Tryptophan Synthetase α Chain of Escherichia coli and Its Structural Gene

Abstract

The tryptophan synthetase of Escherichia coli is a tetrameric protein consisting of two pairs of nonidentical polypeptide chains (Yanofsky and Crawford, 1972). These polypeptide chains, designated α and β, are specified by structural genes trpA and trpB, respectively, which lie adjacent to one another at the operator-distal end of the tryptophan operon. Mutants deficient in the terminal reaction in tryptophan biosynthesis are generally altered in trpA, trpB, or both. Extensive genetic studies have been performed with these mutants and fine-structure genetic maps have been prepared for each of the genes (Yanofsky and Crawford, 1972; Yanofsky and Horn, 1972; Yanofsky et al., 1964, 1967). Many trpA mutants have also been analyzed biochemically to determine the positions in the tryptophan synthetase α chain affected by mutation. Comparison of the linear correspondence between the genetic map of trpA and the positions in the α chain altered by mutation has demonstrated that the map and polypeptide are colinear (Figure 1) (Yanofsky and Horn, 1972; Yanofsky et al., 1964, 1967). Furthermore, relative distance on the genetic map was found to approximate distance between amino acid residues in the polypeptide chain. These findings suggest that the nucleotide sequence of a gene corresponds to the linear sequence of amino acids in its specified polypeptide chain.