PSII-I protein plays an essential role in dimerization of photosystem II

Abstract

PSII-I is a small membrane-spanning protein, which is associated with the PSII reaction center complex. It is universally found in cyanobacteria as well as green plants, although its function is still not known. In this communication, We attempted to elucidate the role of the PSII-I protein in the thermophilic cyanobacterium, Thermosynechococcus (formerly Synechococcus) elongatus strain BP-1 by targeted disruption and biochemical analysis. The psbI gene encoding PSII-I was cloned by PCR and used for gene disruption. The psbI-disruptant could grow autotrophically like wild type. Oxygen-evolving activity of PSII particles was approximately 20% lower in the mutant than in wild type. MonoQ column chromatography of the crude PSII particles revealed that the PSII dimer is missing in the psbI-disruptant. The molecular size of the monomer PSII from the mutant was confirmed by gel filtration chromatography. These suggest that PSII-I is specifically involved in dimerization of PSII complex or stabilization of the PSII dimer. The crude PSII from the psbI-disruptant exhibited lower efficiency of light-harvesting than that of wild type. These findings imply that dimerization supports structural integrity of the functional PSII or in supramolecular interaction between PSII and antennae complexes.