46) There is at least one nonsense suppressing tRNA known that can suppress more

Question

46) There is at least one nonsense suppressing tRNA known that can suppress more than one type of non-sense codon: Use the wobble rules to help solve this problem

a) Anticodon___________                             

b) Stop codons _______ & ________      

c) amino acid(s)________    ____________

                                                                                                                                                                                                ____________

Explanation / Answer

a)

Nonsense suppressors are produced by base substitution mutations in the DNA corresponding to the anticodon of a tRNA that cause the anticodon to pair with one of the terminarion (or "nonsense") codons, UAG (Amber), UAA (Ochre), or UGA (Opal). Examples of nonsense suppressors produced by single base substitutions in E. coli. Because of the wobble rules amber suppressing tRNAs can only read UAG codons, but ochre-suppressing tRNAs can read both UAA and UAG codons.

Suppressor E changes the anticodon CUG --> CUA in tRNAGln and supP changes anticodon CAA --> CUA in tRNALeu.

b)

Stop codons : The three codons UAG (Amber), UAA (Ochre), and UGA (Opal) are usually translation termination signals.

c)

Two possible events can occur when a ribosome encounters a nonsense codon in a strain with a nonsense suppressor: (1) termination of peptide elongation can occur if the appropriate release factors associate with the ribosome, or (2) an amino acid can be inserted into the growing peptide chain if the suppressor tRNA associates with the ribosome.

The efficiency of suppression depends upon how well the suppressor tRNA is charged with the appropriate amino acid, the concentration of the suppressor tRNA in the cell, and the "context" of the nonsense codon in the mRNA -- especially the base on the 3' side of the codon. In fact, UGA is misread by tRNATrp about 1-3% of the time even in wild-type cells. Although suppressor tRNAs are often inefficient, the amount of protein produced is often sufficient to repair the mutant phenotype. The amino acids like Gly, Trp, Tyr, Gln. Ser and Lys are involved in nonsense suppressor mutations as these amino acids are coded by more than 2 codons.

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