Modifying the genetic code: Converting nonsense codons into sense codons by targeted pseudouridylation

All three translation termination codons, or nonsense codons, contain a uridine residue at the first position of the codon(1–3). Here, we demonstrate that pseudouridylation [conversion of uridine into pseudouridine (Ψ) (4)] of nonsense codons suppresses translation termination both in vitro and in vivo. In vivo targeting of nonsense codons is accomplished by the expression of an H/ACA RNA capable of directing the isomerization of uridine to Ψ within the nonsense codon. Thus, targeted pseudouridylation represents a novel approach for promoting nonsense suppression in vivo. Remarkably, we also show that pseudouridylated nonsense codons code for amino acids with similar properties. Specifically, ΨAA and ΨAG code for serine and threonine, whereas ΨGA codes for tyrosine and phenylalanine, thus suggesting a novel mode of decoding. Our results suggest that RNA modification, as a naturally occurring mechanism, may offer a new way to expand the genetic code.