Role of DNA topology in transcription of coliphage λ in vivo: II. DNA topology protects the template from exonuclease attack

Abstract

We have examined the ability of two topological forms of coliphage λ DNA template, a linear form and a circular form, to serve as a source of λ gene function in vivo. If infecting λ DNA enters an environment containing λ gam product, or an environment deficient in recBC nuclease activity, both linear (half DNA molecule) and circular (whole DNA molecule) templates efficiently provide λ gene function. Entry of λ DNA into an environment lacking λ gam product results in loss of template potential for the linear molecule in both rec + and recA hosts, while the template potential of the circular DNA molecule is unaffected. We conclude that (a) λ gam product protects linear λ DNA forms from attack by recBC nuclease; the Escherichia coli recA product affords no such protection for linear A DNA; (b) the role of template topology in λ messenger RNA synthesis is indirect but very essential. Although the linear molecule will suffice as template for transcription per se, circular topology is required for protection of the template from recBC nuclease activity until λ gam product protection is established.