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Nonsense suppressors change the effects of nonsense mutations by altering how stop codons are read. The largest class of nonsense suppressors are mutations in Category:tRNA genes that alter the codon-anticodon interaction to allow a stop codon to be read by that tRNA. This will only suppress the mutation if the amino acid inserted restores a functional protein product.

Nonsense suppression is defined genetically based on reversing an observable phenotype. Note that this does not mean full restoration of normal function. Nonsense suppression is often incomplete ("leaky") and pleiotropic.

This table is incomplete and may have inaccuracies in the synonym lists for amber vs ochre. Need to add the engineered suppressors from Kleina et al^[3] and Normanly et al^[4]. Others?

tRNA suppressors

tRNA suppressors are often, but not always ^[5], caused by changes in the anticodon loop of the tRNA to allow recognition of a stop codon. Due to wobble rules, amber suppressors are specific for amber (UAG) codons, while ochre suppressors recognize both Ochre (UAA) and Amber (UAG) codons via G=U and A=U basepairing. The suppressor tRNA is no longer able to recognize its cognate codons, which means that in order to be viable, suppressor strains need to have another tRNA that recognizes the unmutated codons. This can be accomplished using strains that are merodiploid for tRNAs that are present in single copy in E. coli, or through the use of multicopy plasmids expressing the suppressor tRNA.

Because the suppressor tRNA is in competition with translation termination factors, the efficiency of suppression is usually less than 100%. In general, ochre suppressors are less efficient than amber suppressors; this is probably due to selection for suppressors that can survive the pleiotropic effects of widespread translational readthrough. Because more E. coli genes terminate in UAA than UAG, amber suppressors tend to be less sickly than ochre suppressors.

non-tRNA nonsense suppressors

UGA suppressors have been isolated^[6] in prfB, which encodes peptide release factor RF2.

See Also

• http://www.sci.sdsu.edu/~smaloy/MicrobialGenetics/topics/rev-sup/nonsense-suppressors.html Stan Maloy's page on nonsense suppressors

References

1. ↑ WEIGERT MG & GAREN A (1965) AMINO ACID SUBSTITUTIONS RESULTING FROM SUPPRESSION OF NONSENSE MUTATIONS. I. SERINE INSERTION BY THE SU-1 SUPPRESSOR GENE. J Mol Biol 12: 448-55 PubMed EcoliWiki page
2. ↑ ^{2.0 2.1 2.2} Weigert MG et al. (1965) Amino acid substitutions resulting from suppression of nonsense mutations. II. Glutamine insertion by the Su-2 gene; tyrosine insertion by the Su-3 gene. J Mol Biol 14: 522-7 PubMed EcoliWiki page
3. ↑ Kleina LG et al. (1990) Construction of Escherichia coli amber suppressor tRNA genes. II. Synthesis of additional tRNA genes and improvement of suppressor efficiency. J Mol Biol 213: 705-17 PubMed EcoliWiki page
4. ↑ Normanly J et al. (1990) Construction of Escherichia coli amber suppressor tRNA genes. III. Determination of tRNA specificity. J Mol Biol 213: 719-26 PubMed EcoliWiki page
5. ↑ citation needed
6. ↑ Chang Z et al. (1990) Novel UGA-suppressors in Escherichia coli K-12. Jpn J Genet 65: 71-81 PubMed EcoliWiki page