THE ROLE OF BACTERIORHODOPSIN IN PHOTORECEPTION IN HALOBACTERIUM-HALOBIUM

Bacterio-opsin gene is introduced to a ''blind'' Halobacterium halobium mutant which lacks all the four retinal proteins (bacteriorhodopsin, halorhodopsin and sensory rhodopsins I and II) and,as a consequence,shows no attractant response to a long wavelength light and no repellent response to the short wavelength light. The resulting transformant cells acquire the capability to sense the light stimuli. However, decrease in intensity of not only orange and red lights but also of blue light is shown to induce the repellent effect. To induce the light response, much larger changes in the light intensity are necessary in the bacteriorhodopsin-containing transformant than in a mutant, containing sensory rhodopsins. The light sensitivity of the mutant is increased by adding 1 mM cyanide or decreased by arginine, i.e by agents reducing or increasing the light-independent generation of DELTAmu(H)+BAR, respectively. At very high light intensities, even a large decrease in the illumination fails to induce the repellent effect but the addition of the DELTAmu(H)+BAR-lowering agent (protonophorous uncoupler) sensitizes the cell to the light stimuli. Mutation resulting in substitution of the Asp-96 by Asn in bacteriorhodopsin and as a consequence, in inhibition of its H+ pump activity, is found to reduce the bacteriorhodopsin-mediated photoreception. Azide, which is known to activate the mutant H+ pump, increases the photosensitivity of the mutant cells. Moreover, azide abolishes the DELTApsi-decreasing and repellent effects on the blue light on the strain possessing a mutant bacteriorhodopsin with replacement of Asn for Asp in position 96 of the polypeptide chain, under orange background illumination. It is assumed that the bacteriorhodopsin-mediated photoreception is due to the light-supported generation Of DELTAmu(H)+BAR, which is monitored by a special DELTAmu(H)+BAR sensor, protometer''.