The complex consists of three subunits in most bacteria, and nine in mitochondria: both bacterial and mitochondrial complexes contain cytochrome b and cytochrome c1 subunits, and an iron–sulfur 'Rieske' subunit, which contains a high potential 2Fe-2S cluster. The mitochondrial form also includes six other subunits that do not possess redox centres. Plastoquinone-plastocyanin reductase (b6f complex), present in cyanobacteria and the chloroplasts of plants, catalyses the oxidoreduction of plastoquinol and cytochrome f. This complex, which is functionally similar to ubiquinol-cytochrome c reductase, comprises cytochrome b6, cytochrome f and Rieske subunits.