Role of exonuclease and beta protein of phage lambda in genetic recombination. V. Recombination of lambda DNA in vitro.

The sequential action of lambda exonuclease and polynucleotide ligase upon redundant joint molecules is sufficient to produce intact polynucleotide chains and heat-stable, biologically active molecules of lambda DNA, whereas the action of ligase alone is insufficient. These results (a) confirm the previously described mechanism of single-strand assimilation, including a subsidiary mechanism by which the further action of lambda exonuclease is arrested when a redundant strand is completely assimilated, and (b) represent a simulation of the steps in genetic recombination that follow the formation of biparental complexes (synapsis). lambda exonuclease is postulated to catalyze a concerted reaction that includes exposure of complementary sequences, formation of heteroduplex regions, and elimination of redundant branches.