Electronic connection between Q(o) and Q(i) quinone catalytic sites of dimeric cytochrome bc(1), is a central feature of the energy-conserving Q cycle. While both the intra- and inter-monomer electron transfers were shown to connect the sites in the enzyme, mechanistic and physiological significance of the latter remains unclear. Here, using a series of mutated hybrid cytochrome bc(1)-like complexes, we show that inter-monomer electron transfer robustly sustains the function of the enzyme in vivo, even when the two subunits in a dimer come from different species. This indicates that minimal requirement for bioenergetic efficiency is to provide a chain of cofactors for uncompromised electron flux between the catalytic sites, while the details of protein scaffold are secondary. (C) 2014 The Authors. Published by Elsevier Inc.