Abstract
A 38 kilobase [kb] pair region of the E. coli K12 chromosome containing the replication origin was physically mapped with restriction endonucleases EcoRI and HindIII. Replication starts within or very near a 1.3 kb pair HindIII fragment in the middle of this region and proceeds outward in both directions with apparently equal speed. This pattern was observed in dnaA and dnaC temperature-sensitive (ts) initiation mutants at the start of the synchronous round of replication which occurs after downshift from the nonpermissive to the permissive temperature.