DNA Packaging and Cutting

Abstract

INTRODUCTION
The head of bacteriophage λ is assembled when the products of two separate synthetic pathways come together. One, the DNA replication pathway, produces a linear polymer of λ chromosomes called a concatemer. In the other pathway, λ head proteins assemble to produce the empty capsid shell known as the prohead (Georgopoulos et al., this volume). DNA and proheads interact, and a chromosome from the concatemer is inserted into the cavity of the prohead and is separated from the unencapsidated remainder of the DNA by endonucleolytic cutting.

Knowledge of λ head assembly was presented by Yarmolinsky (1971) and by Kellenberger and Edgar (1971) in The Bacteriophage Lambda. The review of Hohn and Katsura (1977) offers a more recent summary of the subject. Casjens and King (1975), Murialdo and Becker (1978b), and Wood and King (1979) have written comparative reviews on head morphogenesis and DNA packaging. Most recently, the problem of DNA packaging in the double-stranded DNA phages has been updated in an elegant synthesis by Earnshaw and Casjens (1980). Because of space limitations in this paper, we restrict discussion of other viruses to results not emphasized in other reviews.

CHROMOSOME STRUCTURE AND DNA PACKAGING
The linear λ chromosome is injected into the cell, and annealing and ligation of the cohesive ends rapidly cyclize the molecule. Early in the growth cycle, bidirectional replication results in progeny rings. Later, rolling-circle replication produces the concatemers that are the substrates for DNA packaging (Skalka 1977; Furth and Wickner, this volume). DNA packaging generates mature...