Multiple long-range cis interactions generate CTCF insulator-dependent viral chromatin domains in quiescent HSV-1 genomes
In cellular genomes, CTCF insulators impact transcription over small distances in a one-dimensional manner and over much longer distances in a three-dimensional manner by maintaining chromatin loops. We have previously shown that the latent HSV-1 genome contains CTCF insulators that function to regulate lytic transcription of adjacent genes in a one-dimensional manner. Here, we test the hypothesis that HSV-1 CTCF insulators nucleate chromatin loops to regulate the expression of distance separated gene regions through three-dimensional organization of viral genomes. We used 4C-seq methods to identify multiple long-range cis interactions in HSV-1 genomes that generate viral chromatin domains, including those nucleated by the viral CTCF insulator CTRL2. Deletion of the CTRL2 insulator disrupted these viral chromatin domains. Loop-nucleating interactions were quantitated with a novel approach (UMI-4C-seq) that utilizes unique molecular identifiers to label and count chromatin interactions associated with specific viewpoint primers. Cis-interaction peaks across four different viewpoints were quantified. Viral genomes lacking CTRL2 displayed more cis-interaction peaks and wider ranges of interaction lengths compared to wt virus, suggesting altered chromatin organization. Furthermore, differential looping analysis showed viral genomes lacking CTRL2 displayed a more transcriptionally permissive chromatin environment. Thus, the CTRL2 insulator functions as a critical regulator of long-range chromatin interactions and its deletion reshapes the viral chromatin landscape, leading to a more accessible and dynamic regulatory environment that may influence HSV-1 transcriptional programs and latency-associated chromatin states.