During Lytic Infections, Herpes Simplex Virus Type 1 DNA is in Complexes with the Properties of Unstable Nucleosomes

The genomes of herpes simplex virus type 1 (HSV-1) are regularly chromatinized during latency such that their digestion with micrococcal nuclease (MCN) releases nucleosome-sized DNA fragments. In lytically infected cells, in contrast, MCN releases HSV-1 DNA in primarily heterogeneously sized fragments. Consistently, only a small percentage of this HSV-1 DNA coimmunoprecipitates with histones. Most current models propose that histones associate with HSV-1 DNA during lytic infections at low occupancy. However, histone modification or occupation is also proposed to regulate HSV-1 transcription. It remains unclear how the histones associated with a small percentage of HSV-1 DNA may regulate transcription globally. Moreover, the physical properties of the complexes containing histones and HSV-1 DNA are unknown. We evaluated the HSV-1 DNA-containing complexes at 5 h after (lytic) infection by biochemical fractionations. Nuclear HSV-1 DNA did not fractionate as protein-free HSV-1 DNA but as DNA in cellular nucleosomes. Moreover, MCN released HSV-1 DNA in complexes that fractionate as cellular mono- and dinucleosomes by centrifugation followed by sucrose gradients and size-exclusion chromatography. The HSV-1 DNA in such complexes was protected to heterogeneous sizes and was more accessible to MCN than DNA in most cellular chromatin. Using a modified MCN digestion to trap unstable digestion intermediates, HSV-1 DNA was quantitatively recovered in discrete mono- to polynucleosome sizes in complexes fractionating as cellular mono- to polynucleosomes. The HSV-1 DNAs in complexes fractionating as mono- to dinucleosomes were stabilized by cross-linking. Therefore, most HSV-1 DNA forms particularly unstable nucleosome-like complexes at 5 h of lytic infection.