Extremely halophilic archaebacterium
Halobacterium halobium cells show phototaxis to visible hight. The phototaxis requires vitamine A aldehyde, a retinal. Protein pigments containing retinal are the photoreceptors of the phototaxis. Phoborhodopsin (pR) is a photoreceptor of the negative phototaxis. Sensory rhodopsin (sR) is a photoreceptor of the positive phototaxis and S373, a transient photoproduct of sR, is another photoreceptor of the negative phototaxis. These photoreceptors are too unstable to isolate. Haloalkalophilic archaebacterium
Natronobacterium pharaonis cells have pharaonis phoborhodopsin (ppR), which is also a photoreceptor of a negative phototaxis. PPR is solubilized and isolated in an active state, A methylation/demethylation system is involved in the signalling chain from photoreceptor to flagellar motor switch.
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