Abstract
Recently, a third retinal binding chromoprotein was found in cell membrane of Halobacterium halobium. Like bacteriorhodopsin and halorhodopsin, the third pigment shows photoreaction cycling upon illumination, but the decay of its photointermediate is two orders of magnitude slower than that of the two. Behavioral studies have shown that the pigment acts as photoreceptors for both positive and negative phototactic systems in this bacteria. Also, another photorepellent system has been found in the course of the study, which is mediated by a different (the fourth) photoreactive retinal-protein pigment.
