In the Escherichia coli phage-plasmid P4, two partially overlapping replicons with bipartite ori sites coexist. The essential components of the oriI replicon are the  and cnr genes and the ori1 and crr sites; the oriII replicon is composed of the  gene, with the internal ori2 site, and the crr region. The P4  protein has primase and helicase activities and specifically binds type I iterons, present in ori1 and crr. Using a complementation test for plasmid replication, we demonstrated that the two replicons depend on both the primase and helicase activities of the  protein. Moreover, neither replicon requires the host DnaA, DnaG, and Rep functions. The bipartite origins of the two replicons share the crr site and differ for ori1 and ori2, respectively. By deletion mapping, we defined the minimal ori1 and ori2 regions sufficient for replication. The ori1 site was limited to a 123-bp region, which contains six type I iterons spaced regularly close to the helical periodicity, and a 35-bp AT-rich region. Deletion of one or more type I iterons inactivated oriI. Moreover, insertion of 6 or 10 bp within the ori1 region also abolished replication ability, suggesting that the relative arrangement of the iterons is relevant. The ori2 site was limited to a 36-bp P4 region that does not contain type I iterons. In vitro, the  protein did not bind ori2. Thus, the  protein appears to act differently at the two origins of replication.