Replication of coliphage lambda DNA

Abstract

A general scheme of λ phage and plasmid DNA replication in Escherichia coli is presented, and results of in vivo experiments from the authors' laboratory are superimposed. The initiator λO functions in the assembly of the replication complex (RC) at oriλ, making it a stable component of this structure. ClpP / ClpX protease-specific action on λO does not affect the regulation of replication; it only degrades the surplus of synthesized λO. The initiator λO becomes protected from proteolysis at a distinct step of the pathway of RC assembly. The host DnaA initiator-regulated transcriptional activation of oriλ seems to be coupled with RC assembly at the step of chaperone-mediated rearrangement of the pre-primosome. The once-assembled RC is inherited by one of two λ plasmid daughter copies at each round of circle-to-circle (θ) replication. The inherited, old RC-driven replication is also dependent on RNA polymerase and DnaA functions. It seems that DnaA licenses λ plasmid DNA for only one replication round, resembling the putative eukaryotic licensing factor in this respect. The λO binding to oriλ does not seem to play any role in regulation of λ plasmid replication, and the Cro-autoregulatory loop may be deleted. The emerging picture shows λ plasmid circles with RCs bound to their ori, awaiting a signal triggering initiation of replication. The host DnaA initiator-regulated transcriptional activation of oriλ may be involved in signal transmission. Inactivation of DnaA function blocks initiation of λ phage DNA replication, but the lambdoid prophage Rac compensates this defect and all parental phage DNA molecules, after one round of θ replication switch to the σ mode and produce progeny in high yield. We suspect that DnaA-regulated transcriptional activation is involved in installation and adequate positioning of two RCs, required for bidirectional replication, but in the Rac-promoted process only one RC may be installed, leading to unidirectional replication continued in the σ mode. In wild-type cells consumption of DnaA function by the rapidly replicating λ phage DNA may switch replication from bidirectional θ to unidirectional θ, and later to the σ mode; the λ circles produced earlier may play the role of Rac, which is required only when DnaA function has been inactivated prior to phage infection.