PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Absence of the psbH gene product destabilizes photosystem II complex in Thermosynechococcus elongatus BP-1. MASAKO IWAI-OGURO^*,1, MITSUNORI KATAYAMA², YASUNORI INOUE¹, MASAHIKO IKEUCHI², ¹ Department of Applied Biological Science, Faculty of Science and Technology, Noda, Chiba, Japan² Department of Life Sciences (Biology), Meguro, Tokyo, Japan

ABSTRACT- PSII-H is a small hydrophobic protein, which is universally present in the PSII core complex among cyanobacteria and plants. The role of PSII-H was studied by directed mutagenesis and biochemical analysis in the thermophilic cyanobacterium T. elongatus BP-1. The psbH-disruptant could grow photoautotrophically, but the rate of growth was much slower than that of wild type. Oxygen-evolving PSII complexes were successfully isolated from the mutant and wild type. Protein compositions of the active PSII complexes from the mutant were practically the same as wild type except the absence of PSII-H and PSII-X. Moreover, during chromatographic separation the mutant yielded relatively large amount of the inactive PSII complex, which lacked extrinsic proteins of 33-kDa, 12-kDa, and cytochrome c550. There were significant differences in oxygen evolution activities of cells, thylakoids, and PSII complexes between the psbH-disruptant and wild type. At high concentration of 2,6-DCBQ, the mutant showed much lower activity than wild type. Gel filtration chromatography of the PSII complexes showed that both inactive and active PSII complexes from the mutant were mostly isolated as a monomeric form, while the active PSII complex from wild type was in a dimeric form. These results indicate that PSII-H protein is essential for association of PSII-X with the PSII complex, as well as for stabilization of oxygen-evolving dimeric form of the PSII complex.

KEY WORDS: psbH, Thermosynechococcus elongatus BP-1, psbX, photosystem II