在嗜鹽古細菌 Haloarcula marismortui 發現一種新型態的感受型光感蛋白質

Abstract

Photoreceptors play important roles in responding to environmental stimuli like light and they are essential for surviving in microbes. Ion pumps and photosensory are two main types of microbial rhodopsin among known microbial photoreceptors. Proton pump, like bacteriorhodopsin 1 in Halobacteria salinarum, pumps out proton to create a proton gradient for eventually energy generation, while photosensory rhodopsins, on the other hand, mediate repellent or attract signal in response to different wavelength of light. Sensory rhodopsin I (SR I) mediates both attract/repellent responses together with its cognate partner protein, HtrI, while sensory rhodopsin II (SRII) is solely responsible for repellent response when working together with its cognate transducer, HtrII. The genome project for Haloarcula marismortui predicted the existence of six photoreceptors, a most abundance and variety among any single archaeon. Overexpression and absorbance measurements of those six photoreceptors concluded them as three ion pumps, two sensory rhodopsins and one unknown type rhodopsin. Here, we propose that this function-unknown protein as a new type of modulatory photoreceptor based on the results that this SRM can switch between ground state and M-intermediate state when exposed to different light wavelength. Also, we cloned and identified the SRM cognate partner transducer, HtrM, which has two trans-membrane region followed by a very short cytoplasmic region containing a HAMP domain in the C-terminus. Finally, we propose SRM modulates photosensory signal for H. marismortui by changing the ratio of Ground/M-intermediate state in response to different wavelength of light.