Phototaxis in Archaea is regulated by the two receptors sensory rhodopsin I and sensory rhodopsin II which are closely related to the two ion pumps halorhodopsin (HR) and bacteriorhodopsin (BR). These seven helix membrane proteins are activated by light which induces an aW-trans to 13-cis isomerisation of the retinal chromophore bound via a protonated Schiff base to helix G. The signal invoked by these reactions triggers structural changes in cognate halobacterial transducers of rhodopsin. The cytoplasmic domains of these membrane proteins are homologous to that of eubacterial chemo-receptors which activate proteins of the two-component signalling cascade. The similarities between the phototaxis machinery with the two-component signalling chain on the one hand and between the photoreceptors with the ion pumps BR and HR on the other direct the present review. The first part addresses the physiological response of the H. salinarum towards light and the underlying protein network. The next section focuses on the shared properties of receptors and ion pumps such as structural similarities and common principles of the light activated reactions. Finally, the molecular mechanism of signal transfer from the photoreceptor to the transducer is discussed.
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