A histone methylation code for SV40 minichromosomes

Chromatin structure and function is dynamically regulated, in part, by the various post-translational modifications on the DNA-associated histone proteins. These histone modifications can occur alone or in combination at specific loci to create a “histone code” that is directly associated with a distinct DNA-templated process such as transcription or replication.1 Although many studies throughout the last few decades have greatly increased our understanding of histone modification functions in eukaryotes, very little is known about the role of histone modifications within viral genomes. To address this, Milavetz and colleagues have been investigating alterations of histone modifications on the Sv40 minichromosome during lytic infection of mammalian cells.2 The circular double-stranded Sv40 DNA is chromatinized in both infected cells and virions offering a relatively simple but powerful tool for examining histone modifications during viral infection. in this issue of Cell Cycle, Balakrishnan et al. continue to employ the Sv40 minichromosome to investigate a specific histone modification, H4 lysine 20 (H4K20) methylation, during lytic infection. Lysine residues can accept up to three methyl groups and, therefore, can be mono-(me1), di- (me2) or trimethylated (me3).3 Using antibodies that selectively discriminate between the different H4K20 methylated forms in chromatin immunoprecipitations (ChiPs), the authors determined that H4K20me1 was present on Sv40 minichromosomes at all times tested post-infection. Although H4K20me1 coincided with RNA polymerase ii during late infection suggesting a role in transcriptional activation, the authors found H4K20me1 broadly distributed throughout the Sv40 minichromosome. H4K20me1 was also detected during encapsidation and in the newly assembled virions suggesting that this modification may be important for proper packaging of Sv40. This is consistent with recent reports indicating that H4K20me1 is associated with chromatin condensation.4 It was previously postulated that H4K20me1 serves as the preferred substrate for higher degrees of H4K20 methylation.5 Consistent with this theory, the authors provide evidence that H4K20me1 appears to be absent on H4K20me3-associated nucleosomes even though the levels of both modifications are highest during early infection. interestingly, H4K20me1 remains on Sv40 minichromosomes at later time points whereas H4K20me3 rapidly disappears strongly suggesting that these two histone modifications are differentially regulated and have distinct functions. What was the reason for the disappearance of H4K20me3? The authors noticed that the loss of H4K20me3 occurred within the same timeframe as the overall reduction of Sv40 DNA suggesting a link between these two events. indeed, the authors demonstrated that Sv40 minichromosomes targeted for degradation were selectively enriched for H4K20me3 suggesting that H4K20me3 may serve as a rapid host defense mechanism, not unlike DNA methylation, to signal for inactivation and/or destruction of the viral DNA. This seems likely as mammalian H4K20me3 is enriched within repetitive and foreign DNA sequences typically associated with transcriptionally inert chromatin.6 One unexpected observation made by the authors was the near absence of H4K20me2 on the Sv40 minichromosomes. This was surprising given that the majority of human H4 molecules are dimethylated and these are widely distributed throughout the genome7 The reasons for these differences remain unclear. Another interesting observation was the presence of H4K20me1 in replicating and newly replicated Sv40 minichromosomes. This was unexpected as H4K20me1 levels in mammalian chromatin are lowest during DNA replication and maximal during mitosis.8 The authors provide two possible explanations for this observation: that H4K20me1 is introduced into parental Sv40 minichromosomes to permit the replication machinery to proceed through chromatin or that H4K20me1 occurs following replication. Further investigation is required to elucidate the role of H4K20me1 in viral replication. Collectively, Milavetz and colleagues have provided novel insights into how alterations of an expanding histone code within Sv40 minichromosomes are likely to influence its function during lytic infection.