Polypeptides of the cytochrome b-f complex

The Cyt b-f complex contains the redox components Cyt /, Cyt 6-563 and the Rieske Fe-S protein, which in spinach have been identified as polypeptides of 33, 23 and 20 kDa respectively [95,98]. The spinach complex contains in addition a polypeptide of 17 kDa with no known redox function. The reported sizes of these polypeptides estimated by SDS-gel electrophoresis vary somewhat between different laboratories, presumably beca.use of slightly different electrophoretic procedures. The estimated size of the Cyt / polypeptide varies between different plants, even when analysed in the same electrophoresis system, although the gene sequences predict polypeptides of very similar relative molecular mass. 

Additional polypeptides ascribed to the Cyt b-f complex are a bound form of ferredoxin-NADP reductase (FNR) [99] and one or more smaller polypeptides [100]. An association of the complex with ferredoxin-NADP reductase may be expected in view of the reported role of FNR in cyclic electron flow from PS I to the Cyt complex [101]. FNR remains associated with the complex during the early stages of the purification of the complex but there is no evidence that it is an intrinsic component of the complex necessary for plastoquinol-plastocyanin oxido-reductase. The presence of small polypeptides in the complex requires further investigation. Polypeptides of about 5 kDa have been reported to be associated with the spinach complex [100]. 

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