4300 Nucleic Acids Research, 2006, Vol. 34, No. 15 Cbf5/TruB Catalysis PUA Catalysis Zn THUMP Catalysis Figure 5. Domain organization of Cbf5 and Pus10. Schematic representation of predicted domains of Cbf5, TruB, Pus4 and Pus10 proteins. Conserved catalytic, PUA, Zn and THUMP domains are indicated. See text for discussion. may have had the ability to also act on rRNA (and some dependence on guide RNAs and accessory proteins) in a common ancestor of eukaryotes and archaea. After the diver- gence of eukaryotes and archaea, a gene duplication in eukaryotes may have allowed separation of the functions in Cbf5 and Pus4. An analogous situation exists in the cases of the RNA-guided machinery catalyzing the formation of 20O-methylribose in both rRNA and tRNA (42,43). As we have indicated, our results suggest the possibility that Cbf5 and Pus10 provide redundant function in formation of the highly conserved tRNA U55 modiﬁcation in archaea. 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