Fig. 2. Genetic systems producing heterodimeric cytochromes bc1.
A. One-plasmid system is genetically less stable possibly due to high frequency intra-molecular rearrangements between petB genes (encoding cytochrome b) duplications carried by the plasmid, but produces only heterodimeric cytochromes bc1 [29]. petB-B encodes cytochrome b-b, formed by linking two cytochrome b copies by a dodecameric peptide. petA and petC refer to the structural genes of Fe/S protein and cytochrome c1, respectively. S indicates the Strep-tag epitope added to the carboxyl end of the second copy of cytochrome b. B. Two-plasmids system is genetically more stable, as only one copy of petABC (structural genes encoding cytochrome bc1) is carried by each plasmid. Various subunits of cytochrome bc1 are produced and re-assorted independently, leading the cells harboring this system to produce a combination of homodimeric (M1/M1 and M2/M2) and heterodimeric (M1/M2) cytochrome bc1 that are differentially tagged by the Flag (F) and Strep (S) epitopes, [32], as shown. Apparently no high frequency inter-molecular rearrangements are observed in this system, but the heterodimeric cytochromes bc1 thus produced need to be separated from homodimeric variants by affinity chromatography via appropriate epitope tags.