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

Abstract

There are two modes of bacteriophage k DNA replication following infection of its host, Escherichia coli. Early after infection, replication occurs according to the theta (teta or circle-to-circle) mode, and is later switched to the sigma (sigma 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 wildtype cells the replication starting from orik proceeds both bidirectionally and unidirectionally, whereas in bacteria devoid of a functional DnaA protein, replication from orik is predominantly unidirectional. The regulation of directionality of replication from orik is mediated by positive control of lambda pR promoter activity by DnaA, since the mode of replication of an artificial lambda replicon bearing the ptet promoter instead of pR was found to be independent of DnaA function. These findings and results of density-shift experiments suggest that in dnaA mutants infected with lambda, phage DNA replication proceeds predominantly according to the unidirectional teta mechanism and is switched early after infection to the sigma mode. It is proposed that in wild-type E. coli cells infected with lambda, phage DNA replication proceeds according to a bidirectional teta mechanism early after infection due to efficient transcriptional activation of ori lambda, stimulated by the host DnaA protein. After a few rounds of this type of replication, the resulting increased copy number of teta genomic DNA may cause a depletion of free DnaA protein because of its interaction with the multiple DnaA-binding sites in lambda DNA. It is proposed that this may lead to inefficient transcriptional activation of orik resulting in unidirectional teta replication followed by sigma type replication.