Transcription of the herpes simplex virus latency-associated transcript promotes the formation of facultative heterochromatin on lytic promoters.

An important question in virology is the mechanism(s) by which persistent viruses such as the herpesviruses and human immunodeficiency virus (HIV) establish a latent infection in specific types of cells. In the case of herpesviruses, herpes simplex virus (HSV) infection of epithelial cells results in a lytic infection, whereas latent infection is established in sensory neurons. Recent studies have shown the importance of chromatin structure in the regulation of latent infection for both HSV and HIV. For HSV, we have shown previously that the viral latency-associated transcript (LAT) promotes lytic gene silencing and the association of one heterochromatin marker, dimethylation of lysine 9 on histone H3 (H3K9me2), with viral lytic genes. In this study, we further defined the structure of latent viral chromatin by examining the heterochromatin markers on histones associated with the HSV latent genome. We detected the H3K9me2, H3K9me3, and H3K27me3 modifications, with H3K27me3, which is indicative of facultative heterochromatin, exhibiting the highest enrichment on all viral promoters tested. A modification associated with cellular centromeric heterochromatin, H4K20me3, was not detected. A mutant virus containing a 1.8-kbp deletion within the LAT region showed reduced levels of the facultative heterochromatin marker (H3K27me3) along with H3K9me3 during latency, whereas a viral mutant defective for the LAT promoter showed a specific reduction in H3K27me3. Cellular long, noncoding RNAs induce facultative heterochromatin, and this study shows that transcription of a viral noncoding RNA can also induce facultative heterochromatin to promote lytic gene silencing during latency.