Abstract
PSII-H is present in photosystem II (PSII) core complex in cyanobacteria and plants. The role of PSII-H was studied by directed mutagenesis and biochemical analysis in the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1. The photoautotrophic growth of the psbH-disruptant was much slower than that of wild type. Cells and thylakoids from the mutant showed about half the oxygen-evolving activity of wild type. While the PSII complex of wild type was mainly collected as dimer by anion exchange chromatography, the PSII complex of the psbH-disruptant was not recovered as dimer but as two monomeric forms. One was an inactive PSII complex lacking 33 kDa, cyt c550, and 12 kDa extrinsic proteins. The other was a PSII complex retains the oxygen-evolving activity and the extrinsic proteins. Moreover, PSII-X was absent in the PSII complexes of the psbH-disruptant. These results imply that PSII-H stabilizes the dimeric PSII complex associated with PSII-X and extrinsic proteins.
