Function and association of CyanoP in photosystem II of Synechocystis sp PCC 6803

The CyanoP protein is a cyanobacterial homolog of the PsbP protein, which is an extrinsic subunit of photosystem II (PSII) in green plant species. The molecular function of CyanoP has been investigated in mutant strains of Synechocystis but inconsistent results have been reported by different laboratories. In this study, we generated and characterized a Synechocystis mutant in which entire region of the CyanoP gene was eliminated. After repeated subculture in CaCl2-depleted medium, growth retardation was clearly observed for a CyanoP knockout mutant of Synechocystis sp. PCC 6803 (a dagger P). The PSII-mediated oxygen-evolving activity of the a dagger P cells was more susceptible to depletion of CaCl2 than that of wild-type cells. The 77 K fluorescence emission spectra indicated that energy coupling between phycobilisome and PSII was perturbed in both wild-type and a dagger P cells under CaCl2-depleted conditions, and was more evident for the a dagger P mutant. To examine the association of CyanoP with PSII complexes, we tested several detergents for solubilization of thylakoid membranes and showed that CyanoP was partly included in fractions containing large protein complexes in gel-filtration analysis. These results indicate that CyanoP constitutively stabilizes PSII functionality in vivo.