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. {ECO:0000269|PubMed:10087258; ECO:0000269|PubMed:12645924; ECO:0000269|PubMed:9829824}.