A chimeric infection program and epigenetic conflict underpin endogenous giant virus latency

Abstract

Giant endogenous viral elements (GEVEs) are ubiquitous in eukaryotic genomes, but how the virus adapts to the host and is capable of reactivation despite host defense mechanisms remains unclear. Here we examined this process in the green alga Chlamydomonas reinhardtii and its latent giant virus, punuivirus, which is integrated into the host genome. We reveal a chimeric genomic architecture in which punuivirus latency genes are enriched in spliceosomal introns and 6-methyladenine marks, mirroring host transcriptional regulation, while virulence genes are predominantly controlled by a conserved viral promoter. Using single-cell RNA-seq, we show that nearly 70% of cells express viral transcripts, but only a small subpopulation commits to viral production, expressing virulence genes with host transcriptional shutoff. Moreover, we find that the intact punuivirus GEVE remains refractory to host 5-methylcytosine (5mC) silencing, whereas a naturally occurring deletion variant is associated with widespread 5mC accumulation across the GEVE and elsewhere in the host genome. The deleted region encodes a highly expressed ten-eleven translocation dioxygenase (TET), and epigenetic profiling of intact and deletion strains implicates this GEVE gene in removing 5mC marks, thereby maintaining the virus in a transcriptionally permissive latent state. Our study provides novel insights into the molecular underpinnings of host-virus co-existence in a giant virus-alga interaction.