The photosystem II (PSII) complex of photosynthetic oxygen evolving membranes comprises a number of small proteins whose functions remain unknown. Here we report that the low molecular weight protein encoded by the psbJ gene is an intrinsic component of the PSII complex. Fluorescence kinetics, oxygen flash yield, and thermoluminescence measurements indicate that inactivation of the psbJ gene in Synechocystis 6803 cells and tobacco chloroplasts lowers PSII-mediated oxygen evolution activity and increases the lifetime of the reduced primary acceptor Q A Ϫ (more than a 100-fold in the tobacco ⌬psbJ mutant). The decay of the oxidized S 2,3 states of the oxygen-evolving complex is considerably accelerated, and the oscillations of the Q B Ϫ /S 2,3 recombination with the number of exciting flashes are damped. Thus, PSII can be assembled in the absence of PsbJ. However, the forward electron flow from Q A Ϫ to plastoquinone and back electron flow to the oxidized Mn cluster of the donor side are deregulated in the absence of PsbJ, thereby affecting the efficiency of PSII electron flow following the charge separation process. The photosystem II complex (PSII)1 of the thylakoid membrane is involved in the photochemical process resulting in water oxidation, oxygen evolution, and reduction of plastoquinone. PSII comprises a core complex formed by a few proteins binding the ligands required for primary photochemistry and electron transfer and the chlorophyll-binding antennae as well as a number of low molecular weight proteins whose functions have not yet been identified (1, 2). The process of light-induced charge separation and reduction of a quinone acceptor, similar to that performed by PSII, is also carried out by photosynthetic bacteria that do not evolve oxygen and perform cyclic electron flow around the bacterial photochemical reaction center (RC) (3). While in both cases a small number of proteins binding the appropriate ligands are capable of light-driven charge separation and electron transfer, the linear electron flow performed by PSII using water as a source of electrons is more complex and may require regulatory steps that are not demanded by the cyclic electron flow of the bacterial RC. Thus it is plausible that proteins of PSII, besides those acting as energy-transferring antennae (4), may play some role in the regulation of the forward and backward electron flow within the PSII core complex. Recombination of the primary charge-separated pair due to back electron flow may lead to the generation of singlet oxygen considered to be the cause of PSII oxidative stress and turnover of its core proteins (5, 6). Indeed cytochrome b 559 , an essential component of the PSII core complex is considered to play such a regulatory role by diverting electrons from the reducing side of PSII and channeling them to the oxidized donor side of the complex (7). This protein is missing from the RC complex of purple anoxygenic photosynthetic bacteria. The psbE and psbF genes encoding the ␣ and ␤ subunits of cytochrome b 559 , respectiv...