Effect of bicarbonate on the S₂ multiline EPR signal of the oxygen-evolving complex in photosystem II membrane fragments

Removal of bicarbonate from spinach photosystem II BBY particles by means of washing in a CO₂-free medium results in the loss of their capability to accumulate the S₂ multiline EPR signal upon continuous illumination at 190 K. Addition of 1 mM NaHCO₃ before illumination leads to a 50-60% restoration of the multiline signal. Similarly, in BBY particles depleted of Mn by treatment with 1 mM 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 (~ 30%) of the S₂ multiline EPR signal of the Mn cluster, while in the absence of NaHCO₃ no restoration is observed. The results provide further evidence that bicarbonate is essential for maintaining the Mn-containing oxygen-evolving complex of PS II in a functionally active form.