Functions as a general transcriptional regulator of cellular and viral mRNAs mainly by mediating changes on the host RNA polymerase II. One change, which is UL13 independent, is the rapid loss of Pol II forms bearing Ser-2 phosphorylation. A second change, which is UL13 dependent, is the appearance of an intermediate form of Pol II that differs from the normal hypo- and hyperphosphorylated forms. These Pol II modifications immediately inhibit host genome transcription, leading to cell cycle deregulation and loss of efficient antiviral response. Recruits also cellular transcription elongation factors to viral genomes for efficient transcription elongation of viral genes. Interacts with host FACT complex members SSRP1 and SUPT16H; these interactions are required for the relocalization of the FACT complex in infected cells and its association with viral genomes. Localizes in small nuclear bodies early in infection then moves to a more diffuse distribution in viral compartments as infection progresses. Additional isoforms seem to exist. Tyrosine phosphorylated. ICP22 and protein US1.5 mRNAs are transcribed from two different promoters on the US1 gene. Belongs to the herpesviridae ICP22 family.