Junction-resolving enzyme that selectively binds and cleaves four-way (Holliday) DNA junctions present after viral genomic replication. These intermediates are created during DNA repair, processing of stalled replication forks and homologous genetic recombination. Introduces two nicks on the two non-crossing strands, at 5' sides of the junction. Participates also together with gp6 in the degradation of host chromosome to provide nucleotides for phage DNA synthesis. Endonucleolytic cleavage to 5'-phosphooligonucleotide end-products. Homodimer.