Origin and Bidirectional Orientation of DNA Replication in Coliphage Lambda

Abstract

In the lysogenic, integrated state coliphage λ DNA is replicated as a part of the bacterial chromosome. After prophage induction or after infection, replication of λ DNA becomes autonomous and much more rapid than that of the host genome.

According to current evidence, several elements are required for autonomous λ DNA replication. These include (1) at least part of the host replication system, (2) the products of λ genes O and P, (3) the site on the λ genome where replication originates and which will be designated as the origin of replication (ori) or the replicator, and (4) activation of the replicator, which requires rightward transcription of any part of the short λ DNA segment containing the ori site (see Thomas and Bertani, 1964; Dove et al., 1969, and this volume). The first two requirements can be satisfied in trans, whereas requirements (3) and (4) can be fulfilled only in cis.

The present study was undertaken to determine the molecular location of the origin of autonomous λ DNA replication (ori), the orientation of the in situ replication of the induced but not excised λ prophage, and the effect of mutations in the phage or host genome on λ DNA replication. It was found that (a) the ori site is located just to the left of gene O at approximately 80.2 ± 0.5% of the λ genome length (measured from the left terminus of the mature phage DNA), and (b) the orientation of replication is bidirectional, proceeding both to the left...