Abstract:

Haloarchaea survive extreme environmental conditions with the assistance from several microbial rhodopsins, including but not limited to bacteriorhodopsin, halorhodopsin and sensory rhodopsins. Here we will discuss when facing variation in pH and illumination intensities, those microbial rhodopsins appeared to evolve and adjust accordingly for optimal functional exertion. First, our atomic structure unveiled a bacteriorhodopsin adopted extra hydrogen bondings on the periplasmic loop and became both functional and optically stable under the otherwise challenging low pH environment. Secondly, to overcome low proton concentration in the alkaline habitat, a halorhodopsin intensified its interior hydrogen bonding network around retinal binding pocket to maintain the proton-assisted chloride transportation. Lastly, a never observed three sensory rhodopsins composed a red-green-blue trichromatic “vision” system to extend their phototaxis wavelength to the blue side for ~20 nm when compared to most haloarchaea, and such extension comes from a sensory rhodopsin, SRM to absorb 503 nm and likely abolishes the photo-repellent responses of SRII; it consequently can increase the time length for energy harvesting via bacteriorhodopsin(s) for this particular haloarchaeon.