Chapter 12, Problem 33P

Summary Introduction

To analyze:

Charles Yanofsky in $\text{1950s}$ and $\text{1960s}$ describe the nature of tryptophan synthesis in E.coli. In Yanogsky’s one experiment, he identified glycine (GLY) as the wild-type amino acid in position $211$ of tryptophan synthetase, the product of trpA gene. He recognized two independent missense mutants with defective tryptophan synthetase at these positions that resulted from base pair substitutions. One mutant encoded arginine(Arg) and other 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 possess glutamine, and a base pair substitution mutant encodes a stop codon. Spot the most likely wild type codons for position $\text{211,}\text{235 and 243}$ with proper justification.

Introduction:

Base pair substitution is a type of mutation where one nucleotide base pair is replaced by another. There are many possible types of replacements in base pairs-

Transition type: One Purine nucleotide by another Purine.

One Pyrimidine nucleotide by another Pyrimidine.

Transversion type: One Purine is replaced by Pyrimidine or vice versa.

Again, there are two possible types of mutation by the replacement of nucleotide base pair:

Silent or Synonymous Mutation: In this case, the amino acid remains the same even after the replacement of the nucleotide base pair, therefore, function of the protein is not affected.

Missense Mutation: In this type of mutation, the replacement of nucleotide base pair causes a change in amino acid resulting in the loss of protein function.

Charles Yanofsky found many Missense mutations while characterizing tryptophan synthetase, the product of trp A gene.