Effect of Cultivation Conditions on the Expression of the Exiguobacterium sibiricum Proteorhodopsin Gene

Recombinant proteorhodopsin ESR of the gram-positive bacterium Exiguobacterium sibiricum isolated from permafrost deposits in northeastern Siberia binds retinal and acts as a light-dependent proton pump, but not much is known about its expression under natural conditions. In this work, expression of the esr gene in E. sibiricum cultures grown under various conditions was studied by quantitative PCR. It has been discovered that cultivation on poor media at low temperatures contributes to a significant increase in the content of the corresponding mRNA. The data obtained are confirmed by the results of the analysis of the membrane fraction of cells using label-free quantitative chromatography-mass spectrometry. Also, at 10°C, increased content of phytoene desaturases, which are involved in the biosynthesis of carotenoids, is observed. However, we were unable to detect the presence of a functional retinal-containing protein in the cells, presumably due to the lack of an enzymatic retinal synthesis system in E. sibiricum. The possible functions of ESR in E. sibiricum cells are discussed in connection with the characteristics of the extreme habitat of the bacterium. The results of this study contribute to expanding the understanding of the molecular mechanisms of microbial adaptation to environmental conditions and the potential role of microbial rhodopsins in these processes.