A four-subunit cytochrome bc1 complex complements the respiratory chain of Thermus thermophilus

Several components of the respiratory chain of the eubacterium Thermus thermophilus have previously been characterized to various extent, while no conclusive evidence for a cytochrome bc1 complex has been obtained. Here, we show that four consecutive genes encoding cytochrome bc1 subunits are organized in an operon-like structure termed fbcCXFB. The four gene products are identified as genuine subunits of a cytochrome bc1 complex isolated from membranes of T. thermophilus. While both the cytochrome b and the FeS subunit show typical features of canonical subunits of this respiratory complex, a further membrane-integral component (FbcX) of so far unknown function copurifies as a subunit of this complex. The cytochrome c1 carries an extensive N-terminal hydrophilic domain, followed by a hydrophobic, presumably membrane-embedded helical region and a typical heme c binding domain. This latter sequence has been expressed in Escherichia coli, and in vitro shown to be a kinetically competent electron donor to cytochrome c552, mediating electron transfer to the ba3 oxidase. Identification of this cytochrome bc1 complex bridges the gap between the previously reported NADH oxidation activities and terminal oxidases, thus, defining all components of a minimal, mitochondrial-type electron transfer chain in this evolutionary ancient thermophile.