The ICP22 protein of Herpes Simplex Virus 1 promotes RNA Polymerase II activity on Viral Immediate Early Genes

To determine the role of herpes simplex virus (HSV-1) ICP22 in viral transcription we performed precise nuclear run-on followed by deep sequencing (PRO-Seq) to map active RNA polymerase II (Pol II) on viral and cellular genomes in cells infected with a viral mutant lacking the entire ICP22-encoding α22 (US1/US1.5) gene, or a virus derived from the deletion mutant but bearing a restored α22 gene. At 3 hours post infection (hpi), the lack of ICP22 reduced Pol II activity at promoter proximal pause (PPP) sites on the α4 and α0 genes, and on the bodies of the α4, α0, and α27 genes. The decreased activity at α0 and α4 PPP sites at 3 hpi was distinguishable from effects caused by treatment with a viral DNA polymerase inhibitor. The ICP22 mutant had multiple defects at 6 hpi, including lower viral DNA replication and reduced Pol II activity on viral genes of all temporal classes. Between 3 and 6 hpi the repair virus, like wild type HSV-1, redirected Pol II activity from cellular genes to viral genes. In the absence of ICP22, the opposite occurred inasmuch as Pol II activity returned from the viral genome to cellular genes. These data indicate that ICP22 acts to increase Pol II activity at the PPP sites and bodies of viral immediate early genes at early times post infection, and directly or indirectly helps retain Pol II activity on the viral genome later in infection. Importance Using a mutant lacking the full US1/US1.5 gene, this study establishes a role for ICP22 in increasing Pol II activity on viral immediate early genes by enhancing transcription initiation and/or elongation onto immediate early gene bodies. Unlike truncation mutants studied previously, the full null virus is unable to sustain DNA replication and Pol II activity on late genes, precluding later stages of the viral life cycle. ### Competing Interest Statement JDB is a Scientific Advisory Board Member for Virios, which neither reviewed nor funded the current study.