Abstract Blocking in vivo synthesis of retinal by addition of nicotine to the culture medium leads to the loss of photobehavior in Halobacterium halobium. Addition of rrans-retinol or frans-retinol2 (3,4-dehy-droretinol) restores the responses to light decreases in the green-yellow spectral range. Action spectra of the reconstituted retinal- and retinal2-photosystem show maximal sensitivity at 565 and 580 nm, respectively. Addition of retinol or retinol2 also restores the formation of bacteriorhodopsin (BR) or bacteriorhodopsin2 (BR2= 3,4-dehydroretinal-bacterio-opsin complex). The absorption spectra of BR and BR2, measured in isolated membranes, as well as in living bacteria, show maxima at 568 nm (BR) and at about 600 nm (BR2), respectively. Comparison of the action spectrum of the retinal2-containing sensory photosystem with the absorption spectrum of BR2 suggests that a retinal pigment different from BR is responsible for the photosensory behavior to green-yellow light.
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