Component of the cytochrome b6-f complex, which mediates electron transfer between photosystem II (PSII) and photosystem I (PSI), cyclic electron flow around PSI, and state transitions. a plastoquinol + 2 H(+)(in) + 2 oxidized [plastocyanin] = a plastoquinone + 4 H(+)(out) + 2 reduced [plastocyanin] Binds 1 [2Fe-2S] cluster per subunit. The 4 large subunits of the cytochrome b6-f complex are cytochrome b6, subunit IV (17 kDa polypeptide, PetD), cytochrome f and the Rieske protein, while the 4 small subunits are PetG, PetL, PetM and PetN. The complex functions as a dimer. The transmembrane helix obliquely spans the membrane in one monomer, and its extrinsic C-terminal domain is part of the other monomer. The Rieske iron-sulfur protein is a high potential 2Fe-2S protein. Belongs to the Rieske iron-sulfur protein family.