On the functional significance of the polypeptide PsbY for photosynthetic water oxidation in the cyanobacterium Synechocystis sp. strain PCC 6803

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Abstract
Recent investigations have revealed that the cyanobacterial photosystem II complex contains more than 26 polypeptides. The functions of most of the low-molecular-mass polypeptides, including PsbY, have remained elusive. Here we present a comparative characterization of the wild-type Synechocystis sp. strain PCC 6803 and a PsbY-free mutant derived from it. The results show that growth of the PsbY-free mutant was comparable to that of the wild-type when cells were cultivated in complete BG11 medium or under initial manganese or chloride limitation, and when illuminated at 20 or 200 μE m−2 s−1. However, while growth rates of both the wild-type and the PsbY-free mutant were reduced when cells were cultivated in BG11 medium in the absence of calcium, the reduction was significantly greater in the case of the PsbY-free mutant. This differential effect on growth of the mutant relative to the wild-type in CaCl2 deficient medium was detected when the cells were illuminated with high-intensity light (200 μE m−2 s−1) but not when light levels were lower (20 μE m−2 s−1). The differential effect on growth was associated with lower O2 evolving activity in the mutant compared to wild-type cells. The mutant was also found to be more sensitive to photoinhibition, and showed an altered pattern of fluorescence emission at 77 K. In addition, mass spectrometric analysis revealed that PsbY-free cells cultivated in CaCl2 sufficient medium (in which no growth reduction was observed) had a significantly higher O2 evolution from hydrogen peroxide and a lower O2 evolution from water under flash light illumination than wild-type cells. These results imply that photosystem II is slightly impaired in the PsbY-free mutant, and that the mutant is less capable of coping with low levels of Ca2+ than the wild-type.

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