THE PHOTOSYNTHETIC OXYGEN-EVOLVING COMPLEX REQUIRES CHLORIDE FOR ITS REDOX STATE S-2-]S-3 AND S-3-]S-0 TRANSITIONS BUT NOT FOR S-0-]S-1 OR S-1-]S-2 TRANSITIONS

The Cl- requirement in the redox cycle of the oxygen-evolving complex (OEC) was determined by measurements of flash-induced UV absorbance ch anges in Cl--depleted and Cl--reconstituted photosystem II membranes, On the first flash after dark adaptation the spectrum and amplitude of those changes, known to reflect the oxidation of Mn-III to Mn-IV on t he S-1-->S-2 transition, were the same in the presence or absence of C l-. On the second and later flashes, however, absorbance changes in Cl --depleted samples revealed only electron transfer from tyrosine to qu inone which reversed slowly in the dark by charge recombination and di d not produce the S-3-state. A rapid method was developed to remove Cl - after producing the S-3-state by two flashes. The lifetime of the S- 3-state was found to be unaffected by Cl--depletion, in contrast to th e 20-fold stabilization of the S-2 lifetime by Cl- removal, and the Cl --depleted S-3-state did not proceed to S-0 on flash illumination, How ever, when the same Cl--depletion procedure was applied after producin g the S-0-state by three flashes, further advance to S-2 by two additi onal flashes was not impaired by the absence of Cl-. The requirement f or Cl- only on the S-2-->S-3 and S-3-->S-0 transitions can be rational ized by the hypothesis that Cl- is required for electron transfer betw een manganese ions within the oxygen-evolving complex.