tRNA suppressors can arise by mutations in a gene encoding a tRNA. For example, the wild-type tyrT gene encodes a tRNA that recognizes a 5' UAC 3' codon in the mRNA and inserts tyrosine into the growing polypeptide chain. A mutation in the gene changes the anticodon so that it recognizes the stop codon 5' UAG 3' in the mRNA and, instead of terminating, inserts a tryrosine at that position in the polypeptide chain. The mutant form of the tyrT gene is called supF. The mutation in the tyrT DNA sequence and the resulting change in the tRNA is shown below:
tyrT DNA sequence:
tyrT and supF tRNAs:
Some other examples of tRNA suppressors from E. coli are shown in the cartoons below. The tRNAs are shown as cloverleaf structures with the anticodon highlighted in yellow, but due to interactions between nucleotides in tRNA molecules,the three-dimensional structure of tRNAs is L-shaped with the anticodon loop at one end of the L and the 3'OH of the tRNA where the amino acid is attached is located at the other end of the L.
In the following figures the base substitution resulting in the suppressor phenotype is underlined and the normal base at this position indicated below the arrow. Note that the suppressor phenotype can result from mutations that change the anticodon or from mutations that affect other regions of the tRNA. In addition, a variety of other types of mutations that are not shown can also produce a suppressor tRNA.
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Last modified September 17, 2000