Abstract
Light represents one of the most important energy sources for life. Previous studies had identified photoreceptors from halophilic archaea that mediate four different physiological functions when responding to light. These photoreceptors belong to the rhodopsin family and can be classified as ion transporter or sensory receptor for phototaxis responses. Bacteriorhodopsin (BR) is the first well studied archaeal rhodopsin, which was identified as a light-driven proton pump responsible for energy generation under high intensity light illumination or under low oxygen condition. Haloarcula marismortui genome sequencing was completed in 2004 and a total of six rhodopsin genes were predicted, the most numerous in a single archaeon. Two genes－bop and xop1, were identified and determined to encode proteins called HmBRI and HmBRII, respectively, which formed a unique two-proton pump system. The goal of this thesis was set to compare protein features and functions between HmBRI and II to further understand the possible explanation why there are two BRs in the same cell. First, the HmRBI and II genes were expressed in E. coli and the optimum conditions were obtained and a new heat treatment procedure was introduced. Secondly, several assays for protein function probing were performed, including UV-vis absorption spectrum scanning, duration of photocycle and direct proton pump measurements, both HmBRI and HmBRII were proton pump and response to the same wavelength were thus concluded. Thirdly, protein sequence analysis, absorption spectra, photocycle measurements, proton pump activity and protein structure prediction analysis were carried out to extend our understanding of these two proteins. Finally, protein crystallization was performed to study the protein structures and needle-shaped crystals were successfully obtained. Even no clear conclusion was obtained; this study established the basic characteristics of those two BR proteins for any further study or development of industry applications.
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