A nonsense suppressor is a tRNA mutation that suppresses the protein truncation resulting from a nonsense mutation. The nonsense suppressor is a tRNA gene (from Escherichia coli) which has an anticodon mutated to recognize the nonsense (stop) codon.
As a result, when the ribosome reaches the codon in question, the nonsense suppressor will sometimes bind and incorporate its amino acid (instead of the regular release factor binding and causing termination), allowing translation to continue. The frequency of this event is the ratio of the nonsense suppressor expression to the expression of the "correct" release factor for that stop codon. This is a useful genetic tool, but can also result in problematic side effects, since all identical stop codons in the genome will also be suppressed to the same degree. Genes with different or multiple stop codons will be unaffected.
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