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In theEscherichia coli phage-plasmid P4, two partially overlapping replicons with bipartiteori sites coexist. The essential components of theoriI replicon are the α andcnr genes and theori1 andcrr sites; theoriII replicon is composed of the α gene, with the internalori2 site, and thecrr region. The P4 α protein has primase and helicase activities and specifically binds type I iterons, present inori1 andcrr . Using a complementation test for plasmid replication, we demonstrated that the two replicons depend on both the primase and helicase activities of the α protein. Moreover, neither replicon requires the host DnaA, DnaG, and Rep functions. The bipartite origins of the two replicons share thecrr site and differ forori1 andori2 , respectively. By deletion mapping, we defined the minimalori1 andori2 regions sufficient for replication. Theori1 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 inactivatedoriI . Moreover, insertion of 6 or 10 bp within theori1 region also abolished replication ability, suggesting that the relative arrangement of the iterons is relevant. Theori2 site was limited to a 36-bp P4 region that does not contain type I iterons. In vitro, the α protein did not bindori2 . Thus, the α protein appears to act differently at the two origins of replication.

Characterization of the oriI and oriII Origins of Replication in Phage-Plasmid P4