Experiments conducted by Charles Yanofsky in the 1950s and 1960s helped characterize the nature of tryptophan synthesis in E. coli. In one of Yanofsky's experiments, he observed that a mutation in the gene caused premature termination of translation due to a stop codon at position 243. Which of the following base-pair substitutions could result in a stop codon at position 243 of tryptophan synthetase?