Molecular Genetics

Nonsense mutations can be suppressed by mutant tRNAs, called suppressors.

Facts

• Termination of translation occurs whenever the codons UGA, UAA, UAG are encountered. ORFs often end in a series of two or more stop codons.
• Some mutations in coding sequences change sense codons to stop codons. These are called nonsense mutations and are: ochre (UAA), amber (UAG), umber (UGA).
• Second site mutations reverting the effects of nonsense mutations are known.
• Many of the reverting mutations are in tRNA genes.
• The mutant tRNAs, called suppressor tRNAs, are charged with amino acids and recognize stop codons, thus by-passing the stop codon and continuing the polypeptide chain elongation.

Interpretations

• Suppressor tRNA genes are useful genetic engineering tools because of their small size and because their presence can be a selectable marker using nonsense mutations in selectable genes.

Further information

• Nonsense suppressor mutations also suppress the polar effects of the mutations on transcription.
• The umber codon is also known as the opal codon.

This is page 2421 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2000