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The function of the extrinsic 23 kDa polypeptide (OEC23) in Photosystem II (PS
II) is to retain Ca(2+) and Cl(-) during the S-state turnover of the Mn cluster
in photosynthetic oxygen evolution. Recombinant OEC23s from several plant
species were produced in Escherichia coli to characterize the molecular
mechanism of the OEC23 function then used in reconstitution experiments. One
tobacco isoform, OEC23 (2AF), had much less oxygen-evolving activity than the
spinach and cucumber OEC23s when PS II activities were reconstituted in
salt-washed spinach PS II particles. The fact that the OEC23s had similar
binding affinities for PS II particles suggests different ion-retention
capacities for the individual OEC23s: The chimeric OEC23s produced between
spinach and 2AF and those produced between cucumber and 2AF show that 58
N-terminal amino acid residues are important for PS II activity. Further
dissection of the sequence and site-directed mutagenesis indicated the
importance of 20 N-terminal amino acid residues for activity, in particular the
asparagine at the 15th position. In spinach the N15D mutation decreased PS II
activity, whereas in 2AF the D15N mutation increased it. This shows the
importance of the N-terminal sequence of OEC23 in ion retention during the
water-splitting process.
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