Pseudouridine (abbreviated Ψ) is the C-glycoside isomer of the nucleoside uridine, and it is the most prevalent of the over one hundred different modified nucleosides found in RNA . Ψ is found in all species and in all classes of RNA except mRNA . Ψ is formed by enzymes called Ψ synthases, which post-transcriptionally isomerize specific uridine residues in RNA.

References

1. Grosjean, H. and Benne, R., Modification and Editing of RNA. 1998, Washington, DC: ASM Press.