The pioneering studies of Beadle and Tatum with Neurospora crassal led to the concept that there is a 1: 1 relationship between gene and enzyme. Subsequent studies on the structure of proteins2 and genetic material' permitted a restatement of this relationship in molecular terms;4 the linear sequence of nucleotides in a gene specifies the linear sequence of amino acids in a protein. Several years ago studies were initiated with the A gene-A protein system of the tryptophan synthetase of Escherichia coli with the intention of examining this concept of a colinear relationship between gene structure and protein structure. A large number of mutant strains which produced altered A proteins were isolated, and genetic and protein primary structure studies were performed with these strains to locate the positions of the alterations within the A gene and the A protein.5-'0 It was hoped that with information of this type it would be possible to determine whether a genetic map and the primary structure of the corresponding protein were colinear. Recently, Kaiser" has demonstrated the correspondence of the genetic map with the sequence of blocks of nucleotides in DNA. Thus if a colinear relationship could be established between a genetic map and the primary structure of a protein, it would be reasonable to conclude that this relationship extends to the nucleotide sequence corresponding to the genetic map. In previous reports on studies with the tryptophan synthetase A protein, conclusive evidence was presented for the colinearity of a segment of the A gene and a segment of the A protein.'2 13 The present communication deals with more extensive data with 16 mutants with mutational alterations in one segment of the A gene and the A protein. Materials and Methods.-Mutant strains: Of the A-protein mutants examined in detail in this paper, strains A23, A27, A28, A36, A46, A58, A78, A90, A94, A95, A169, A178, and A187 were isolated following ultraviolet irradiation of the K-12 wild-type strain of E. coli, and strain A223 was isolated following treatment of the wild-type strain with ethylmethanesulfonate. Mutant A446 (previously designated PR8)'4 and mutant A487 were initially isolated as spontaneous secondsite reversions and subsequently were separated from the original A mutants with which they had been associated. Strains anth,and anthare blocked prior to anthranilic acid in the tryptophan pathway and respond to anthranilic acid, indole, or tryptophan. VIR and VIR trypdeletion mutants were isolated by treatment of Ti-sensitive populations of the various mutants with phage Tlh+. All of the V/' mutations mentioned are very closely linked to the A gene, and the VR trypdeletions include the V1R locus and some segment of the A gene."5 A stock of unrestricted T1 phage (uTi)'65 was kindly supplied by J. R. Christensen. Protein studies: The altered A proteins were isolated and examined for primary structure changes as described previously.6 7, 9 The ordering of the tryptic peptides mentioned in the paper will be described in detail elsewhere. 17,18 Genetic studies: Recombination experiments were performed with the temperate-transducing phage Plkc.'9 Recombination distances between A mutants were obtained by determining the frequency of appearance of tryp+ transductants. Transduction from his-) his+ was scored in each experiment for internal reference, and the ratio of tryp+/his+ transductants calculated.20