Herpes simplex virus 1 ICP8 mutant lacking annealing activity is deficient for viral DNA replication

Significance Herpes simplex virus (HSV) is a ubiquitous human pathogen. The mechanism of HSV DNA replication is poorly understood. ICP8, the multifunctional HSV single-strand DNA binding protein (SSB), is essential for viral growth and DNA replication. ICP8 is also capable of efficiently annealing complementary ssDNA and has been classified as a single-strand annealing protein (SSAP); however, the role of annealing during infection is still unclear. In this paper, we characterize a mutant of ICP8 that retains many of its functions as a replicative SSB but lacks annealing activity. We show that ICP8 annealing activity is essential for viral DNA replication. These findings support our hypothesis that HSV-1 uses recombination-dependent mechanisms to replicate its DNA.