MECHANISM OF PHOTOSENSORY ADAPTATION IN HALOBACTERIUM-SALINARIUM

Phototaxis in Halobacterium salinarium is the result of an interplay o f sensory rhodopsin excitation and adaptation to the stimulus backgrou nd. Adaptation to orange light, received by sensory rhodopsin I was pr obed by measuring the behavioral response of cells to a step-like decr ease in intensity. Cells were able to adapt to an intensity range of m ore than four orders of magnitude. The data were analysed on the basis of theoretical fluence rate response relationships calculated from th e photocycle kinetics of the complex of sensory rhodopsin I with its t ransducer HtrI. Independent of the stimulus background, the cellular r esponse was shown to be a function of the absolute number of photorece ptor complex molecules turned over by the light stimulus. Receptor dea ctivation was identified as the underlying mechanism of adaptation and was sufficient to account for the experimental results. We suggest th at reversible methylation of the transducer protein HtrI provides the chemical mechanism of sensory adaptation in H. salinarium and also exp lains the different sensitivity of the cells to orange and UV light.