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Journal of Bacteriology, January 2004, p. 90-97, Vol. 186, No. 1
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.1.90-97.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Flavin Mononucleotide-Binding Flavoprotein Family in the Domain Archaea

Department of Biochemistry and Molecular Biology, Eberly College of Science, The Pennsylvania State University, University Park, Pennsylvania 16802-4500

Received 6 May 2003/ Accepted 2 October 2003

The protein (AfpA, for archaeoflavoprotein) encoded by AF1518 in the genome of Archaeoglobus fulgidus was produced in Escherichia coli and characterized. AfpA was found to be a homodimer with a native molecular mass of 43 kDa and containing two noncovalently bound flavin mononucleotides (FMNs). The cell extract of A. fulgidus catalyzed the CO-dependent reduction of AfpA that was stimulated by the addition of ferredoxin. Ferredoxin was found to be a direct electron donor to purified AfpA, whereas rubredoxin was unable to substitute. Neither NADH nor NADPH was an electron donor. Ferricyanide, 2,6-dichlorophenolindophenol, several quinones, ferric citrate, bovine cytochrome c, and O₂ accepted electrons from reduced AfpA, whereas coenzyme F₄₂₀ did not. The rate of cytochrome c reduction was enhanced in the presence of O₂ suggesting that superoxide is a product of the interaction of reduced AfpA with O₂. Although AF1518 was previously annotated as encoding a decarboxylase involved in coenzyme A biosynthesis, the results establish that AfpA is an electron carrier protein with ferredoxin as the physiological electron donor. The genomes of several diverse Archaea contained afpA homologs clustered with open reading frames annotated as homologs of genes encoding reductases involved in the oxidative stress response of anaerobes from the domain Bacteria. A potential role for AfpA in coupling electron flow from ferredoxin to the putative reductases is discussed. A search of the databases suggests that AfpA is the prototype of a previously unrecognized flavoprotein family unique to the domain Archaea for which the name archaeoflavoprotein is proposed.

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