A truncated haemoglobin implicated in oxygen metabolism by the microaerophilic food-borne pathogen Campylobacter jejuni

Abstract

Of the three groups of haemoglobins identified in micro-organisms (single-domain globins, flavohaemoglobins and truncated globins), the last group is the least well understood. The function of the truncated haemoglobin (Ctb) encoded by Cj0465c in the microaerophilic food-borne bacterial pathogenCampylobacter jejuniwas investigated by constructing actbmutant and characterizing its phenotype. The effects of thectbmutation on the kinetics of terminal oxidase function inC. jejuniwere investigated using oxyleghaemoglobin and oxymyoglobin as sensitive reporters of O2consumption. TheVmaxofctbmutant cells for O2, calculated using either globin, was greater than that of wild-type cells at extracellular O2concentrations up to ∼1 μM, suggesting a role for Ctb in moderating O2supply for reduction by high-affinity terminal oxidases. However, cells mutated inctbwere disadvantaged when grown under conditions of high aeration, as revealed by measurements of growth yields and rates in batch culture. Furthermore, the rate at whichctbmutant cells consumed O2in an O2electrode (10–200 μM O2) was approximately half the rate displayed by wild-type cells, reflecting a role for Ctb in respiration at physiologically relevant external O2concentrations. However, a lack of sensitivity of the mutant to paraquat or H2O2indicated that increased oxidative stress under such conditions was not the cause of these phenotypes. O2affinities of cells (Kmvalues of approximately 40 nM and 1 μM) were unaffected by mutation of either Ctb or the full-lengthC. jejuniglobin, Cgb. Although the gene encoding Ctb was found to be upregulated byS-nitrosoglutathione (GSNO) and the NO-donating compoundS-nitroso-N-acetylpenicillamine (SNAP), actbmutant did not display sensitivity to a number of nitrosative stress-generating compounds. The authors conclude that Ctb is involved in moderating O2flux withinC. jejuni.