correct incorrect abortive amino acids amino acids chain ending mutant DNA
5't insertion of a nucleotide during gene replication mutant DNA;
5't insertion of a nucleotide during gene replication mutant mRNA2
cxoooocooo even framesliift mutations are also known). In E. roli, suppressor genes are known for each of the three stop codons. They act by reading a slop codon as if it were a signal for a specific amino acid. There are, for example, three well-characterized genes that suppress the UAG codon. One suppressor gene inserts serine, another glutamine, and a third tyrosine at the nonsense position. In each of the three UAG suppressor mutants, the anticodon of a tRNA species specific for one of these amino acids has been altered. For example, the tyrosine suppressor arises by a mutation within a tRNA1^ gene that changes the anticodon from GUA (S'-AUG-S') to CUA [3'-AUC-5'), thereby enabling it to recognize UAG codons (Figure 15-7), The serine and glutamine suppressor tRNAs also arise by single base changes m their anticodons.
The discovery that cells with nonsense suppressors contain mutation-ally-altered tRNAs raised the question of how their codons corresponding to these tRNAs could continue to be read normally. In the case of the tyrosine UAG suppressor, the answer comes from the discovery that throe separate genes code for tRNA1^. One codes for the major tRNA1^
a mutated gene containing nonsense codon
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