藍綠藻 Synechocystis PCC 6803中光系統二細胞色素b559的結構與功能研究

Abstract

Part I: The functional role of Cyt b559 in PSII was investigated in H22Kα and Y18Sα Cyt b559 mutants of the cyanobacterium Synechocystis sp. PCC6803. H22Kα and Y18Sα Cyt b559 mutant carries one amino acid substitution on and near one of heme axial ligands of Cyt b559 in PSII, respectively. Both mutants grew photoautotrophically, assembled stable PSII, and exhibited the normal period-four oscillation in oxygen yield. However, both mutants showed several distinct chlorophyll a fluorescence properties and were more susceptible to photoinhibition than wild type. EPR results indicated the displacement of one of the two axial ligands to the heme of Cyt b559 in H22Kα mutant reaction centers, at least in isolated reaction centers. The maximum absorption of Cyt b559 in Y18Sα mutant PSII core complexes was shifted to 561 nm. Y18Sα and H22Kα mutant PSII core complexes contained predominately the low potential form of Cyt b559. The findings lend support to the concept that the redox properties of Cyt b559 are strongly influenced by the hydrophobicity and ligation environment of the heme. When the Cyt b559 mutations placed in a D1-D170A genetic background that prevents assembly of the manganese cluster, accumulation of PSII is almost completely abolished. Overall, our data support a functional role of Cyt b559 in protection of PSII under photoinhibition conditions in vivo. Part II: The physiological function of cytochrome (Cyt) b559 in photosystem II (PSII) was investigated using H22Kα and Y18Sα Cyt b559 mutants of the cyanobacterium Synechocystis sp. PCC6803. Both mutant cells grew photoautotrophically and assembled stable PSII-like wild-type cells under normal light conditions (light intensity about 30 μmol photons m-2 s-1). However, both mutant cells suffered from photoinhibition under high light conditions (light intensity about 200 μmol photons m-2 s-1). Western blot analysis showed that the FtsH-PSII super-complex was significantly increased but PSII sub-complexes (PSII/RC47 heterodimer, PSII monomer and RC47 sub-complexes) were largely decreased in both Cyt b559 mutant cells under high light conditions. Our results suggest that Cyt b559 is important for maintaining normal photosynthetic growth and PSII activity in Synechocystis sp. PCC6803 cells under light stress conditions. In addition, H22Kα/ΔftsH2 and Y18Sα/ΔftsH2 double mutant cells showed severe impairments in their photosynthetic growth and PSII activities. Thylakoid membranes of the H22Kα/ΔftsH2 double mutant showed significant reductions in D1, D2 and Cyt b559 α protein contents compared to those of H22Kα mutants. In contrast, thylakoid membranes of ΔftsH2 mutant showed significant increases in PSII protein contents compared to those of wild-types. Our results demonstrate that FtsH2 protease may plays an important role in repairing PSII and maintaining its function in Synechocystis Cyt b559 mutant cells.