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. Required for maintenance of the germline stem cell lineage during spermatogenesis. a uridine in RNA = a pseudouridine in RNA Component of the small nucleolar ribonucleoprotein particle containing H/ACA-type snoRNAs (H/ACA snoRNPs). Isoform C is expressed both maternally and zygotically. Isoform A is expressed at the same level in adult females but at a much lower level in ovaries. Belongs to the pseudouridine synthase TruB family.