The DNA sequence of the Salmonella typhimurium metC gene and its flanking regions was determined. The metC gene contains an open reading frame of 1185 nucleotides encoding a polypeptide of 395 amino acids with a predicted molecular weight of 42,874 daltons. S1 nuclease mapping experiments located the transcription start site of the metC gene. The nucleotide sequence and the deduced amino acid sequence for the metC genes of S. typhimurium and Escherichia coli were compared. Although there are 279 nucleotide replacements, most do not change the amino acid sequence. Nucleotide sequence analysis of the flanking regions of the S. typhimurium metC gene shows that there is an open reading frame upstream and an open reading frame downstream of the gene. The existence of the divergently transcribed upstream open reading frame (designated ORF1) was confirmed by the construction of an ORF1-lacZ fusion. The transcription start site of ORF1 was determined by S1 nuclease mapping.