Characterization of a cytochrome a1 that functions as a ubiquinol oxidase in Acetobacter aceti

The terminal oxidase for ethanol oxidation in Acetobacter aceti was purified as a complex consisting of four subunits (subunits I, II, III, and IV) with molecular masses of 72, 34, 21, and 13 kDa, respectively. Spectrophotometric analysis and catalytic properties determined with the purified enzyme showed that it belonged to a family of cytochrome a1 (ba)-type ubiquinol oxidases. A polymerase chain reaction with two oligonucleotides designed for amino acid sequences that are conserved in subunit I of the aa3-type cytochrome c oxidases from various origins and of an Escherichia coli o (bo)-type ubiquinol oxidase was used for cloning the cytochrome a1 gene. A 0.5-kb fragment thus amplified was used as the probe to clone a 4.5-kb KpnI fragment that contained a putative open reading frame for the whole subunit I gene. The molecular weight and amino acid composition of the product of this open reading frame (cyaA) were the same as those of the purified protein from A. aceti. The amino acid sequence of CyaA was homologous to that of subunit I of the E. coli o-type ubiquinol oxidase. Nucleotide sequence analysis of the region neighboring the cyaA gene revealed that the genes (cyaB, cyaC, and cyaD) encoding the other three subunits (subunits II, III, and IV) were clustered upstream and downstream of the cyaA gene in the order cyaB, cyaA, cyaC, and cyaD and with the same transcription polarity, forming an operon. As expected from the enzymatic properties, CyaB, CyaC, and CyaD showed great similarity in amino acid sequence to the corresponding sununits of the E. coli o-type ubiquinol oxidase and as(3)-type cytochrome c oxidases.