INTEGRATION HOST FACTOR OF ESCHERICHIA-COLI REVERSES THE INHIBITION OF R6K PLASMID REPLICATION BY PI-INITIATOR PROTEIN

Integration host factor (IHF) protein is the host-encoded protein known to bind and affect replication of the gamma-origin of Escherichia coli plasmid R6K. We examined the ability of R6K origins to replicate in cells lacking either of the two subunits of IHF. As shown previously, the gamma-origin cannot replicate in IHF-deficient cells. HOwever, this inability to replicate was relieved under the following conditions: underproduction of the wild-type-pi-replication protein of R6K or production of normal levels of mutant-pi-proteins which exhibit relaxed replication control. The copy number of plasmids containing the primary R6K origins (alpha and beta) is substantially reduced in IHF-deficient bacteria. Furthermore, replication of these plasmids is completely inhibited if the IHF-deficients strains contain a helper plasmid producing additional wild-type-pi-protein. IHF protein has previously been shown to bind to two sites within the gamma-origin. These sites flank a central repeat segment which binds-pi-protein. We propose a model in which IHF binding to its sites reduces the replication inhibitor activity of pi-protein at all three R6K origins.