Volume 1

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

    52/668 (page 30)
    The suppressor tRNA which inserts tyrosine in response to UAG has been sequenced and comparison with the wild type molecule shows that the mutation is located in the anticodon and enables the suppressor to recognise UAG instead of the usual UAU or UAC. A further mutation in the anticodon of this rRNA changes its coding properties again so that it recognises both UAA and UAG. Such experiments confirm the sequences assigned to the amber and ochre nonsense codons; and this form of suppression is discussed on page 211. Biochemical experiments with synthetic templates and with both nonsense mutant and wild type RNA phages have demonstrated that the effect of nonsense codons is mediated by protein factors which recognise their sequences as the signals for releasing completed proteins from transfer RNA (see page 111). That all three nonsense codons may be used as termination signals at the ends of genes has been shown by determination of the nucleotide sequences of RNA phages (see page 86); we do not know for certain the extent to which each of the three nonsense triplets is used within the bacterial cell, but this problem is discussed on page 114. The chain terminating role of the codons UAA, UAG and UGA has been defined conclusively only in bacteria. However, it seems very likely that at least the UAA and UAG codons and probably also the UGA triplet have the same meaning in eucaryotic cells. Biochemical experiments have failed to identify any transfer RNAs which respond to the ochre and amber codons (see for example figure 5.20) ; and mammalian cells contain a protein terminator factor which responds to all three nonsense codons. Genetic experiments have been possible only with yeast, in which Stewart et al. (1972) and Stewart and Sherman (1972, 1973) identified two classes of mutant in cytochrome c which cause complete inactivation; these appear to be nonsense mutants and their codons may be deduced from the amino acids which can be inserted when a second mutation generates a reversion to give an active protein. One class appears to comprise UAA and the other UAG. Hatfield (1972) has found that the UGA triplet appears to stimulate binding of mammalian arg-tRNA and ser-tRNA in the Nirenberg assay; but this alone cannot be taken as evidence that this codon represents an amino acid, since binding (to ribosomes of E.coli) may reflect insufficiently stringent conditions of incubation. Ochre and amber codons therefore appear likely to have a universal meaning of chain termina tion; the role of the UGA codon cannot at present be defined, although genetic experiments with yeast show that it does not code for tryptophan. Evolution of the Code Universal Nature of the Code The genetic code appears to be universal, so that the codon meanings which have been defined in E.coli are the same in all living organisms. The coding