REPLICATION OF COLIPHAGE-LAMBDA DNA

A general scheme of lambda phage and plasmid DNA replication in Escher ichia coli is presented, and results of in vivo experiments from the a uthors' laboratory are superimposed. The initiator lambda O functions in the assembly of the replication complex (RC) at ori lambda, making it a stable component of this structure. ClpP/ClpX protease-specific a ction on lambda O does not affect the regulation of replication; it on ly degrades the surplus of synthesized lambda O. The initiator AO beco mes protected from proteolysis at a distinct step of the pathway of RC assembly. The host DnaA initiator-regulated transcriptional activatio n of ori lambda seems to be coupled with RC assembly at the step of ch aperone-mediated rearrangement of the pre-primosome. The once-assemble d RC is inherited by one of two lambda plasmid daughter copies at each round of circle-to-circle (theta) replication. The inherited, old RC- driven replication is also dependent on RNA polymerase and DnaA functi ons. It seems that DnaA licenses lambda plasmid DNA for only one repli cation round, resembling the putative eukaryotic licensing factor in t his respect. The lambda O binding to ori lambda does not seem to play any role in regulation of lambda plasmid replication, and the Cro-auto regulatory loop may be deleted. The emerging picture shows lambda plas mid circles with RCs bound to their ori, awaiting a signal triggering initiation of replication. The host DnaA initiator-regulated transcrip tional activation of ori lambda may be involved in signal transmission . Inactivation of DnaA function blocks initiation of lambda phage DNA replication, but the lambdoid prophage Rac compensates this defect and all parental phage DNA molecules, after one round of theta replicatio n switch to the sigma mode and produce progeny in high yield. We suspe ct that DnaA-regulated transcriptional activation is involved in insta llation and adequate positioning of two RCs, required for bidirectiona l replication, but in the Rac-promoted process only one RC may be inst alled, leading to unidirectional replication continued in the sigma mo de. In wild-type cells consumption of DnaA function by the rapidly rep licating lambda phage DNA may switch replication from bidirectional th eta to unidirectional theta, and later to the sigma mode; the lambda c ircles produced earlier may play the role of Rac, which is required on ly when DnaA function has been inactivated prior to phage infection.