolet light, and in positive phototaxis the cells are attracted to green-red light. The extent of the responses are measured by monitoring the changes in the reversal frequency of the swimming direction of cells using a computer-linked automated method as described previously (Takahashi, T., and Y. Kobatake, 1982, Cell. Struct. Funct., 7:183-192). When the intensity of the background light (illumination for the observation) was dramatically reduced, the sensitivity of the cells to the repellent light decreased markedly. This result has been previously explained by Bogomolni and Spudich (1982, Proc. Natl. Acad. Sci. USA, 79:6250-6254), who proposed that the photoreceptor for negative phototaxis is the long-lifetime intermediate in the photocycle of slow-rhodospin. The behavioral response in the negative phototaxis is dependent upon the intensity of the actinic light and the background light. This agrees quantitatively with our model based on the aforementioned hypothesis.