A nonsense mutation is one that converts a codon for an amino acid to a stop codon, thus terminating the polyeptide chain prematurely. It turns out, however, that there are mutations in other regions of the genome separate from the nonsense mutation that can suppress the effects of the nonsense mutation. That is, suppressor mutations occurring elsewhere in the genome can cause the gene containing the premature stop codon to still make complete proteins. When the suppressor mutation occurs in a second gene, it is called intergenic suppression.

Figure 27.27 shows how intergenic mutation in a tRNA gene suppress a nonsense mutation. In the example, the nonsense mutation of a tyrosine gene leads to the synthesis of a protein truncated at the point of the mutation. Mutation at a second site--in the anticodon loop of a tyrosine-tRNA--creates a tRNA that can base pair with the original nonsense mutation and insert tyrosine into the polypeptide chain during translation. This inhibits early translation termination and allows the complete protein to be made.

Suppressor mutations can correct missense and frameshift mutations, too (see here). In a missense mutation a codon has been changed so that a different amino acid is coded for at the point of the mutation. In a frame shift mutation a base has been inserted or deleted in a coding region, thus altering the reading frame of all subsequent codons. Suppressor mutations correct missense mutations similar to the way they correct nonsense mutations. Frameshift mutations, on the other hand, can be alleviated by suppressor mutations that have mutated tRNAs with 2 or 4 bases in the anticodon loop.

Suppressor mutations have consequences, however, in that the mutated tRNAs are not confined in action solely to the mutated gene. That is, the mutated tRNAs will interfere with the normal termination of other proteins.