Nonsense suppressors

Gozalbo D, Hohmann S (1990) Nonsense suppressors partially revert the decrease of the mRNA level of a nonsense mutant allele in yeast. Curr Genet 17 77—79... 

When a mutation introduces a termination codon in the interior of a gene, translation is prematurely halted and the incomplete polypeptide is usually inactive. These are called nonsense mutations. The gene can be restored to normal function if a second mutation either (1) converts the misplaced termination codon to a codon specifying an amino acid or (2) suppresses the effects of the termination codon. Such restorative mutations are called nonsense suppressors they generally involve mutations in tRNA genes to produce altered (suppressor) tRNAs that can recognize the termination codon and insert an amino acid at that position. Most known suppressor tRNAs have single base substitutions in their anticodons. 

A suppressor of frame-shift mutations in Salmonella is a tRNA containing at the anticodon position the nucleotide quartet CCCC instead of the usual CCC triplet anticodon.442 443 It has eight unpaired bases in the anticodon loop instead of the usual seven. Other frame-shift repressor tRNAs have been identified in E. cob, 444 Salmonella, and yeast.445 Not all suppressor genes encode tRNAs. For example, a UGA suppressor from E. coli is a mutant 16S rRNA from which C1054 has been deleted.446 A general nonsense suppressor in yeast is homologous to yeast elongation factor EF-la as well as to E. coli EF-Tu.447... 

Most nonsense suppressor genes are mutants of tRNA genes. In view of this fact, how can cells survive the presence of such mutations ... 

Nonsense suppressors are produced by tethering a nonnatural amino acid to a stop (or nonsense ) anticodon in tRNA. As a result, the stop codon in an mRNA sequence is converted from a protein synthesis termination site to a site at which the nonnatural amino acid may specifically be inserted. DNA base substitutions that correspond to the stop anticodon of tRNA may thus be made in order to specifically incorporate nonnatural amino acids into proteins. 

In the mouse muscle nicotinic receptor (nAChR), NB-tyrosine was incorporated at positions 93 and 198 of the a subunit. These are conserved residues crucial for acetylcholine binding. The mutagenized mRNA and the relative nonsense suppressor tRNA charged with the NB-Tyr were injected into Xenopus oocytes. The channel was successfully expressed and incorporated into the egg membrane [116]. In the following voltage-clamp study, a train of about 20 near-UV laser pulses (300-350 nm) was able to increase acetylcholine-induced conductance across the membrane with about 5% of decaged Tyr residues in any one flash. 

Soll, L. and Berg, P. (1969) Recessive lethals, a new class of nonsense suppressors in Escherichia coli. Proc. Nat. Acad. Sci. 63, 392. 

Capecchi, M. and Gussin, G. N. (1965) Suppression in vitro Identification of a serine tRNA as a nonsense suppressor. Science, 149,417-422. 

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