Deregulation of electron flow within photosystem II in the absence of the PsbJ protein

The photosystem II (PSII) complex of photosynthetic oxygen evolving membran es comprises a number of small proteins whose functions remain unknown. Her e we report that the low molecular weight protein encoded by the psbJ gene is an intrinsic component of the PSII complex. Fluorescence kinetics, oxyge n flash yield, and thermoluminescence measurements indicate that inactivati on of the psbJ gene in Synechocystis 6803 cells and tobacco chloroplasts lo wers PSII-mediated oxygen evolution activity and increases the lifetime of the reduced primary acceptor Q(A)(-) (more than a 100-fold in the tobacco D elta psbJ mutant). The decay of the oxidized S-2,S-3 states of the oxygen-e volving complex is considerably accelerated, and the oscillations of the Q( B)(-)/S-2,S-3 recombination with the number of exciting flashes are damped. Thus, PSII can be assembled in the absence of PsbJ. However, the forward e lectron flow from Q(A)(-) to plastoquinone and back electron flow to the ox idized Mn cluster of the donor side are deregulated in the absence of PsbJ, thereby affecting the efficiency of PSII electron flow following the charg e separation process.