Characterization of the gene family encoding alternative oxidase from Candida albicans

Candida albicans possesses a cyanide-resistant respiratory pathway mediated by alternative oxidase (AOX), which seems to be encoded by a gene family w ith two members. Cloning and expression of AOX1a, one of the genes encoding alternative oxidase from C. albicans, has previously been reported [Huh an d Kang (1999) J. Bacteriol. 181, 4098-4102]. In the present study we report the isolation of another gene coding for alternative oxidase, designated A OX1b. AOX1b contains a continuous open reading frame that encodes a polypep tide consisting of 365 amino acids. Interestingly, AOX1a and AOX1b were fou nd to be located in tandem on one of the chromosomes of C. albicans. The pr esence of cyanide in the culture medium remarkably retarded the growth of t he aox1a/aox1a mutants. The growth of the aox1b/aox1b mutants and the aox1a /aox1a aox1b/aox1b double mutants was almost completely inhibited in the sa me medium. beta -Galactosidase reporter assays indicated that, whereas AOX1 a was expressed constitutively, the expression of AOX1b was dependent on gr owth phase and was induced by treatment with cyanide, antimycin A, H2O2, me nadione and paraquat. Growth of the cells in media with non-fermentable car bon sources also enhanced the expression of AOX1b. CaSLN1, which encodes a histidine kinase, seems to be involved in the regulation of AOX expression in C. albicans on the basis of the observation that the activity of cyanide -resistant respiration and the expression level of AOX in the casln1/casln1 mutants were found to be significantly low under normal conditions and sli ghtly increased in the presence of respiratory inhibitors compared with the wild-type strain. Like AOX1a, AOX1b could also be functionally expressed i n AOX-deficient Saccharomyces cerevisiae and confer cyanide-resistant respi ration on the organism.