Regulation in Phage with Duplications of the Immunity Region

Abstract

The autonomously replicating plasmid λdv is a DNA molecule about one-eighth the size of the λ virulent genome from which it was derived. Lambda dv contains only the immunity region of λ and the genes necessary for phage DNA synthesis (see Fig. 1). Cells carrying λdv appear immune to λ but are lysed by certain hybrid phage such as λimm²¹ (in which the immunity region of phage 21 has replaced that of λ). During λimm²¹ infection of λdv carrier cells, recombinants arise which carry genes originating from the λdv plasmid (Matsubara and Kaiser, 1968). The majority of these recombinants appear to have added a λdv unit to the genome of the infecting phage (Kellenberger-Gujer, this volume). The structure of these genomes has permitted an examination of the role of the λN gene product in phage multiplication. Before discussing the regulatory properties of the duplication phage, it will be necessary to describe their genetic structure.

THE FORMATION OF DUPLICATION PHAGE
Approximately 0.2% of the progeny phage from infection of cells carrying λdv with phage λimm²¹sus P are P⁺ recombinants. To show that recombination between an infecting phage and the λdv plasmid can add λdv to the phage genome, I used λb538imm²¹susP901, a 21% deletion relative to wild-type λ (λ⁺), to infect λdv carrier strain KM424, anticipating that the 21% deletion would compensate for the addition of λdv during packaging.

The density distribution of P⁻ recombinants from the λb538imm²¹susP901 phage infection of KM424 is given in Fig. 2. This is...