Further tests of a recombination model in which χ removes the RecD subunit from the RecBCD enzyme of Escherichia coli

When one of two infecting λ phage types in a replication-blocked cross is χ+ and DNA packaging is divorced from the RecBCD-χ interaction, complementary χ-stimulated recombinants are recovered equally in mass lysates only if the χ+ parent is in excess in the infecting parental mixture. Otherwise, the χ0 recombinant is recovered in excess. This observation implies that, along with the χ0 chromosome, two χ+ parent chromosomes are involved in the formation of each χ+ recombinant. The trimolecular nature of χ+-stimulated recombination is manifest in recombination between λ and a plasmid. When λ recombines with a plasmid via the RecBCD pathway, the resulting chromosome has an enhanced probability of undergoing λ×λ recombination in the interval into which the plasmid was incorporated. These two observations support a model in which DNA is degraded by Exo V from cos, the sequence that determines the end of packaged λ DNA and acts as point of entry for RecBCD enzyme, to χ, the DNA sequence that stimulates the RecBCD enzyme to effect recombination. The model supposes that χ acts by ejecting the RecD subunit from the RecBCD enzyme with two consequences. (1) ExoV activity is blocked leaving a highly recombinagenic, frayed duplex end near χ, and (2) as the enzyme stripped of the RecD subunit travels beyond χ it is competent to catalyze reciprocal recombination