Naithani S, Hou J M, Chitnis P R
Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
Photosynth Res. 2000;63(3):225-36. doi: 10.1023/A:1006463932538.
PsaK and PsaM are small, integral membrane proteins, which are associated with the Photosystem I complexes of cyanobacteria. The complete genome sequence of Synechocystis sp. PCC 6803 has revealed the presence of two unlinked psaK genes: psaK1 (ssr0390) and psaK2 (sll0629). To investigate structural and functional roles of the PsaK1, PsaK2 and PsaM polypeptides in Synechocystis sp. PCC 6803, we generated targeted mutants that lack the functional psaK1, psaK2 or psaM genes. Inactivation of psaK1, psaK2 or psaM did not affect photoautotrophic growth, photosynthetic activity and accumulation of other subunits of the Photosystem I complex. The psaK1 (-), psaK2 (-) and psaK1 (-) psaK2 (-) mutants showed normal levels of Photosystem I trimers, whereas the lack of PsaM resulted in a 75% reduction in the recovery of trimers compared to the wild type. A 6.2 kDa polypeptide was observed in the Photosystem I preparations from the wild type, but not from the psaK2 (-) strain, suggesting the presence of PsaK2 in the Photosystem I complexes. Using reverse-transcription and polymerase chain reaction, we confirmed the expression of the psaK2 gene in Synechocystis sp. PCC 6803. To conclude, both psaK1 and psaK2 are expressed in Synechocystis sp. PCC 6803 and the absence of both proteins results in only a small reduction in Photosystem I electron transport. The PsaM subunit is required for the formation of stable Photosystem I trimers.
PsaK和PsaM是小型整合膜蛋白，与蓝细菌的光系统I复合物相关。聚球藻属PCC 6803的全基因组序列显示存在两个不连锁的psaK基因：psaK1（ssr0390）和psaK2（sll0629）。为了研究聚球藻属PCC 6803中PsaK1、PsaK2和PsaM多肽的结构和功能作用，我们构建了缺失功能性psaK1、psaK2或psaM基因的靶向突变体。psaK1、psaK2或psaM的失活不影响光合自养生长、光合活性以及光系统I复合物其他亚基的积累。psaK1（-）、psaK2（-）和psaK1（-）psaK2（-）突变体显示光系统I三聚体水平正常，而与野生型相比，缺乏PsaM导致三聚体回收率降低75%。在野生型的光系统I制剂中观察到一种6.2 kDa的多肽，但在psaK2（-）菌株中未观察到，这表明光系统I复合物中存在PsaK2。通过逆转录和聚合酶链反应，我们证实了聚球藻属PCC 6803中psaK2基因的表达。总之，psaK1和psaK2在聚球藻属PCC 6803中均有表达，两种蛋白的缺失仅导致光系统I电子传递略有降低。PsaM亚基是形成稳定的光系统I三聚体所必需的。
