Pseudouridine (Ψ) is the most abundant modified nucleoside in RNA and has been referred to as the ‘fifth nucleoside’. Pseudouridine is found in a plethora of positions within tRNAs as well as in a number of positions in rRNAs and in snRNAs. Pseudouridine is a C-glycosyl pyrimidine that consists of uracil having a β-D-ribofuranosyl residue attached at position 5. The C-glycosyl isomer of the nucleoside uridine. It has a role as a fundamental metabolite. 

[1] RNA Modification, Katherine M. McKenney, ... Juan D. Alfonzo, The Enzymes, 2017.

[2] Carlile TM, et al. Pseudouridine profiling reveals regulated mRNA pseudouridylation in yeast and human cells. Nature. 2014 Nov 6;515(7525):143-6.