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Figure 2. Alignment of the amino acid sequences of the four archaeal rhodopsins in the order BR, HR, SRI, and HsSRII. Numbers present the sequence position in BR. Amino acids forming the retinal binding site are marked by a white background. The two crucial positions in the cytoplasmic channel (96) and extracellular channel (85) are highlighted by a black background. The cytoplasmic side of the membrane is at the top. The amino acid sequences were taken from the SwissProt data bank. Accession numbers: BR, Ml 1720; HR, P16102; SRI, X51682; HsSRII, U62676.

considerable homologies to those of BR and HR. Positions defining the retinal binding site are usually identical; however, the site which in BR is intimately involved in the proton uptake from the cytoplasm (Asp96) is changed to an aromatic amino acid. It has been proposed that this change interferes with an efficient reprotonation of the Schiff base. Indeed, the NpSRII mutant F86D displays an unperturbed M-decay, although the overall turnover is unchanged [39,40],

The sequence determinations revealed upstream of the sopl and sopll loci open reading frames correspond to the halobacterial transducer of rhodopsin (Htr) [37,38,41], Both sop and their corresponding Htr genes are under the control of the same promoter. Sequence alignments with the chemotactic receptors from enteric bacteria revealed considerable homologies, especially in regions important for signalling and adaptation processes. This observation connects the well-known two-component system with the signalling chain in phototaxis, combining our knowledge of the two separate fields of archaeal phototaxis and eubacterial signal transduction.

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