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 and play a central role in ribosomal RNA processing. The H/ACA snoRNP complex also mediates pseudouridylation of other types of RNAs. Catalyzes pseudouridylation at position 93 in U2 snRNA. Also catalyzes pseudouridylation of mRNAs; H/ACA-type snoRNAs probably guide pseudouridylation of mRNAs. uridine in 5S rRNA = pseudouridine in 5S rRNA uridine in snRNA = pseudouridine in snRNA a uridine in mRNA = a pseudouridine in mRNA Component of the small nucleolar ribonucleoprotein particles containing H/ACA-type snoRNAs (H/ACA snoRNPs). Belongs to the pseudouridine synthase TruB family.