Host virus interactions in the initiation of bacteriophage λ DNA replication: Recruitment of Escherichia coli DnaB helicase by λ P replication protein

The bacteriophage λ P protein promotes replication of the phage chromosome by recruiting a key component of the cellular replication machinery to the viral origin. Specifically, P protein delivers one or more molecules of Escherichia coli DnaB helicase to a nucleoprotein structure formed by the λ O initiator at the λ replication origin. Using purified proteins, we have examined the features of the pivotal host virus interaction between P and DnaB. These two proteins interact in vitro to form a P DnaB protein complex that can be resolved by sedimentation or by chromatography on DEAE-cellulose from the individual free proteins. The sedimentation coefficient of the P·DnaB complex, 13 S, suggests a size larger than that of free DnaB hexamer (M_r = 313,600). The P·DnaB complex isolated by glycerol gradient sedimentation contains approximately three protomers of P/DnaB hexamer, consistent with a molecular weight of 393,000. The isolated P· DnaB complex functions in vitro in the initiation of λ DNA replication. Interaction of P with DnaB strongly suppressed both the intrinsic DN A-dependent ATPase activity of DnaB, as well as the capacity of DnaB to assist E. coli primase in the general priming reaction. Formation of a P· DnaB protein complex also blocked DnaB from functioning in the initiation of E. coli DNA replication in vitro. The physical and functional properties of λ P protein suggest that it is a viral analogue of the E. coli DnaC replication protein. Like P, DnaC also binds to DnaB (Wickner, S., and Hurwitz, J. (1975) Proc. Natl. Acad. Sci. U. S. A. 72, 921-925), but unlike P, DnaC stimulates DnaB-mediated general priming. When viral P and bacterial DnaC replication proteins were placed in direct competition with one another for binding to DnaB, the viral protein was clearly predominant. For example, a 5-fold molar excess of DnaC protein only partially reversed the inhibitory effect of P on general priming. Furthermore, when a preformed DnaC DnaB protein complex was incubated briefly with P protein, it was readily converted into a P·DnaB protein complex and the bulk of the bound DnaC was released as free protein. It is likely that the capacity of the λ P protein to outcompete the analogous host protein for binding to the bacterial DnaB helicase is the critical molecular event enabling infecting phage to recruit cellular replication proteins required for initiation of DNA synthesis at the viral origin.