Genetic Suppression

The effect of a mutation on the phenotype can be reversed so that the original wild type phenotype is brought back. This reversal may be due to true reversion or suppersion.

In a true reversion there is a reversal of the original genetic change. A C→A mutation would change the codon GCU (alanine) to GAU (aspartate). This may result in the enzyme formed becoming inactive. In a true reversion a reverse mutation from A→C would restore the condon for alanine (GAU→GCU). Such a mutation is called a back mutation.

Intragenic Suppression. Suppression mutations are of two types, intragenic suppression and extragenic suppression. In intragenic suppression a mutation in a gene is suppressed by another mutation in the same gene.

(1) Intracodon suppression. A codon that has undergone a change as a result of mutation may undergo another mutation to a codon that is less harmful to enzyme function. Thus mutation of GCU (alanine) to GAU (aspartate) may results in an inactive enzyme. A second mutation A→U would give the codon GUU for valine and may restore enzyme activity partially or fully.

(2) Reading from mutations. A second mutation at a different site in the gene may neutralize the effects of the first mutation. This results in an altered enzyme which differs from the wild type by only a few amino acids. Thus the addition of a base a few steps away from an earlier deletion can suppress the effects of the deletion.

Extragenic or intergenic suppression. If the deleterious effects of a mutation in a gene are overcome by a mutation in another gene, the process is called extragenic or intergenic suppression. In the strict sense the term suppressor mutations refers to intergenic events only, and not intragenic events. The essential feature of intergenic suppression is that the interacting mutational events take place in two separate genes. These two genes may even be located on different chromosomes.

The termination of polypeptide chain synthesis is brought about by converts a codon (UAA, UAG or UGA). A mutation which converts a codon specifying an amino acid into a termination codon (nonsense mutation) results in the formation of an incomplete polypeptide chain. Such chains are usually inactive. The effect of a nonsense mutation can be suppressed by mutations in the other genes (intergenic suppression). Such suppressor mutations result in viable proteins.