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

Information on the role of λ exonuclease in genetic recombination has been sought by testing the enzymic activity on certain internal sites in DNA as well as termini. Large excesses of λ exonuclease failed to release 32P from 5'-phosphoryl-labeled nicks or to attack the hydrogen-bonded circular form of λ DNA (Hershey circle) which contains two intrinsic nicks. This was true at pH 7.5 or 9.6, and was unaffected by the addition of β protein. λ exonuclease did not readily reinitiate digestion at a terminal gap produced by prior digestion of phage P22 DNA by the same enzyme. At pH 7.5, λ exonuclease did not detach from a molecule of DNA until the susceptible portion of that molecule was largely degraded.