Oxygen Reactivity of Both Respiratory Oxidases in Campylobacter jejuni : the cydAB Genes Encode a Cyanide-Resistant, Low-Affinity Oxidase That Is Not of the Cytochrome bd Type

Abstract

ABSTRACT The microaerophilic bacterium Campylobacter jejuni is a significant food-borne pathogen and is predicted to possess two terminal respiratory oxidases with unknown properties. Inspection of the genome reveals an operon ( cydAB ) apparently encoding a cytochrome bd -like oxidase homologous to oxidases in Escherichia coli and Azotobacter vinelandii . However, C. jejuni cells lacked all spectral signals characteristic of the high-spin hemes b and d of these oxidases. Mutation of the cydAB operon of C. jejuni did not have a significant effect on growth, but the mutation reduced formate respiration and the viability of cells cultured in 5% oxygen. Since cyanide resistance of respiration was diminished in the mutant, we propose that C. jejuni CydAB be renamed CioAB ( c yanide- i nsensitive o xidase), as in Pseudomonas aeruginosa . We measured the oxygen affinity of each oxidase, using a highly sensitive assay that exploits globin deoxygenation during respiration-catalyzed oxygen uptake. The CioAB-type oxidase exhibited a relatively low affinity for oxygen ( K m = 0.8 μM) and a V max of >20 nmol/mg/s. Expression of cioAB was elevated fivefold in cells grown at higher rates of oxygen provision. The alternative, ccoNOQP -encoded cyanide-sensitive oxidase, expected to encode a cytochrome cb′ -type enzyme, plays a major role in the microaerobic respiration of C. jejuni , since it appeared to be essential for viability and exhibited a much higher oxygen affinity, with a K m value of 40 nM and a V max of 6 to 9 nmol/mg/s. Low-temperature photodissociation spectrophotometry revealed that neither oxidase has ligand-binding activity typical of the heme-copper oxidase family. These data are consistent with cytochrome oxidation during photolysis at low temperatures.