The Role of Exonuclease and β Protein of Phage λ in Genetic Recombination

The amount of λ exonuclease that binds to DNA is increased when single stranded interruptions are made in the DNA either by pancreatic DNase or by the decay of 3H label. Free ends of DNA are not required for binding, since λ exonuclease binds to the hydrogen-bonded circular form of λ DNA (Hershey circle) which has two intrinsic single stranded interruptions. The strength of binding to ends is greater than the binding to secondary sites: nicks introduced by pancreatic DNase provide neither new substrate nor sites that competitively inhibit the action of exonuclease at ends. The binding of λ exonuclease to DNA can also be shown by the trapping of DNA on nitrocellulose filters that is dependent on λ exonuclease. A preparation of β protein did not cause the trapping of DNA but appeared to augment the trapping caused by exonuclease.