Gene structure in the tryptophan operon of Escherichia coli: Nucleotide sequence of trpC and the flanking intercistronic regions

We have determined the DNA sequence for the portion of the Escherichia coli tryptophan (trp) operon spanning trpC, which codes for the bifunctional enzyme N-(5′-phosphoribosyl)-anthranilic acid isomerase/indole-3-glycerol phosphate synthetase. The coding region consists of 1356 nucleotides, directing the synthesis of a polypeptide 452 amino acids in length. The predicted protein sequence is consistent with the amino acid composition of the pure enzyme, and with all known partial peptide sequences derived from this molecule. The enzyme is of particular functional interest, because it contains the catalytic activities for two sequential reactions in tryptophan biosynthesis in a single polypeptide chain.The nucleotide sequences of the junctions between trpC and its flanking genes, trpD and trpB, have also been determined. The trpD-trpC junction consists of six untranslated nucleotides and translation of trpC initiates at the second of two adjacent AUG codons. The trpC termination codon is separated from trpB by 11 nucleotides. The short non-translated regions flanking trpC distinguish it from trpA and trpD, whose initiation codons overlap the termination codons of the preceding genes (trpB and trpE), respectively. These differences in the intercistronic regions may reflect functional relationships between the products of adjacent genes in the operon.