The DNA sequence of the Escherichia coli metK gene has been determined. Protein sequence data for purified S-adenosylmethionine synthetase have also been obtained and confirm that metK is the structural gene for S-adenosylmethionine synthetase in E. coli. The sequence of the amino-terminal 35 residues of purified S-adenosylmethionine synthetase localizes the beginning of the coding region of the DNA. The open reading frame extends 1152 bases and codes for a 384-residue protein of Mr = 41,941. The gene is transcribed clockwise on the E. coli chromosome. The DNA region 5' to the coding region was found to contain symmetrical sequences suggestive of operator structures and homologous to sequences upstream from other met genes sharing the same regulatory mechanism.