Suppressor mutations: "Two wrongs make a right""

Most mutations in protein-coding genes result in replacement of one amino acid by another (sense mutations) or premature termination (non-sense mutations) if a coding triplet mutates to a termination triplet.

However, the translation apparatus is also subject to mutational change. Notably, 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 transition mutation (C T) in the second triplet gives a mutated 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 -

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

    The mutated tDNA gene therefore produces a tRNA molecule that will read the the mutated Gln codon UAG as a Tyr codon, instead of as a 'stop' codon.The polypeptide will have an amino acid substitution (Gln Tyr). 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 suppressor 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.

    Amber suppressor mutants were first discovered in bacteriophage conditional mutants.

Homework: why is this mutation called an "amber supressor"?

All text material ©2015 by Steven M. Carr