Experiments 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 identified glycine (Gly) as the wild-type amino acid in position 211 of tryptophan synthetase, the product of the trpA gene. He identified two independent missense mutants with defective tryptophan synthetase at these positions that resulted from base-pair substitutions. One mutant encoded arginine (Arg) and another encoded glutamic acid (Glu). At position 235, wild-type tryptophan synthetase contains serine (Ser) but a base-pair substitution mutant encodes leucine (Leu). At position 243, the wild-type polypeptide contains glutamine and a base-pair substitution mutant encodes a stop codon. Identify the most likely wild-type codons for positions 211, 235, and 243. Justify your answer in each case.

Key Concepts

Codons and Amino Acids

Codons are sequences of three nucleotides in mRNA that correspond to specific amino acids during protein synthesis. Each amino acid is encoded by one or more codons, which are determined by the genetic code. Understanding the relationship between codons and their respective amino acids is crucial for identifying wild-type codons based on the amino acid substitutions observed in mutants.

Missense Mutations

Missense mutations are a type of point mutation where a single nucleotide change results in the coding of a different amino acid. This can affect protein function, as seen in Yanofsky's experiments where specific amino acids were replaced in tryptophan synthetase. Recognizing how these mutations alter the amino acid sequence helps in deducing the original codons that would produce the wild-type protein.

Base-Pair Substitution

Base-pair substitutions are mutations that involve the replacement of one nucleotide with another in the DNA sequence. This can lead to changes in the corresponding mRNA and ultimately the amino acid sequence of a protein. In the context of the question, understanding how these substitutions affect the codons at specific positions is essential for determining the wild-type codons for tryptophan synthetase.