Herpes Simplex Virus 1 DNA Polymerase RNase H Activity Acts in a 3′-to-5′ Direction and Is Dependent on the 3′-to-5′ Exonuclease Active Site

The catalytic subunit (Pol) of herpes simplex virus 1 (HSV-1) DNA polymerase has been extensively studied both as a model for other family B DNA polymerases and for its differences from these enzymes as an antiviral target. Among the activities of HSV-1 Pol is an intrinsic RNase H activity that cleaves RNA from RNA-DNA hybrids. There has long been a controversy regarding whether this activity is due to the 3′-to-5′ exonuclease of Pol or whether it is a separate activity, possibly acting on 5′ RNA termini. To investigate this issue, we compared wild-type HSV-1 Pol and a 3′-to-5′ exonuclease-deficient mutant, D368A Pol, for DNA polymerase activity, 3′-to-5′ exonuclease activity, and RNase H activity in vitro . Additionally, we assessed the RNase H activity using differentially end-labeled templates with 5′ or 3′ RNA termini. The mutant enzyme was at most modestly impaired for DNA polymerase activity but was drastically impaired for 3′-to-5′ exonuclease activity, with no activity detected even at high enzyme-to-DNA substrate ratios. Importantly, the mutant showed no detectable ability to excise RNA with either a 3′ or 5′ terminus, while the wild-type HSV-1 Pol was able to cleave RNA from the annealed RNA-DNA hairpin template, but only detectably with a 3′ RNA terminus in a 3′-to-5′ direction and at a rate lower than that of the exonuclease activity. These results suggest that HSV-1 Pol does not have an RNase H separable from its 3′-to-5′ exonuclease activity and that this activity prefers DNA degradation over degradation of RNA from RNA-DNA hybrids. IMPORTANCE Herpes simplex virus 1 (HSV-1) is a member of the Herpesviridae family of DNA viruses, several of which cause morbidity and mortality in humans. Although the HSV-1 DNA polymerase has been studied for decades and is a crucial target for antivirals against HSV-1 infection, several of its functions remain to be elucidated. A hypothesis suggesting the existence of a 5′-to-3′ RNase H activity intrinsic to this enzyme that could remove RNA primers from Okazaki fragments has been particularly controversial. In this study, we were unable to identify RNase H activity of HSV-1 DNA polymerase on RNA-DNA hybrids with 5′ RNA termini. We detected RNase H activity on hybrids with 3′ termini, but this was due to the 3′-to-5′ exonuclease. Thus, HSV-1 is unlikely to use this method to remove RNA primers during DNA replication but may use pathways similar to those used in eukaryotic Okazaki fragment maturation.