Pseudouridine in RNA: What, where, how, and why

Pseudouridine (5-ribosyluracil) is a ubiquitous yet enigmatic constituent o f structural RNAs (transfer, ribosomal, small nuclear, and small nucleolar) . Although pseudouridine (Psi) was the first modified nucleoside to be disc overed in RNA, and is the most abundant, its biosynthesis and biological ro les have remained poorly understood since its identification as a "fifth nu cleoside" in RNA, Recently, a combination of biochemical, biophysical, and genetic approaches has helped to illuminate the structural consequences of Psi in polyribonucleotides, the biochemical mechanism of U-->Psi isomerizat ion in RNA, and the role of modification enzymes (Psi synthases) and bos H/ ACA snoRNAs, a class of eukaryotic small nucleolar RNAs, in the site specif ic biosynthesis of Psi. Through its unique ability to coordinate a structur al water molecule via its free N1-H, Psi exerts a subtle but significant "r igidifying" influence on the nearby sugar-phosphate backbone and also enhan ces base stacking. These effects may underlie the biological role of most ( but perhaps not all) of the Psi residues in RNA. Certain genetic mutants la cking specific Psi residues in tRNA or rRNA exhibit difficulties in transla tion, display slow growth rates, and fail to compete effectively with wild- type strains in mixed culture. In particular, normal growth is severely com promised in an Escherichia coli mutant deficient in a pseudouridine synthas e responsible for the formation of three closely spaced Psi residues in the mRNA decoding region of the 23S rRNA, Such studies demonstrate that pseudo uridylation of RNA confers an important selective advantage in a natural bi ological contest.