THE BOX H-RNA PSEUDOURIDINE SYNTHASE(ACA SNORNAS CARRY CBF5P, THE PUTATIVE RIBOSOMAL)

Many or all of the sites of pseudouridine (Psi) formation in eukaryoti c rRNA are selected by site-specific base-pairing with members of the box H + ACA class of small nucleolar RNAs (snoRNAs). Database searches previously identified strong homology between the rat nucleolar prote in Nap57p, its yeast homolog Cbf5p, and the Escherichia coli Psi synth ase truB/P35. We therefore tested whether Cbf5p is required for synthe sis of Psi in the yeast rRNA. After genetic depletion of Cbf5p, format ion of Psi in the pre-rRNA is dramatically inhibited, resulting in acc umulation of the unmodified rRNA. Protein A-tagged Cbf5p coprecipitate s all tested members of the box H + ACA snoRNAs but not box C + D snoR NAs or other RNA species. Genetic depletion of Cbf5p leads to depletio n of all box H + ACA snoRNAs. These include snR30, which is required f or pre-rRNA processing. Depletion of Cbf5p also results in a pre-rRNA processing defect similar to that seen on depletion of snR30. We concl ude that Cbf5p is likely to be the rRNA Psi synthase and is an integra l component of the box H + ACA class of snoRNPs, which function to tar get the enzyme to its site of action.