Vanselow Christopher, Weber Andreas P M, Krause Kirsten, Fromme Petra
Department of Chemistry and Biochemistry, Arizona State University, Tempe 85287-1604, AZ, USA.
Biochim Biophys Acta. 2009 Jan;1787(1):46-59. doi: 10.1016/j.bbabio.2008.10.004. Epub 2008 Oct 22.
Currently, there are very little data available regarding the photosynthetic apparatus of red algae. We have analyzed the genes for Photosystem I in the recently sequenced genome of the red alga Galdieria sulphuraria. All subunits that are conserved between plants and cyanobacteria were unambiguously identified in the Galdieria genome: PsaA, PsaB, PsaC, PsaD, PsaE, PsaF, PsaI, PsaJ, PsaK and PsaL. From the plant specific subunits, PsaN and PsaO were identified but the sequence homology was much lower than for the subunits that are present in plants and cyanobacteria. The subunit PsaX, which is specific for thermophilic cyanobacteria, is not present in the Galdieria genome, whereas PsaM is a plastid-encoded protein as in other red algae. The sequences of the core subunits of PSI were further analyzed by mapping of the conserved areas in the crystal structures of cyanobacterial and plant PSI. The structural comparison shows that PSI from the red alga Galdieria may represent a common ancestral structure at the interface between cyanobacterial and plant PSI. Some subunits have a "zwitter" structure that contains structural elements that show similarities with either plant or cyanobacterial PSI. The structure of PsaL, which is responsible for the trimerization of PSI in cyanobacteria, lacks a short helix and the Ca(2+) binding site, which are essential for trimer formation indicating that the Galdieria PSI is a monomer. However the sequence homology to plant PsaL is low and lacks strong conservation of the interaction sites with PsaH. Furthermore, the sites for interaction of plant PSI with the LHCI complex are not well conserved between plants and Galdieria, which may indicate that Galdieria may contain a PSI that is evolutionarily much more ancient than PSI from green algae, plants and the current cyanobacteria.
目前，关于红藻光合机构的数据非常少。我们分析了红藻嗜盐栖热菌（Galdieria sulphuraria）最近测序基因组中的光系统I基因。在嗜盐栖热菌基因组中明确鉴定出了植物和蓝细菌之间保守的所有亚基：PsaA、PsaB、PsaC、PsaD、PsaE、PsaF、PsaI、PsaJ、PsaK和PsaL。从植物特有的亚基中，鉴定出了PsaN和PsaO，但序列同源性远低于植物和蓝细菌中存在的亚基。嗜盐栖热菌基因组中不存在嗜热蓝细菌特有的亚基PsaX，而PsaM如在其他红藻中一样是一种质体编码蛋白。通过绘制蓝细菌和植物光系统I晶体结构中的保守区域，进一步分析了光系统I核心亚基的序列。结构比较表明，红藻嗜盐栖热菌的光系统I可能代表了蓝细菌和植物光系统I之间界面处的一种共同祖先结构。一些亚基具有“两性离子”结构，包含与植物或蓝细菌光系统I显示出相似性的结构元件。负责蓝细菌中光系统I三聚化的PsaL结构缺乏短螺旋和Ca(2+)结合位点，而这两者对于三聚体形成至关重要，这表明嗜盐栖热菌光系统I是单体。然而，其与植物PsaL的序列同源性较低，并且缺乏与PsaH相互作用位点的强保守性。此外，植物光系统I与LHCI复合体相互作用的位点在植物和嗜盐栖热菌之间保守性不佳，这可能表明嗜盐栖热菌可能含有一种在进化上比绿藻、植物和当前蓝细菌的光系统I古老得多的光系统I。
