Index >> Gene Mutation >> Suppressor Genes

Suppressor Genes -

It has been seen that in intergenic suppression the effects of a harmful mutation are neutralized by a second mutation in another gene.

Genes whose activity results in suppression of mutations in other genes are called suppressor genes.

There are suppressor genes for each of the three termination codons, UAG (amber), UAA (ochre) and UGA (opal).

The amber mutants of the bacteriophage T4 cannot grow on most strains of E. coli. They can, however, grow on certain strains called permissive strains.

These permissive strains are mutant strains which contain suppressor genes.

They act by producing changes in the anticodons of certain tRNA species. The tRNAs with modified anticodons can now read the termination codon UAG.

One suppressor gene inserts serine in the termination position.

Another inserts glutamine, while the third inserts tyrosine. The tyrosine-inserting suppressor gene acts by changing the tyrosine tRNA anticodon from 3' AUG 5' to 3' AUC 5'.

Opal suppressor genes which suppress the opal termination codon UGA have also been found.

Here suppression takes place by the insertion of tryptophan in the termination position (UGA). Normal tryptophan tRNA reads only the UGG codon.

The suppressor mutation causes it to read the termination codon UGA, at the same time retaining its ability to read the tryptophan codon UGG.

Suppression of UAG and UGA is about 50%. UAA suppression is less efficient and is about 1-5%.