The cartoons below show the effect of a nonsense mutation and nonsense suppressor on translation of a hypothetical gene (shown as an open reading frame starting with ATG and ending with TAA). In each panel three codons are shown -- the start codon, a codon in the middle of the gene, and the stop codon at the end of the gene. To insert an amino acid at the corresponding position in the protein, when the ribosome reaches that codon a tRNA carrying an amino acid must bind to the codon. In Supo cells there is not tRNA that recognized the nonsense codons UAG, UAA, or UGA. Instead, a protein release factor binds to the ribosome and dissociates the ribosome, releasing the ribosomal subunits and the newly synthesized protein. In cells with a nonsense suppressor tRNA, the tRNA can bind to the nonsense codon and insert an amino acid (Ser in the case of supD shown below), allowing synthesis of the full length protein. However, the suppression is not 100% efficient -- sometimes a release factor binds to the nonsense codon and terminates translation, releasing a truncated protein.
(A) Wild-type gene in supo strain:
(B) Amber mutant in supo strain:
(C) Amber mutant in supD amber suppressor strain:
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Last modified February 5, 2000