感覺型視紫質 SRM 和其傳導元 HtrM 對嗜鹽古生菌光趨性之影響

Abstract

古生菌的感覺型視紫質 (Sensory Rhodopsin, SR) 可藉由感受不同可見光波長之光源，並藉由其對應之傳導元 (Transducer, Htr) 傳遞訊息，以調控嗜鹽古生菌之趨、避光反應。目前發現之SRs共有三種型態 : SRI, SRII及SRM；其感受之可見光波長分別分佈於紅藍綠光波段。基因體的序列中，嗜鹽古生菌Natronomonas pharaonis 僅具有調控吸收 ~498 nm 而驅動避光的NpSRII、 Halobacterium salinarum 擁有SRII及吸收~590 nm 來調控趨光的SRI 。本實驗室先前發表基因體的 Haloarcula marismortui，為一三色感光系統的嗜鹽古生菌，除了SRI, SRII以外，尚有一個功能未定的 SRM。SRI 及 SRII 之傳導元藉由類同細菌中化學趨性的 two-component system ，將光訊號轉化為化學訊息傳遞至鞭毛，進而調控菌體之泳動 ; 然而， SRM的傳導元 HtrM 結構上缺乏了許多構件，因此我們推測 SRM-HtrM應由其他機制調控嗜鹽古生菌之光趨性。本篇研究，先建立 SRM 及SRM-HtrM 在二色感光系統 H. salinarum菌株中的表現及確立其功能，再以兩種本篇研究發展之光趨性研究方法 (ELISA Reader 之量測及顯微鏡之觀察)，量化 Ｈ. salinarum 及其轉形株的光趨反應。初步結果顯示， SRM-HtrM減少了 H. salinarum在綠光及藍光下的避光反應; 並且，在 HtrM 缺乏的轉形株中，該現象並沒有被觀察到。因此，我們認為 HtrM 的存在，對於 SRM-HtrM 複合蛋白質在 H. salinarum中調控的趨光反應是重要的。未來，可以嘗試將 SRM-HtrM 複合蛋白質嵌入磷脂中，分析其與化學趨性相關蛋白質之間的交互作用，以解出其分子機制。

A group of photoreceptors, sensory rhodopsin (SR), regulates phototaxis in haloarchaea through absorbing diverse range of visible light and relaying the signals to the cell by their cognate transducer (Htr). To date, there are three types of SRs identified: SRI, SRII and SRM, and they response to red, blue and green light, respectively. Among the annotated archaeal genomes, Natromonas pharaonis solely own the NpSRII, which response blue light and mediates photorepellent; Halobacterium salinarum holds HsSRII and HsSRI, and HsSRI is known to absorb ~590 nm of light to mediate photoattractant. However, in Haloarcula marismortui, there exists three SRs, namely SRI, SRII and functionally unknown SRM. Previous studies showed that SRI and SRII transduce photo signal to flagellum through transducer and chemotaxis proteins, similar to two-component system of chemotaxis in bacteria. However, it is speculated that the SRM-HtrM complex regulates the phototaxis responses through new pathway as SRM-HtrM lacks many structural components seen in other transducers. In this study, we transplanted SRM and SRM-HtrM into H. salinarum cells and compare the phototaxis responses of H. salinarum and its transformants under different wavelengths of light through two new measurements developed in this study. It is found that SRM-HtrM decreased the photorepellent response of H. salinarum in green and blue light; but not in SRM transformant. The importance of HtrM in SRM-HtrM signaling was concluded. In future, the molecular mechanism of SRM-HtrM can be examined through measurements of the interaction between lipid-reconstituted SRM-HtrM with related chemotaxis proteins.