File:Nonsense and Frameshift Suppression.png

English: Mechanism of nonsense and frameshift suppression. (A) Nonsense suppression, an amber suppressor tRNA recognizes the desired UAG stop codon and incorporates an UAA. (B) Competition for nonsense suppression arises from a protein release factor (RF1, for UAG, in prokaryotes and eRF1 in eukaryotes) and causes termination of the protein sequence. (C) Frameshift suppression, a frameshift suppressor tRNA recognizes the desired four-base codon and incorporates an UAA. The CGGG codon is shown, but any four- or five-base codon is considered frameshift suppression. (D) Competition arises from endogenous tRNA that recognizes the first three bases of the quadruplet codon, which results in a –1 frameshift (at the same position shown in C) and results in truncation by stop codons (shown in green) presented after the -1 frameshift. (E) An undesired suppression event can occur where the frameshift suppressor tRNA recognizes another cognate four-base codon in the mRNA sequence, which results in the incorporation of an UAA and a +1 frameshift. The +1 frameshift causes truncation by stop codons (shown in green) presented after the +1 frameshift. mRNAs are written from 5’ to 3’ going from right to left.