The chloroplast cytochrome b 6 f complex can exist in monomeric and dimeric states

The cytochrome b 6 f complex isolated from spinach chloroplast membranes can be resolved into two forms, a monomeric and a dimeric form, by centrifugation on sucrose gradients. The conversion of the dimeric form of the complex into the monomeric form could be prevented by cross-linking with the homobifunctional reagent, dithiobis(succinimidylpropionate) but not by cross-linking with disuccinimidyltartrate or glutaraldehyde. SDS-PAGE analyses of the monomeric and dimeric forms of the cytochrome complex showed the presence of specific cross-linked products in each respective form of the complex. For example, the monomeric form contained a cross-linked product of cytochrome f, cytochrome b 6 f and subunit IV while the dimeric form contained a cross-linked dimer of cytochrome b 6 f. The presence of the former in the isolated cytochrome b 6 f complex prepared by the method of Hurt and Hauska (Eur J Biochem 117: 591-599, 1981) indicates the presence of the monomer in his preparation.