RESOLUTION OF AN EARLY RECA-RECOMBINATION INTERMEDIATE BY A JUNCTION-SPECIFIC ENDONUCLEASE

The nucleoprotein filament formed on a circular single strand by Esche richia coli RecA protein in vitro can pair with homologous duplex DNA even when the latter lacks a free homologous end, but subsequent progr ession of the reaction through strand exchange requires an end in at l east one strand of the duplex DNA, We purified from E. coli an endonuc lease activity that cleaves the outgoing strand of duplex DNA at the j unction of homologous and heterologous sequences in three-stranded Rec A-recombination intermediates, This endonuclease activity also cleaves specifically at the junctions of duplex and single-stranded regions i n synthetic double-stranded oligonucleotides whose central portion con sists of unpaired heterologous sequences, These activities are consist ent with a role in recombination and repair of DNA.