THE PRIMARY STRUCTURE OF SENSORY RHODOPSIN-II - A MEMBER OF AN ADDITIONAL RETINAL PROTEIN SUBGROUP IS COEXPRESSED WITH ITS TRANSDUCER, THE HALOBACTERIAL TRANSDUCER OF RHODOPSIN-II

The blue-light receptor genes (sopII) of sensory rhodopsin (SR) II,wer e cloned from two species. the halophilic bacteria Haloarcula vallismo rtis (vSR-II) and Natronobacterium pharaonis (pSR-II). Upstream of bot h sopII gene loci, sequences corresponding to the halobacterial transd ucer of rhodopsin (Htr) IT were recognized. In N. pharaonis, psopII an d phtrII are transcribed as a single transcript. Comparison of the ami no acid sequences of vHtr-II and pHtr-II with Htr-I and the chemotacti c methyl-accepting proteins from Escherichia coli revealed considerabl e identities in the signal domain and methyl-accepting sites. Similari ties with Htr-I in Halobacterium salinarium suggest a common principle in the phototaxis of extreme halophiles. Alignment of all known retin al protein sequences from Archaea identifies both SR-IIs as an additio nal subgroup of the family. Positions defining the retinal binding sit e are usually identical with the exception of Met-118 (numbering is ac cording to the bacteriorhodopsin sequence), which might explain the ty pical blue color shift of SR-II to approximate to 490 nm, In archaeal retinal proteins, the function can be deduced from amino acids in posi tions 85 and 96, Proton pumps are characterized by Asp-85 and Asp-96; chloride pumps by Thr-85 and Ala-96; and sensors by Asp-85 and Tyr-96 or Phe-96.