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Q40 - Reconstructing Codon 38 of Tryptophan Synthetase from Revertant Analysis

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Translation & the Genetic Code

About 50 years ago, Charles Yanofsky studied the sequence of the tryptophan synthetase of E. coli. The wild type protein (1) has a glycine in position 38. Yanofsky isolated two inactive trp mutants: 2 and 3. Mutant 2 had Arg instead of Gly at position 38, and mutant 3 had Glu at position 38. Mutants 2 and 3 were plated on minimal medium (without tryptophan). Colonies appearing correspond to spontaneous mutations that restored tryptophan synthetase function. The amino acid at position 38 was identified as described in figure A: mutant 2 (Arg) reverted to strains 4 (Ile), 5 (Thr), 6 (Ser) and 7 (Gly); mutant 3 (Glu) reverted to strains 8 (Gly), 9 (Ala) and 10 (Val). Assume that each amino acid replacement results from a single base-pair change.

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. Mutant 2 results from a base substitution at the first position of the codon.
B. Strains 7 and 8 likely have the same sequence as the wild type strain.
C. The codon in strain 10 is 5'GUA3'
D. The codon in strain 6 is 5'AGC3'.
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Question reproduced from IBO 2016, Theoretical Paper A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)