The herpes simplex virus type 1 (HSV-1) alkaline nuclease, encoded by the UL12 gene, plays an important role in HSV-1 replication, as a UL12 null mutant displays a severe growth defect. The HSV-1 alkaline exonuclease UL12 interacts with the viral single-stranded DNA binding protein ICP8 and promotes strand exchange in vitro in conjunction with ICP8. We proposed that UL12 and ICP8 form a two-subunit recombinase reminiscent of the phage lambda Red ␣/␤ recombination system and that the viral and cellular recombinases contribute to viral genome replication through a homologous recombination-dependent DNA replication mechanism. To test this hypothesis, we identified cellular interaction partners of UL12 by using coimmunoprecipitation. We report for the first time a specific interaction between UL12 and components of the cellular MRN complex, an important factor in the ATM-mediated homologous recombination repair (HRR) pathway. This interaction is detected early during infection and does not require viral DNA or other viral or cellular proteins. The region of UL12 responsible for the interaction has been mapped to the first 125 residues, and coimmunoprecipitation can be abolished by deletion of residues 100 to 126. These observations support the hypothesis that cellular and viral recombination factors work together to promote efficient HSV-1 growth.The herpes simplex virus (HSV) genome replicates in the nucleus of an infected host cell, resulting in the production of longer-than-unit-length head-to-tail concatemers of viral DNA. Production of infectious virus requires the processing of concatemeric DNA into unit-length genomes by the packaging machinery followed by encapsidation into preassembled capsids (63). Several lines of evidence suggest that recombination plays a role in concatemer formation (69). Genomic inversions, proposed to occur through recombination, can be detected very early during infection, and these inversions require sequence homology (2,23,56). In addition, replication intermediates in HSV type 1 (HSV-1)-infected cells adopt a complex nonlinear structure that does not migrate in a pulsed-field gel, even after digestion with an enzyme that cuts once per unit length of the genome (1,33,51,71). Electron micrographs have revealed that replication intermediates are branched and contain Y-and X-shaped junctions (19,52). Furthermore, high levels of recombination have been reported not only between coinfecting viral strains (4, 17, 50, 62, 66) but also in plasmids containing repeated sequence elements (10, 11). These observations, taken together, are not consistent with a simple rolling circle mechanism of replication and suggest that, reminiscent of the bacteriophages T4 and lambda, HSV-1 utilizes a recombination-dependent replication mechanism to generate concatemeric viral DNA.We have previously reported that virus-encoded proteins are capable of participating in recombination events in vitro. The viral 5Ј-3Ј alkaline exonuclease (UL12) and the single-strand binding protein (ICP8) together mediate stra...