Abstract: How pseudouridylation (Psi), the most common and evolutionarily conserved modification of rRNA, regulates ribosome activity is poorly understood. Medically, Psi is important because the rRNA Psi synthase, DKC1, is mutated in X-linked dyskeratosis congenita (X-DC) and Hoyeraal-Hreidarsson (HH) syndrome. Here, we characterize ribosomes isolated from a yeast strain in which Cbf5p, the yeast homolog of DKC1, is catalytically impaired through a D95A mutation (cbf5-D95A). Ribosomes from cbf5-D95A cells display decreased affinities for tRNA binding to the A and P sites as well as the cricket paralysis virus internal ribosome entry site (IRES), which interacts with both the P and the E sites of the ribosome. This biochemical impairment in ribosome activity manifests as decreased translational fidelity and IRES-dependent translational initiation, which are also evident in mouse and human cells deficient for DKC1 activity. These findings uncover specific roles for Psi modification in ribosome-ligand interactions that are conserved in yeast, mouse, and humans.CI - Copyright (c) 2011 Elsevier Inc. All rights reserved.