Mutations resulting in a defective mini-F replicon in the presence of both origins, oriI and oriII.

Within the region of the origin of replication (oriII) of plasmid mini-F, we introduced a set of deletions around the XhoI site (45.080F) and an insertion of 4 nucleotides at the Bg/II site (45.213F). These alterations had the effect that replication from oriII as well as from the alternative origin, oriI, was abolished. The analysis suggests that an AT-rich region which may function as entry site for DnaB-DnaC protein is an essential structural requirement for replication of plasmid mini-F.