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.