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. Binds 2 heme groups. One heme group is bound covalently by a single cysteine link, the other one non-covalently. 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. Heme 1 (or BH or b566) is high-potential and absorbs at about 566 nm, and heme 2 (or BL or b562) is low-potential and absorbs at about 562 nm. Belongs to the cytochrome b family. PetB subfamily.