3′ to 5′ Exonuclease Activity of Herpes Simplex Virus Type 1 DNA Polymerase Modulates Its Strand Displacement Activity

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Abstract
Using a minicircle DNA primer-template, the wild-type catalytic subunit of herpes simplex virus type 1 (HSV-1) DNA polymerase (pol) was shown to lack significant strand displacement activity with or without its processivity factor, UL42. However, an exonuclease-deficient (exo − ) pol (D368A) was capable of slow strand displacement. Although UL42 increased the rate (2/s) and processivity of strand displacement by exo − pol, the rate was slower than that for gap-filling synthesis. High inherent excision rates on matched primer-templates and rapid idling-turnover (successive rounds of excision and polymerization) of exo-proficient polymerases correlated with poor strand displacement activity. The results suggest that the exo activity of HSV-1 pol modulates its ability to engage in strand displacement, a function that may be important to the viability and genome stability of the virus.