SUMMARY
Alternative oxidase (AOX), a ubiquinol oxidase, introduces a branch point into the respiratory electron transport chain, bypassing complexes III and IV and resulting in cyanide-resistant respiration. Previously, AOX was thought to be limited to plants and some fungi and protists but recent work has demonstrated the presence of AOX in most kingdoms of life, including animals. In the present study we identified AOX in 28 animal species representing nine phyla. This expands the known taxonomic distribution of AOX in animals by 10 species and two phyla. Using bioinformatics we found AOX gene sequences in members of the animal phyla Porifera, Placozoa, Cnidaria, Mollusca, Annelida, Nematoda, Echinodermata, Hemichordata and Chordata. Using reverse-transcriptase polymerase chain reaction (RT-PCR) with degenerate primers designed to recognize conserved regions of animal AOX, we demonstrated that AOX genes are transcribed in several animals from different phyla. An analysis of full-length AOX sequences revealed an amino acid motif in the C-terminal region of the protein that is unique to animal AOXs. Animal AOX also lacks an N-terminal cysteine residue that is known to be important for AOX enzyme regulation in plants. We conclude that the presence of AOX is the ancestral state in animals and hypothesize that its absence in some lineages, including vertebrates, is due to gene loss events.
FOOTNOTES
Supplementary material available online at http://jeb.biologists.org/cgi/content/full/212/16/2627/DC1
We thank the following individuals for the provision of animal tissues: Dani Biaggio, Sheila Rush, Joanne Wolf, Ben Speers-Roesch, Pablo Jaramillo and Drs. Sally Leys, Rich Palmer, Maydianne Andrade, John Youson, Colin Montpetit, Doug Fudge, Jim Ballantyne, and Gord McDonald. We thank Drs. Chris Guglielmo, Louise Milligan and Denis Maxwell for the generous use of equipment and supplies. We thank Dr Sasan Amirsadeghi and Ms Dorothy Zhao for helping to generate the AOX sequence information from C. gigas. We thank two anonymous reviewers for their helpful comments on the manuscript. This work was supported by an NSERC Post-Doctoral Fellowship to A.E.M, and NSERC grants to G.C.V. and J.F.S.
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