Early replicative intermediates of Escherichia coli chromosome isolated from a membrane complex

The replication origin region of the Escherichia coli chromosome was isolated from an outer membrane fraction. The chromosome was further purified by centrifugation in a cesium chloride gradient. Early replicative intermediates were enriched in the preparation when cytosine‐1‐beta‐arabinofuranoside was added to the culture at the time of initiation of chromosome replication. DNA fragments with an eye structure having two branches of less than 400‐500 bp in length were associated with components that were removed by phenol treatment. We conclude that the replication fork usually proceeds counter‐clockwise toward the unc operon in the earliest period of replication.