Only oriC and its flanking region are recovered from the complex formed at the time of initiation of chromosome replication in Escherichia coli.

The oriC region of Escherichia coli constructs a specific complex to associate with the outer membrane fraction (oriC complex). The oriC complex was periodically formed before as well as in the short period after initiation of DNA replication. Using the DNA extracted from outer membrane fractions of the cells just after initiation as a probe, the whole E. coli genomic library was assayed by plaque hybridization. DNA regions that hybridized with the probe corresponded to about a 100-kb length of chromosome DNA which included oriC. In addition, clones located in a counterclockwise direction from oriC were more preferentially hybridized among these positive clones. Thus, we conclude that the oriC region is a unique locus of the chromosome that binds to the outer membrane at the time of initiation of chromosome replication.