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. 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 (By similarity). A longer mRNA that is not produced by splicing has been shown to be transcribed in barley and maize. It is not known if this mRNA is translated. Belongs to the cytochrome b family. PetD subfamily.