Multicomponent Machines in RNA Modification: H/ACA Ribonucleoproteins

Pseudouridylation, the isomerization of uridine to pseudouridine, is the most frequent posttranscriptional modification of RNA, such that pseudouridine has even been termed the fifth nucleotide. Whereas eubacteria employ single protein enzymes to identify and modify target uridines, archaebacteria and eukaryotes additionally evolved more complex modification machines, H/ACA ribonucleoproteins (RNPs). Each H/ACA RNP consists of a short RNA and the same four core proteins, one of which is the pseudouridine synthase related to the bacterial single protein enzymes. In this chapter, we will give an overview of these multicomponent machines with emphasis on the eukaryal systems that have acquired additional functions and that are the subject of the inherited bone marrow failure syndrome dyskeratosis congenita.