Identification of a third rhodopsin-like pigment in phototactic Halobacterium halobium

Mutant Halobacterium halobium strains deficient in all previously reported rhodopsin-like pigments show phototaxis responses comparable to those of wild-type strains. Spectroscopic analysis reveals the presence of a third retinal-containing pigment in the cells and their membrane fractions. It undergoes a photoreaction cycle with a half-time of approximately equal to 1 sec at room temperature and at physiological light intensities the photostationary state of the pigment consists of two species, one absorbing in the 580- to 590-nm region and the other at 373 nm, both of which are photoactive. Illumination of the long-wavelength species generates the 373-nm intermediate, which upon photoexcitation reconverts to the long-wavelength form. Therefore, changes in the relative light intensities in the long- and short-wavelength regions of the visible spectrum cause opposing shifts in the photostationary state. The spectral sensitivity of this pigment correlates with the color-discriminating phototaxis sensitivities of this organism and strongly suggests that it is the sensory photoreceptor.