The nucleotide sequence of a 3120 bp region of the E. coli chromosome that includes the entire ptr gene has been determined. The proposed coding region for Protease III is 2889 nucleotides long, which would encode a protein consisting of 962 amino acids with a calculated molecular mass of 107,719 daltons. The predicted primary structure of the protein includes a 23-residue signal sequence, cleavage of which would give rise to a mature protein of molecular mass 105,124 daltons. At its 3' end, the ptr gene overlaps the start of the recB coding sequence by 8 bases, suggesting that these genes may form part of an operon.