Yu J, Smart L B, Jung Y S, Golbeck J, McIntosh L
DOE Plant Research Laboratory, Michigan State University, East Lansing 48824, USA.
Plant Mol Biol. 1995 Oct;29(2):331-42. doi: 10.1007/BF00043656.
In photosystem I (PSI) of oxygenic photosynthetic organisms the psaC polypeptide, encoded by the psaC gene, provides the ligands for two [4Fe-4S] clusters, FA and FB. Unlike other cyanobacteria, two different psaC genes have been reported in the cyanobacterium Synechocystis 6803, one (copy 1) with a deduced amino acid sequence identical to that of tobacco and another (copy 2) with a deduced amino acid sequence similar to those reported for other cyanobacteria. Insertion of a gene encoding kanamycin resistance into copy 2 resulted in a photosynthesis-deficient strain, CDK25, lacking the PsaC, PsaD and PsaE polypeptides in isolated thylakoid membranes, while the PsaA/PsaB and PsaF subunits were found. Growth of the mutant cells was indistinguishable from that of wild-type cells under light-activated heterotrophic growth (LAHG). A reversible P700+ signal was detected by EPR spectroscopy in the isolated thylakoids during illumination at low temperature. Under these conditions, the EPR signals attributed to FA and FB were absent in the mutant strain, but a reversible Fx signal was present with broad resonances at g = 2.079, 1.903, and 1.784. Addition of PsaC and PsaD proteins to the thylakoids gave rise to resonances at g = 2.046, 1.936, 1.922, and 1.880; these values are characteristic of an interaction-type spectrum of FA- and FB-. In room-temperature optical spectroscopic analysis, addition of PsaC and PsaD to the thylakoids also restored a 30 ms kinetic transient which is characteristic of the P700+ [FA/FB]- backreaction. Expression of copy 1 was not detected in cells grown under LAHG and under mixotrophic conditions. These results demonstrate that copy 2 encodes the PsaC polypeptide in PSI in Synechocystis 6803, while copy 1 is not involved in PSI; that the PsaC polypeptide is necessary for stable assembly of PsaD and PsaE into PSI complex in vivo; and that PsaC, PsaD and PsaE are not needed for assembly of PsaA-PsaB dimer and electron transport from P700 to Fx.
在产氧光合生物的光系统I（PSI）中，由psaC基因编码的psaC多肽为两个[4Fe-4S]簇FA和FB提供配体。与其他蓝细菌不同，已报道集胞藻6803中有两个不同的psaC基因，一个（拷贝1）推导的氨基酸序列与烟草的相同，另一个（拷贝2）推导的氨基酸序列与其他蓝细菌报道的相似。将编码卡那霉素抗性的基因插入拷贝2中产生了一个光合缺陷型菌株CDK25，在分离的类囊体膜中缺乏PsaC、PsaD和PsaE多肽，而发现了PsaA/PsaB和PsaF亚基。在光激活异养生长（LAHG）条件下，突变细胞的生长与野生型细胞没有区别。在低温光照期间，通过电子顺磁共振（EPR）光谱在分离的类囊体中检测到可逆的P700+信号。在这些条件下，突变菌株中不存在归因于FA和FB的EPR信号，但存在一个可逆的Fx信号，在g = 2.079、1.903和1.784处有宽共振峰。向类囊体中添加PsaC和PsaD蛋白会在g = 2.046、1.936、1.922和1.880处产生共振峰；这些值是FA-和FB-相互作用型光谱的特征。在室温光学光谱分析中，向类囊体中添加PsaC和PsaD也恢复了一个30毫秒的动力学瞬变，这是P700+[FA/FB]-反向反应的特征。在LAHG和混合营养条件下生长的细胞中未检测到拷贝1的表达。这些结果表明，拷贝2在集胞藻6803的PSI中编码PsaC多肽，而拷贝1不参与PSI；PsaC多肽对于PsaD和PsaE在体内稳定组装到PSI复合物中是必需的；并且PsaC、PsaD和PsaE对于PsaA-PsaB二聚体的组装以及从P700到Fx的电子传递不是必需的。
