Genetics: Analysis and Principles 5th Edition by Robert Brooker

Exercise 14

Mutations in tRNA genes can create tRNAs that recognize stop codons. Because stop codons are sometimes called nonsense codons, the types of mutations that affect tRNAs are called nonsense suppressors. For example, a normal tRNA Gly has an anticodon sequence CCU that recognizes a glycine codon in mRNA (GGA) and puts in a glycine during translation. However, a mutation in the gene that encodes tRNA Gly could change the anticodon to ACU. This mutant tRNA Gly would still carry glycine, but it would recognize the stop codon UGA. Would this mutation affect attenuation of the trp operon Explain why or why not. Note: To answer this question, you need to look carefully at Figure 14.12 and see if you can identify any stop codons that may exist beyond the UGA stop codon that is found after region 1.FIGURE 14.12 Sequence of the trpL mRNA produced during attenuation. A second method of regulation of the trp operon is attenuation, which occurs when tryptophan levels are sufficient for protein synthesis. A short mRNA is made that includes the trpL coding sequence. As shown here, this mRNA has several regions that are complementary to each other. The possible hydrogen bonding between regions 1 and 2, 2 and 3, and 3 and 4 is also shown, but each region can hydrogen bond to only one other region at a given time. For example, if region 2 is hydrogen bonded to region 1, it cannot simultanteously hydrogen bond with region 3. The last U in the purple attenuator sequence is the last nucleotide that would be transcribed during attenuation. At very low tryptophan concentrations, however, transcription occurs beyond the end of trpL and proceeds through the trpE gene and the rest of the trp operon.