Häder Donat-P, Iseki Mineo
Department of Biolology, Friedrich-Alexander Universität, Erlangen-Nürnberg, Neue Str. 9, 91096, Möhrendorf, Germany.
Faculty of Pharmaceutical Sciences, Toho University, Miyama 2-2-1, Funabashi, Chiba, 274-8510, Japan.
Adv Exp Med Biol. 2017;979:207-235. doi: 10.1007/978-3-319-54910-1_11.
Motile microorganisms such as the green Euglena gracilis use a number of external stimuli to orient in their environment. They respond to light with photophobic responses, photokinesis and phototaxis, all of which can result in accumulations of the organisms in suitable habitats. The light responses operate synergistically with gravitaxis, aerotaxis and other responses. Originally the microscopically obvious stigma was thought to be the photoreceptor, but later the paraxonemal body (PAB, paraflagellar body) has been identified as the light responsive organelle, located in the trailing flagellum inside the reservoir. The stigma can aid in light direction perception by shading the PAB periodically when the cell rotates helically in lateral light, but stigmaless mutants can also orient with respect to the light direction, and negative phototaxis does not need the presence of the stigma. The PAB is composed of dichroically oriented chromoproteins which is reflected in a pronounced polarotaxis in polarized light. There was a long debate about the potential photoreceptor molecule in Euglena, including carotenoids, flavins and rhodopsins. This discussion was terminated by the unambiguous proof that the photoreceptor is a 400 kDa photoactivated adenylyl cyclase (PAC) which consists of two α- and two β-subunits each. Each subunit possesses two BLUF (Blue Light receptor Using FAD) domains binding FAD, which harvest the light energy, and two adenylyl cyclases, which produce cAMP from ATP. The cAMP has been found to activate one of the five protein kinase s found in Euglena (PK.4). This enzyme in turn is thought to phosphorylate proteins inside the flagellum which result in a change in the flagellar beating pattern and thus a course correction of the cell. The involvements of PAC and protein kinase have been confirmed by RNA interference (RNAi). PAC is responsible for step-up photophobic responses as well as positive and negative phototaxis, but not for the step-down photophobic response, even though the action spectrum of this resembles those for the other two responses. Analysis of several colorless Euglena mutants and the closely related Euglena longa (formerly Astasia longa) confirms the results. Photokinesis shows a completely different action spectrum. Some other Euglena species, such as E. sanguinea and the gliding E. mutabilis, have been investigated, again showing totally different action spectra for phototaxis and photokinesis as well as step-up and step-down photophobic responses.
诸如纤细裸藻这种能够运动的微生物会利用多种外部刺激在其环境中定向移动。它们对光会产生避光反应、光动力学反应和趋光性反应，所有这些反应都可能导致生物体在适宜的栖息地聚集。光反应与趋地性、趋氧性及其他反应协同作用。最初，显微镜下明显可见的眼点被认为是光感受器，但后来副鞭毛体（PAB，旁鞭毛体）被确定为光反应细胞器，位于储液泡内的拖尾鞭毛中。当细胞在侧光中呈螺旋状旋转时，眼点可通过周期性遮挡副鞭毛体来辅助光方向感知，但无眼点突变体也能相对于光方向定向，且负趋光性并不需要眼点的存在。副鞭毛体由二向色性取向的色素蛋白组成，这在偏振光下表现为明显的偏振趋光性。关于裸藻中潜在的光感受器分子存在长期争论，涉及类胡萝卜素、黄素和视紫红质等。当明确证明光感受器是一种400 kDa的光活化腺苷酸环化酶（PAC）时，这场讨论才结束，该酶由两个α亚基和两个β亚基组成。每个亚基都有两个结合FAD的BLUF（利用FAD的蓝光受体）结构域，用于收集光能，还有两个腺苷酸环化酶，可将ATP转化为cAMP。已发现cAMP能激活裸藻中发现的五种蛋白激酶之一（PK.4）。反过来，这种酶被认为会使鞭毛内的蛋白质磷酸化，从而导致鞭毛摆动模式发生变化，进而对细胞的运动轨迹进行校正。RNA干扰（RNAi）已证实了PAC和蛋白激酶的作用。PAC负责增强的避光反应以及正向和负向趋光性，但不负责减弱的避光反应，尽管其作用光谱与其他两种反应的光谱相似。对几种无色裸藻突变体和与之密切相关的长裸藻（原长眼虫）的分析证实了这一结果。光动力学反应呈现出完全不同的作用光谱。人们还对其他一些裸藻物种进行了研究，如血红裸藻和滑行的易变裸藻，结果再次表明趋光性和光动力学反应以及增强和减弱的避光反应具有完全不同的作用光谱。
