Initial cos cleavage of bacteriophage λ concatemers requires proheads and gpFI in in vivo

Abstract

The development of bacteriophage λ and double-stranded DNA viruses in general involves the convergence of two separate pathways: DNA replication and head assembly. Clearly, packaging will proceed only if an empty capsid shell, the prohead, is present to receive the DNA, but genetic evidence suggests that proheads play another role in the packaging process. For example, λ phages with an amber mutation in any head gene or in Fl, the gene encoding the accessory packaging protein gpFl, are able to produce normal amounts of DNA concatemers but they are not cut, or matured, into unit length chromosomes for packaging. Similar observations have been made for herpes simplex 1 virus. In the case of λ, a negative model proposes that in the amber phages, unassembled capsid components are inhibitory to maturation, and a positive model suggests that assembled proheads are required for cutting. We tested the negative model by using a deletion mutant devoid of all prohead genes and Fl in an in vivo cos cleavage assay; in this deleted phage, the cohesive ends were not cut. When λ proheads and gpFl were provided in vivo via a second prophage, cutting was restored, and gpFl was required, results that support the positive model. Phage 21 is a sister phage of λ, and although its capsid proteins share ∼60% residue Identity with λ's, phage 21 proheads did not restore cutting, even when provided with the accessory protein gpFl. Models for the role of proheads and gpFl in cos cutting are discussed.