Equine histones are mobilized within equid alphaherpesvirus 1 (EHV1) replication compartments

Equid alphaherpesvirus 1 (EHV1) is a DNA virus that causes severe disease in equids. Some strains are neurotropic and cause disease in the central nervous system, whereas others are non-neurotropic and can cause negative reproductive outcomes. Mechanisms governing EHV1 pathotype are not understood. However, EHV1 pathotypes have different infection efficiencies and replication kinetics in various cell types. They are also differentially susceptible to epigenetic inhibitors. Aside from this observation little is known about EHV1 chromatin or how its regulation influences infection. To build knowledge of EHV1 chromatin, we characterized equine histone mobility during EHV1 infection of equine cells using fluorescence recovery after photobleaching. We show that non-neurotropic or neurotropic EHV1 equally mobilized canonical (H2A, H2B, H3.1, and H4) and variant (H2A.B, H2A.Z, H2A.X, macroH2A, and H3.3) core and linker H1.2 histones. EHV1 mobilized dynamic histone populations by increasing their free pools and fast chromatin exchange. With the exception of H2A.B, all histones were most mobile within EHV1 replication compartments where EHV1 chromatin is enriched. Such mobility is consistent with highly dynamic viral chromatin. Distinct histone mobilities within EHV1- or infected-cell chromatin also indicate that different mechanisms regulate either chromatin. EHV1 mobilization of histones is similar to that reported for herpes simplex virus 1 (HSV1). This unique chromatin regulatory strategy is thus conserved among alphaherpesviruses in the Varicellovirus and Simplexvirus genera. Importantly, this conservation highlights histone mobilization as a robust chromatin regulatory strategy to promote viral genome accessibility during infection of evolutionarily distinct species.IMPORTANCEDNA viruses are subject to chromatin regulation of their gene expression. Understanding how viruses overcome genome silencing or promote the expression of their genes is important to understand how viruses take over host cells and establish productive infection. We show that EHV1 robustly mobilizes histones within nuclear domains enriched in viral chromatin. Histone mobilization would destabilize chromatin and is consistent with the assembly of dynamic or unstable EHV1 nucleosomes. Histone mobilization is a phenomenon first described for herpes simplex virus 1 (HSV1). Thus, destabilization of chromatin by mobilizing histones is conserved across the Varicellovirus and Simplexvirus genera of alphaherpesviruses. Although this unique chromatin regulatory approach is conserved, we identified differences in histone mobilization by either virus. Knowledge of how alphaherpesviruses mobilize histones during the infection of evolutionarily distinct species will increase our understanding of viral chromatin regulation and support the development of novel therapeutics to silence viral genomes.