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Virus DNA packaging: the strategy used by phage λ

Phage λ, like a number of other large DNA bacterio‐phages and the herpesviruses, produces concatemeric DNA during DNA replication. The concatemeric DNA is processed to produce unit‐length, virion DNA by cutting at specific sites along the concatemer. DNA cutting is coordinated with DNA packaging, the process of translocation of the cut DNA into the preformed capsid precursor, the prohead. A key player in the λ DNA packaging process is the phage‐encoded enzyme terminase, which is involved in (i) recognition of the concatemeric λ DNA; (ii) initiation of packaging, which includes the introduction of staggered nicks at cosN to generate the cohesive ends of virion DNA and the binding of the prohead; (iii) DNA packaging, possibly including the ATP‐driven DNA translocation; and (iv) following translocation, the cutting of the terminal cosN lo complete DNA packaging. To one side of cosN is the site cosB, which plays a role in the initiation of packaging; along with ATP, cosB stimulates the efficiency and adds fidelity to the endo‐nuclease activity of terminase in cutting cosN. cosB is essential for the formation of a post‐cleavage complex with terminase, complex I, that binds the prohead, forming a ternary assembly, complex II. Terminase interacts with cosN through its large subunit, gpA, and the small terminase subunit, gpNul, interacts with cosB. Packaging follows complex II formation. cosN is flanked on the other side by the site cosQ, which is needed for termination, but not initiation, of DNA packaging. cosQ is required for cutting of the second cosN, i.e. the cosN at which termination occurs. DNA packaging in λ has aspects that differ from other λ DNA transactions. Unlike the site‐specific recombination system of λ, for DNA packaging the initial site‐specific protein assemblage gives way to a mobile, translocating complete, and unlike the DNA replication system of λ, the same protein machinery is used for both initiation and translocation during λ DNA packaging.