SENSITIVITY INCREASE IN THE PHOTOPHOBIC RESPONSE OF Halobacterium halobium RECONSTITUTED WITH RETINAL ANALOGS: A NOVEL INTERPRETATION FOR THE FLUENCE‐RESPONSE RELATIONSHIP AND A KINETIC MODELING*

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Abstract
— Phoborhodopsin (also called sensory rhodopsin II) is a photoreceptor protein which mediates photophobic responses of Halobacterium halobium to blue‐green light. Under conditions where the synthesis of the chromophore retinal is inhibited, the photophobic system is reconstituted in vivo by incorporation of all‐trans retinal or retinal analogs into the apoprotein of phoborhodopsin. Retinal analogs which retard the cyclic photoreaction kinetics of phoborhodopsin increase significantly the sensitivity of the photophobic response. This supports the previously reported hypothesis that signal amplification occurs during the lifetime of intermediate states of the photocycle. The sensitivity increase caused by the chromophore substitution is observed in cells at several different growth stages, i.e. the naturally occurring chromophore (all‐trans retinal) does not produce maximal sensitivity at any stage of the culture growth. These results are difficult to interpret in terms of the proposal by Marwan et al. (J. Mol. Biol. 199, 663–664, 1988) that only a single photon is sufficient to cause the photobehavioral response in cells containing native phoborhodopsin. A new interpretation for the fluence‐response curves is described based in part on their Poisson statistical analysis. Further, a kinetic model which relates the receptor photochemical reaction cycle to the behavioral response is developed, which accounts for both the sensitivity increase and the shape of the fluence‐response curves.

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