BACKGROUND: Pseudouridine (Y) is the most abundant post-transcriptionally modified nucleotide found in RNA. Y is clustered in functionally important and evolutionary conserved regions of RNAs in all three domains of life. Pseudouridylation is catalyzed by two distinct mechanisms: an RNA-independent and an RNA-dependent mechanism. The former involves a group of stand-alone protein enzymes, and the latter involves a family of complex enzymes called box H/ACA RNPs, each of which consists of one RNA (box H/ACA RNA) and a set of four core proteins. Over the years, the mechanism of RNA-dependent pseudouridylation has been extensively studied. The crystal structures of partial and complete box H/ACA RNP have been solved. However, the detailed picture of RNA-dependent pseudouridylation is still not entirely clear.

OBJECTIVE: In this work, we review what is known about box H/ACA RNP and the mechanism by which box H/ACA RNP catalyzes RNA-dependent pseudouridylation. We also discuss some examples of the dual nature and redundancy of box H/ACA RNPs that deviate from the usual mechanism.

METHODS: A methodical literature search was performed using the Pubmed central search engine and International Digital Publishing Forum (EPUB) using the following keywords: “pseudouridylation,” “pseudouridine,” and “box H/ACA RNP.” The necessary information was extracted and cited.

RESULTS: A detailed introduction is made including the discovery, mechanism and crystal structure of box H/ACA RNP. Three sequence/structural requirements for box H/ACA RNA-guided pseudouridylation are discussed and the exceptions to those rules are explored.

CONCLUSION: Over the years, box H/ACA RNP-catalyzed pseudouridylation has been extensively studied, generating fruitful results. However, a detailed picture regarding the mechanism of this reaction is still to be deciphered. More work is needed to fully understand box H/ACA RNP-catalyzed pseudouridylation.