THE TERMINAL QUINOL OXIDASE OF THE HYPERTHERMOPHILIC ARCHAEON ACIDIANUS AMBIVALENS EXHIBITS A NOVEL SUBUNIT STRUCTURE AND GENE ORGANIZATION

A terminal quinol oxidase has been isolated from the plasma membrane o f the crenarchaeon Acidianus ambivalens (DSM 3772) (formerly Desulfuro lobus ambivalens), cloned, and sequenced, The detergent-solubilized co mplex oxidizes caldariella quinol at high rates and is completely inhi bited by cyanide and by quinolone analogs, potent inhibitors of quinol oxidases. It is composed of at least five different subunits of 64.9, 38, 20.4, 18.8, and 7.2 kDa; their genes are located in two different operons, doxB, the gene for subunit I! is located together with doxC and two additional small open reading frames (doxE and doxF) in an ope ron with a complex transcription pattern, Two other genes of the oxida se complex (doxD and doxA) are located in a different operon and are c otranscribed into a common 1.2-kb mRNA, Both operons exist in duplicat e on the genome of.-l, ambivalens. Only subunit 1 exhibits clear homol ogy to other members of the superfamily of respiratory heme-copper oxi dases; however, it reveals 14 transmembrane helices, In contrast, the composition of the accessory proteins is highly unusual; none is homol ogous to any known accessory protein of cytochrome oxidases, nor do ho mologs exist in the databases. DoxA is classified as a subunit II equi valent only by analogy of molecular size and hydrophobicity pattern to corresponding polypeptides of other oxidases. Multiple alignments and phylogenetic analysis of the heme-bearing subunit I (DoxB) locate thi s oxidase at the bottom of the phylogenetic tree, in the branch of hem e-copper oxidases recently suggested to be incapable of superstoichiom etric proton pumping, This finding is corroborated by lack of the esse ntial amino acid residues delineating the putative H+-pumping channel, It is therefore concluded that A. ambivalens copes with its strongly acidic environment simply by an extreme turnover of its terminal oxida se, generating a proton gradient only by chemical charge separation.