Microbial rhodopsins are the integral membrane proteins, which perform light-dependent transport of protons and other ions or sensor functions. They are found in archaea, bacteria and eukaryotic algae and share common heptahelical fold with a retinal chromophore. Potential rhodopsin-encoding genes were identified in the genomes of Gram-positive bacteria Exiguobacterium sibiricum, isolated from 3-million year old permafrost (ESR), the thermophilic Exiguobacterium AT1b from the Yellowstone National Park hot spring (ATER) and Exiguobacterium 7-3 , also isolated from permafrost soil (E7_3R). Previously, we have cloned the ESR gene and performed its structural and functional characterization. Amino acid sequence of E7_3R differs from ESR in seven positions, and ATER - in 39 positions. In order to compare the properties of retinal proteins belonging to closely related species from different environments, we have expressed ATER and E7_3R in E. coli cells and studied them by flash photolysis at different pH. We have shown that the photocycle of ATER and E7_3R differs from that of ESR by the shift of pKa of the M intermediate (the deprotonated Schiff base) formation to more alkaline pH and its reduced rate. The observed differences are discussed in relationship with the data about 3D structure of ESR.
retinal protein, proteorhodopsin, Exiguobacterium, Schiff base, proton acceptor, proton donor, photocycle
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