A DNA Fragment Containing the Origin of Replication of the Escherichia coli Chromosome

A 38 kilobase pair region of the Escherichia coli K12 chromosome containing the replication origin has been physically mapped with restriction endonucleases EcoRI and HindIII. Replication starts within or very near a 1.3 kilobase pair HindIII fragment in the middle of this region and proceeds outward in both directions with apparently equal speed. This pattern was observed in both 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.