ANNEALING VS. INVASION IN PHAGE-LAMBDA RECOMBINATION

Genetic recombination catalyzed by X's Red pathway was studied in rec( +) and recA mutant bacteria by examining both intracellular lambda DNA and mature progeny particles. Recombination of nonreplicating phage c hromosomes was induced by double-strand breaks delivered at unique sit es in vivo. In rec(+) cells, cutting only one chromosome gave nearly m aximal stimulation of recombination; the recombinants formed contained relatively short hybrid regions, suggesting strand invasion. In contr ast, in recA mutant cells, cutting the two parental chromosomes at non -allelic sites was required for maximal stimulation; the recombinants formed tended to be hybrid over the entire region between the two cuts , implying strand annealing. We conclude that, in the absence of RecA and the presence of non-allelic DNA ends, the Red pathway of lambda ca talyzes recombination primarily by annealing.