Importance of the N-terminal sequence of the extrinsic 23 kDa polypeptide in Photosystem II in ion retention in oxygen evolution.

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.