Transcriptional activation of the origin of coliphage λ DNA replication is regulated by the host DnaA initiator function

Abstract

The initiator of phage λ DNA replication, the λ O protein, is considered to be an analogue of the initiator of DNA replication (DnaA) of its host, Escherichia coli. Both specifically recognize their origins of replication, oriλ and oriC, respectively, and organize the assembly of specific replication complexes. However, DnaA has an additional activation function, acting on oriC-proximal DnaA-boxes, and regulating transcription initiated at promoters in and around oriC. Here, we demonstrate that λ plasmid replication can be synchronized by a temperature shift-down that caused renaturation of the previously denatured DnaAts protein. Moreover, we show that elimination of the activating DnaA function affects transcriptional activation at oriλ. DnaA may act by binding to DnaA-boxes, situated around the λ p R promoter; there are no such sequences in oriλ. Our results begin to explain in molecular terms why λ plasmid replication is DnaA-dependent [Kur et al., J. Mol. Biol. 198 (1987) 203–210] and why the initiation of phage λ DNA replication is blocked (in E. coli devoid of prophage Rac) after inactivation of DnaA [Wggrzyn et al., Genetics (1995) in press]