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 (C T) in the second triplet gives a 'mutant' DNA sequence
5'- CTA AG
ATT - 3'
3'- GAT TC TAA - 5'
produces the mRNA 5'- CUA AG 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.
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
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
3'- AT -5'
5'- TA -3'
which produces the tRNA 3'- AU -5' anticodon loop
which reads the mRNA 5'- UAG -3'
as - tyr -
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
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.
mutants were first discovered in
bacteriophage conditional mutants.
Homework: why is this mutation called an "amber supressor"?