Plays a central role in ribosomal RNA processing. Probable catalytic subunit of H/ACA small nucleolar ribonucleoprotein (H/ACA snoRNP) complex, which catalyzes pseudouridylation of rRNA. This involves the isomerization of uridine such that the ribose is subsequently attached to C5, instead of the normal N1. Pseudouridine ('psi') residues may serve to stabilize the conformation of rRNAs (By similarity). a uridine in RNA = a pseudouridine in RNA Component of the small nucleolar ribonucleoprotein particle containing H/ACA-type snoRNAs (H/ACA snoRNPs) (By similarity). Component of the telomerase holoenzyme complex at least composed of TERT, CBF5, POT1a and a telomerase RNA template component TER1. Interacts with POT1a. Belongs to the pseudouridine synthase TruB family.