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

There are two modes of bacteriophage lambda DNA replication following infec tion of its host, Escherichia coli. Early after infection, replication occu rs according to the theta (theta or circle-to-circle) mode, and is later sw itched to the sigma (sigma or rolling-circle) mode. It is not known how thi s switch, occurring at a specific time in the infection cycle, is regulated . Here it is demonstrated that in type cells the replication starting from ori lambda proceeds both bidirectionally and unidirectionally, whereas in b acteria devoid of a functional DnaA protein, replication from ori lambda is predominantly unidirectional. The regulation of directionality of replicat ion from ori lambda is mediated by positive control of lambda p(R) promoter activity by DnaA, since the mode of replication of an artificial lambda re plicon bearing the p(tet) promoter instead of p(R) was found to be independ ent of DnaA function. These findings and results of density-shift experimen ts suggest that in dnaA mutants infected with lambda, phage DNA replication proceeds predominantly according to the unidirectional theta mechanism and is switched early after infection to the sigma mode. It is proposed that i n wild-type E. coli cells infected with lambda, phage DNA replication proce eds according to a bidirectional theta mechanism early after infection due to efficient transcriptional activation of ori lambda, stimulated by the ho st DnaA protein. After a few rounds of this type of replication, the result ing increased copy number of lambda genomic DNA may cause a depletion of fr ee DnaA protein because of its interaction with the multiple DnaA-binding s ites in lambda DNA. It is proposed that this may lead to inefficient transc riptional activation of ori lambda resulting in unidirectional theta replic ation followed by sigma type replication.