The Herpes Simplex Virus Type 1 Origin-binding Protein

Origin-dependent replication of the herpes simplex virus type 1 genome requires the virally encoded origin-binding protein, UL9. UL9 binds specifically to the herpes simplex virus type 1 replication origin at two high affinity binding sites on the DNA, Boxes I and II. UL9 also has ATP-dependent DNA helicase and DNA-stimulated ATPase activities that are used to unwind the origin DNA. Origin-specific binding is mediated by the C-terminal domain (C-domain) of the enzyme. ATPase and helicase activities are mediated by the N-terminal domain (N-domain). Previous studies have shown that single-stranded DNA is a good coeffector for ATPase activity. We have analyzed several DNAs for their ability to stimulate the ATPase activity of UL9 and of a truncated UL9 protein (UL9/N) consisting only of the N-domain. We report here that duplex Box I DNA specifically and potently stimulates the ATPase activity of UL9 but not of UL9/N. We also find that removal of the C-domain significantly increases the ATPase activity of UL9. We have incorporated these results into a model for initiation in which the C-domain of UL9 serves to regulate the enzymatic activity of the N-domain.