DNA interactions during bacteriophage lambda site-specific recombination.

Extensive research on site-specific recombination has provided many details, particularly with respect to the protein-DNA interactions. However, very little is known about the molecular mechanism of recombination during synapsis and strand exchange. Presumably, these steps of recombination involve various forms of DNA-DNA, DNA-protein, and protein-protein interactions. One stage at which DNA-DNA interactions may be occurring is at the level of synapsis where the recombining DNAs are juxtaposed. In this paper we have presented evidence that homology-dependent DNA interactions do occur within the overlap region before strand exchange. This interaction is presumably at the synaptic stage of recombination. Furthermore, we have demonstrated that the homology-dependent interactions require that only one strand of attB have homology to attP. Another stage in recombination at which DNA-DNA interactions could occur is during strand exchange where complementary strands from the recombining parents are paired and resealed. We have also presented evidence that homology-dependent DNA interactions occur during strand exchange prior to the resealing of the strands and that disruption of this interaction results in nonreciprocal recombination. Taken together, these results suggest that DNA-DNA interactions during reciprocal site-specific recombination occur during at least two stages in the reaction.