During lytic infection, herpes simplex virus (HSV) DNA is replicated by a mechanism involving DNA recombination. For instance, replication of the HSV-1 genome produces X-and Y-branched structures, reminiscent of recombination intermediates. HSV-1`s replication machinery includes a trimeric helicase/primase is composed of helicase (UL5) and primase (UL52) subunits and a third subunit, UL8. UL8 has been reported to stimulate the helicase and primase activities of the complex in the presence of ICP8, an HSV-1 protein that functions as an annealase, a protein that binds complementary single-stranded (ss)DNA and facilitates its annealing to duplex DNA. UL8 also influences the intracellular localization of the UL5/UL52 subunits, but UL8`s catalytic activities are not known. In this study, we used a combination of biochemical techniques and transmission electron microscopy. First, we report that UL8 alone forms protein filaments in solution. Moreover, we also found that, UL8 binds to ssDNAs >50 nt long and promotes the annealing of complementary ssDNA to generate highly branched duplex DNA structures. Finally, UL8 has a very high affinity for replication fork structures containing a gap in the lagging strand as short as 15 nt, suggesting that UL8 may aid in directing or loading the trimeric complex onto a replication fork. The properties of UL8 uncovered here suggest that UL8 may be involved in the generation of the X-and Y-branched structures that are the hallmarks of HSV replication.Herpes simplex virus type 1 (HSV-1) has a 152-kb linear double-stranded (ds) DNA genome. During HSV-1 infection, the viral genome is released from the capsid into the nucleus and is replicated to form concatemeric DNA that is recognized by the packaging machinery to produce infectious virus. Seven virus-encoded proteins are required for viral DNA synthesis including an origin binding protein (UL9) and six core replication proteins: a two subunit DNA polymerase (UL30 and UL42), a three subunit helicase/primase complex (UL5, UL8 and UL52) and a multifunctional ssDNA binding protein (ICP8) (reviewed in ref.1). The six purified core replication proteins can function to produce linear concatemers in vitro if provided a primed template (2). On the other hand, in vivo, late replicating viral DNA has been reported to adopt a mixture of complex structures such as X-and Y-shaped branches and tangled masses, suggesting that recombination may play a role in the replication of HSV-1 DNA (3-7). Additional support for this concept was provided by Blümel et al. who showed that when SV40 genomes were replicated by the six HSV-encoded replication factors and SV40 large T antigen, concatemers composed of X-shaped DNA structures were observed (8). Since SV40 replication normally produces two circular daughter molecules, the complex DNA generated in the presence of HSV replication proteins suggests that the HSV replication machinery is inherently recombinogenic. A better understanding of the functions and properties of the essential viral replicatio...