Organization and phylogenetic analysis of components of the archaeal four-gene and bacterial five-gene operons. (a) Organization of operons in archaea and bacteria. Selenoprotein genes are shaded. (b) Phylograms of different proteins in these operons. Red indicates Deltaproteobacteria, and green indicates Archaea. Organisms containing the four-gene or five-gene operon are shown in bold. The branch separating other archaea and bacteria in the trees has been shortened for illustration purposes. C, Cys-containing; FrhA, coenzyme F420-reducing hydrogenase α subunit; FrhD, coenzyme F420-reducing hydrogenase δ subunit; FrhG, coenzyme F420-reducing hydrogenase γ subunit; GlpC, Fe-S oxidoreductase; HdrA, heterodisulfide reductase subunit A; U, Sec-containing.