Info

aUA|

IM Normal transcription produces a tRNA with an anticodon AUA (which would pair with the tyrosine codon UAU in translation).

tRNA

If a base substitution introduces an incorrect base (G),...

.the resulting mutant tRNA has anticodon AUC (instead of AUA),.

. which can pair with the stop codon UAG.

17.10 An intergenic suppressor mutation occurs in a different gene from the one bearing the original mutation. (a) The wild-type sequence produces a full-length, functional protein. (b) A base substitution at a site in one gene produces a premature stop codon, resulting in a shortened, nonfunctional protein. (c) A base substitution at a site in another gene, which in this case encodes tRNA, alters the anticodon of tRNATyr so that tRNATyr can pair with the stop codon produced by the original mutation, allowing tyrosine to be incorporated into the protein and translation to continue. Tyrosine replaces the leucine residue present in the original protein.

aUA|

IM Normal transcription produces a tRNA with an anticodon AUA (which would pair with the tyrosine codon UAU in translation).

If a base substitution introduces an incorrect base (G),...

.the resulting mutant tRNA has anticodon AUC (instead of AUA),.

. which can pair with the stop codon UAG.

[T0 Translation continues past the stop codon, Tyr is incorporated into the protein.

remain available to recognize the tyrosine codons. However, we might expect that the tRNAs that have undergone a suppressor mutation would also suppress the normal termination codons at the ends of coding sequences, resulting in the production of longer-than-normal proteins, but this event does not usually take place. Mutations in tRNA genes can also suppress missense and frameshift mutations.

Intergenic suppressors can also work through genic interactions (see p. 000 in Chapter 5). Polypeptide chains which are produced by two genes may interact to produce a functional protein. A mutation in one gene may alter the encoded polypeptide so that the interaction is destroyed and then a functional protein is no longer produced. A suppressor mutation in the second gene may produce a compensatory change in its polypeptide therefore restoring the original interaction. Characteristics of some of the different types of mutations are summarized in Table 17.2.

Table 17.2 Characteristics of different types of mutations ^

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