Chromatinisation of herpesvirus genomes

The double-stranded DNA genomes of herpesviruses exist in at least three alternative global chromatin states characterised by distinct nucleosome content. When encapsidated in virus particles, the viral DNA is devoid of any nucleosomes. In contrast, within latently infected nuclei herpesvirus genomes are believed to form regular nucleosomal structures resembling cellular chromatin. Finally, during productive infection nuclear viral DNA appears to adopt a state of intermediate chromatin formation with irregularly spaced nucleosomes. Nucleosome occupancy coupled with posttranslational histone modifications and other epigenetic marks may contribute significantly to the extent and timing of transcription from the viral genome and, consequently, to the outcome of infection. Recent research has provided first insights into the viral and cellular mechanisms that either maintain individual herpesvirus chromatin states or mediate transition between them. Here, we summarise and discuss both early work and new developments pointing towards common principles pertinent to the dynamic structure and epigenetic regulation of herpesvirus chromatin. Special emphasis is given to the emerging similarities in nucleosome assembly and disassembly processes on herpes simplex virus type 1 and human cytomegalovirus genomes over the course of the viral productive replication cycle and during the switch between latent and lytic infectious stages.