Previous experiments have indicated that the gam gene of bacteriophage λ is responsible for an inhibition of the RecBC DNase—an enzyme that is essential for the major host pathway of genetic recombination. We report here experiments that define the inhibitor as the protein product of the gam gene (“γ-protein”) and that characterize the inhibition reaction with highly purified preparations of γ-protein and RecBC DNase. Genetic characterization was performed with partially purified fractions prepared from cells infected with various λ mutants. An activity that inhibits RecBC DNase was absent in extracts prepared after infection by phage that carry nonsense or deletion mutations in the gam gene; this activity was highly thermolabile in an extract prepared after infection by phage that carry a temperature-sensitive mutation in the gam gene. For biochemical characterization, the γ-protein has been purified more than 800-fold. This highly purified preparation inhibited all of the known catalytic activities associated with the RecBC enzyme, but exhibited no detectable DNase or ATPase activities by itself. These findings are discussed in terms of their implications for regulation of genetic recombination and bacteriophage λ development.

Sakaki, Y., Karu, A. E., Linn, S., & Echols, H. (1973). Purification and Properties of the γ-Protein Specified by Bacteriophage λ: An Inhibitor of the Host RecBC Recombination Enzyme. Proceedings of the National Academy of Sciences, 70(8), 2215–2219.