Figure 4.

Both HSV-1 and IAV block host transcription. Transcription by RNA polymerase II (Pol II) consists of multiple steps: recruitment to the DNA, initiation, mediated by phosphorylation of the serine 5 (Ser5) of the C-terminal domain (CTD) of the large subunit of the Pol II complex by TFIIH, and elongation, mediated by phosphorylation of Ser2 of the CTD by cyclin-dependent kinase 9 (CDK9). Both ICP27 and the IAV RNA-dependent RNA polymerase cause ubiquitination (Ub) and proteasome-mediated degradation of Pol II, but may associate with different phosphorylated forms of the complex. HSV also blocks Pol II recruitment to the DNA via unknown factors and CDK9 activity via ICP22.