Suppressor mutations: wrong + wrong = right

A mutation in the gene for a tRNA molecule that changes its anticodon loop can "suppressnonsense mutations that occur elsewhere in protein-coding genes.

1. Generation of a 'nonsense' mutation

   The wild-type DNA sequence      5'- CTA CAG ATT - 3'
                                   3'- GAT GTC TAA - 5'
   produces the mRNA               5'- CUA CAG AUU - 3'
   which codes for the polypeptide    - leu-gln-iso - [etc]

A 1st position mutation (CT) in the second triplet gives a 'mutant' DNA sequence

   sense strand                    5'- CTA TAG ATT - 3'
                         3'- GAT ATC TAA - 5'
   produces the mRNA               5'- CUA UAG AUU - 3'
   which codes for the polypeptide    - leu- *

because UAG is a 'stop' codon (the so-called amber stop). Chain growth in the polypeptide terminates prematurely. 

  2. Generation of an 'amber suppressor' mutation in a tRNA gene

The portion of the transfer RNA gene (tDNA) that codes for the anticodon loop of tRNAtyr(UAC) , which recognizes the mRNA codon 'UAC' and inserts 'tyr', is

   tDNA gene                 3'- ATG -5'
                             5'- TAC -3'
   which produces the tRNA   3'- AUG -5' anticodon loop
   which reads the mRNA      5'- UAC -3'
   as                           - tyr -

  A 3rd-position mutation (CG) on the 5'3' strand in this region produces a 'mutant' tDNA gene

  tDNA gene                  3'- ATC -5'
                             5'- TAG -3'
  which produces the tRNA    3'- AUC -5'  anticodon loop
  which reads the mRNA       5'- UAG -3'
  as                            - tyr -

Then, the mutated tDNA gene produces a tRNA molecule that will read the the mutated gln codon UAG in part 1 as a tyr codon, instead of as a 'stop' codon.

3. The mutation in the tRNA gene 'suppresses' the 'nonsense' mutation in the coding gene.

    The polypeptide will have an amino acid substitution (glntyr). Translation of the rest of the mRNA  will proceed normally, in the correct reading frame. The mutation in the tDNA gene 'suppresses' the phenotypic effect of the mutation in a protein-coding gene, because the supressor tRNA allows enough of the mRNA to be translated to permit a 'standard' phenotype. Note that other 'stop' codons will be read incorrectly as 'tyr': however, most will be read by 'standard' tRNAs and the protein products will be terminated normally.

All text material © 2010 by Steven M. Carr