EFFECT OF BICARBONATE ON THE S-2 MULTILINE EPR SIGNAL OF THE OXYGEN-EVOLVING COMPLEX IN PHOTOSYSTEM-II MEMBRANE-FRAGMENTS

Removal of bicarbonate from spinach photosystem II BBY particles by me ans of washing in a CO2-free medium results in the loss of their capab ility to accumulate the S-2 multiline EPR signal upon continuous illum ination at 190 K. Addition of 1 mM NaHCO3 before illumination leads to a 50-60% restoration of the multiline signal. Similarly, in BBY parti cles depleted of Mn by treatment with 1 M Tris-HCl (pH 8.0) and 0.5 M MgCl2, re-addition of MnCl2 in the presence of 1 mM NaHCO3 results in a partial restoration (similar to 30%) of the S-2 multiline EPR signal of the Mn cluster, while in the absence of NaHCO3 no restoration is o bserved. The results provide further evidence that bicarbonate is esse ntial for maintaining the Mn-containing oxygen-evolving complex of PS II in a functionally active form. (C) 1998 Federation of European Bioc hemical Societies.