Regulation of the switch from early to late bacteriophage λ DNA replication

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Abstract
There are two modes of bacteriophage λ DNA replication following infection of its host, Escherichia coli. Early after infection, replication occurs according to the theta (θ or circle-to-circle) mode, and is later switched to the sigma (σ or rolling-circle) mode. It is not known how this switch, occurring at a specific time in the infection cycle, is regulated. Here it is demonstrated that in wild-type cells the replication starting from oriλ proceeds both bidirectionally and unidirectionally, whereas in bacteria devoid of a functional DnaA protein, replication from oriλ is predominantly unidirectional. The regulation of directionality of replication from oriλ is mediated by positive control of λ p R promoter activity by DnaA, since the mode of replication of an artificial λ replicon bearing the p tet promoter instead of p R was found to be independent of DnaA function. These findings and results of density-shift experiments suggest that in dnaA mutants infected with λ, phage DNA replication proceeds predominantly according to the unidirectional θ mechanism and is switched early after infection to the σ mode. It is proposed that in wild-type E. coli cells infected with λ, phage DNA replication proceeds according to a bidirectional θ mechanism early after infection due to efficient transcriptional activation of oriλ, stimulated by the host DnaA protein. After a few rounds of this type of replication, the resulting increased copy number of λ genomic DNA may cause a depletion of free DnaA protein because of its interaction with the multiple DnaA-binding sites in λ DNA. It is proposed that this may lead to inefficient transcriptional activation of oriλ resulting in unidirectional θ replication followed by σ type replication.