Implications of exonuclease activity of bacteriophage P2 old protein for lambda exclusion

Temperate bacteriophage P2 has a nonessential gene called old (overcoming lysoginization defection). In the presence of old, the growth of the host (Escherichia coli) with recBC^- genotype is inhibited, and another bacteriophage, lambda, cannot superinfect. The Old protein has been shown to possess an exonuclease activity. Three mutant P2s (old 1, old 17, old 49) which did not exhibit lambda exclusion were obtained and each old gene was cloned into expression vectors to produce hexahistidine-tagged proteins. The proteins were affinity-purified and shown to lose its exonuclease activity on both double-stranded and single-stranded DNA substrates. Thus, it was concluded that the lambda exclusion was related to Old's exonuclease activity.