A robust genetic system for producing heterodimeric native and mutant cytochrome bc(1).

The ubihydroquinone:cytochrome c oxidoreductase, or cytochrome bc1, is central to the production of ATP by oxidative phosphorylation and photophosphorylation in many organisms. Its three-dimensional structure depicts it as a homodimer with each monomer composed of the Fe-S protein, cytochrome b, and cytochrome c1 subunits. Recent genetic approaches successfully produced heterodimeric variants of this enzyme, providing insights into its mechanism of function. However, these experimental setups are inherently prone to genetic rearrangements as they carry repeated copies of cytochrome bc1 structural genes. Duplications present on a single replicon (one-plasmid system) or a double replicon (two-plasmid system) could yield heterogeneous populations via homologous recombination or other genetic events at different frequencies, especially under selective growth conditions. In this work, we assessed the origins and frequencies of genetic variations encountered in these systems and describe an improved variant of the two-plasmid system. We found that use of a recombination-deficient background (recA) minimizes spontaneous formation of co-integrant plasmids and renders the homologous recombination within the cytochrome b gene copies inconsequential. On the basis of the data, we conclude that both the newly improved RecA-deficient and the previously used RecA-proficient two-plasmid systems reliably produce native and mutant heterodimeric cytochrome bc1 variants. The two-plasmid system developed here might contribute to the study of "mitochondrial heteroplasmy"-like heterogeneous states in model bacteria (e.g., Rhodobacter species) suitable for bioenergetics studies. In the following paper (DOI 10.1021/bi400561e), we describe the use of the two-plasmid system to produce and characterize, in membranes and in purified states, an active heterodimeric cytochrome bc1 variant with unusual intermonomer electron transfer properties.