Abstract <p>The interaction of cytochrome <i>c</i> with the manganese-stabilizing protein PsbO and the membranes of photosystem II irradiated with ultraviolet light has been studied. It was found that the PsbO protein irradiated with ultraviolet light is able to reduce cytochrome <i>c</i>; this process is inhibited by superoxide dismutase and stimulated by catalase, which indicates the involvement of reactive oxygen species. A similar reduction of cytochrome <i>c</i> was observed with the addition of solutions of tyrosine and cystine irradiated with ultraviolet light, which confirms the role of amino acid radicals in this process. It has been shown that ultraviolet irradiation causes dimerization of the PsbO protein. Incubation with cytochrome <i>c</i> (24 h) reduces the dimer level, restoring the concentration of the monomer. The membranes of photosystem II irradiated with ultraviolet light also reduced cytochrome <i>c</i> and the reduction activity depended on the presence of PsbO in photosystem II. It has been shown that oxidized cytochrome <i>c</i> binds to the PsbO protein and photosystem II more strongly than the reduced one. The data obtained suggest that ultraviolet irradiation generates reactive groups (tyrosine radicals and thiol compounds) in the PsbO protein (both in isolated form and as part of photosystem II) that reduce cytochrome <i>c</i>. In addition, irradiation of the PsbO protein in solution leads to the formation of intermediates involving reactive oxygen species that oxidize the reduced cytochrome <i>c</i>.</p>