Ochre Mutation (Molecular Biology)

An ochre mutation is a nonsense mutation that changes a sense codon (one specifying an amino acid) into the stop codon UAA, causing premature termination of the polypeptide chain during translation. The mutation, the codon, and the mutant are all called ochre. Ochre mutations arise by single base changes in the codons for seven amino acids (and in the UGA and UAG stop codons, although these would not be nonsense mutations). In principle, mutations in the anticodons of the transfer RNAs that read those seven codons could give rise to ochre suppressors, but suppressors are recovered only if a second tRNA exists that reads the codon. In Escherichia coli, five ochre suppressors that arise by a single base change have been identified. Unlike amber mutations, ochre mutations are suppressed only by ochre suppressors. Ochre suppressors have effects that are much more deleterious than amber suppressors, reflecting the fact that ochre codons are the most frequent stop codon in E. coli and related bacteria. Consequently, most ochre suppressors are weak (inefficient at reading ochre codons) and allow most polypeptide chains to be properly terminated. The term ochre first appears in the literature in 1965 (1), apparently coined by Sydney Brenner to complement amber.