One kind of mutation that can inactivate a gene in any organism is a base-pair change that converts a codon normally encoding an amino acid into a stop codon, such as a change from UAC (encoding tyrosine) to UAG (stop). When such a mutation occurs early in the reading frame, the resulting truncated protein is usually nonfunctional. Such mutations are called nonsense mutations because when the genetic code equating each triplet codon sequence with a single amino acid was being deciphered by researchers, the three stop codons were found not to encode any amino acid—they did not “make sense.”

In genetic studies with the bacterium E. coli, it was discovered that the effect of a nonsense mutation can be suppressed by a second mutation in a tRNA gene. This occurs when the sequence in a tRNA gene that encodes the anticodon is changed to a triplet that is complementary to the original mutant stop codon. For example, if a mutation in tRNA^Tyr changes its anticodon from GUA to CUA, which can base-pair with the UAG stop codon, then the mutant tRNA^Tyr can still be recognized by the tyrosine aminoacyl-tRNA synthetase and coupled to tyrosine. Cells that have both the original nonsense mutation and the second mutation in the anticodon of the tRNA^Tyr gene consequently can insert a tyrosine at the position of the mutant stop codon, allowing protein synthesis to continue past the original nonsense mutation. This mechanism is not highly efficient, so translation of normal mRNAs with a UAG stop codon terminates at the normal position in most instances. If enough of the protein encoded by the original gene with the nonsense mutation is produced to provide its essential functions, the effect of the first mutation is said to be suppressed by the second mutation in the anticodon of the tRNA gene.

This mechanism of nonsense suppression is a powerful tool in genetic studies of bacteria. For example, it allows us to isolate mutant bacterial viruses that cannot grow in normal cells but can grow in cells expressing a nonsense-suppressing tRNA because the mutant virus has a nonsense mutation in an essential gene. Such mutant viruses grown on nonsense-suppressing cells can then be used in experiments to analyze the function of the mutant gene by infecting normal cells that do not suppress the mutation and analyzing what step in the viral life cycle is defective in the absence of the mutant protein.

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