Functional Characterization of Monomeric Photosystem II Core Preparations from Thermosynechococcus elongatus with or without the Psb27 Protein

Abstract

Two monomeric fractions of photosystem II (PS II) core pacticles from the thermophilic cyanobacterium Thermosynechococcus elongatus have been investigated using flash-induced variable fluorescence kinetics and EPR spectroscopy. One fraction was highly active in oxygen evolution and contained the extrinsic protein subunits PsbO, PsbU, and PsbV. The other monomeric fraction lacked oxygen evolving activity as well as the three extrinsic subunits, but the luminally located, extrinsic Psb27 lipoprotein was present. In the monomeric fraction with bound Psb27, flash-induced variable fluorescence showed an absence of oxidizable Mn on the donor side of PS II and impaired forward electron transfer from the primary quinone acceptor, Q_A. These results were confirmed with EPR spectroscopy by the absence of the “split S₁” interaction signal from Y_Z^• and the CaMn₄ cluster and by the absence of the S₂-state multiline signal. A different protein composition on the donor side of PS II monomers with Psb27 was also supported by the lack of an EPR signal from cytochrome c₅₅₀ (in the PsbV subunit). In addition, we did not observe any oxidation of cytochrome b₅₅₉ at low temperature in this fraction. The presence of Psb27 and the absence of the CaMn₄ cluster did not affect the protein matrix around Y_D or the acceptor side quinones as can be judged from the appearance of the corresponding EPR signals. The diminished electron transport capabilities on both the donor and the acceptor side of PS II when Psb27 is present give further indications that this PS II complex is involved in the earlier steps of the PS II repair cycle.