4D Analyses Show That Replication Compartments Are Clonal Factories in Which Epstein–Barr Viral DNA Amplification Is Coordinated ^†

Most DNA viruses must amplify their DNA to form new viral particles. To kickstart their DNA amplification, herpesviruses alter the host cell cycle dynamics by halting G1/S progression. Soon after, the viruses begin amplifying their DNA and halt any detectable cellular DNA synthesis. Viral DNA amplification takes place in specialized regions of the cell known as replication compartments. The genesis and maturation of replication compartments are not well understood. While replication compartments can only be visualized via microscopy, examining DNA synthetic events requires ensemble approaches. We have therefore exploited single-cell assays, including live-cell imaging, fluorescence in situ hybridization (FISH), and EdU-pulse labeling, in combination with computational simulations and ensemble approaches, to study the role of replication compartments in the DNA amplification of the Epstein–Barr virus (EBV). FISH revealed that each replication compartment initially contained a single DNA molecule which did not travel between compartments. DNA amplification lasted for 13–14 h in single cells, as shown by live cell imaging. Replication compartments eventually grew to occupy 30% of the nucleus, which itself grew by 50%. We found that early in the lytic phase, the availability of DNA templates limited DNA synthesis, while late in the lytic phase, the majority of viral DNA molecules no longer served as templates, which correlated with a drop in the levels of the replication protein. The eventual decline in DNA synthesis did not result from encapsidation; only 1–2% of the viral DNA was encapsidated. The levels of viral DNA synthesis in each compartment were similar. Therefore, the number of compartments determined the total amount of DNA synthesized and, consequently, the levels of amplified DNA. This finding was predicted by computational simulations of the amplification of the two distinct EBV derived replicons that we studied. Our results establish that replication compartments represent clonal factories for DNA amplification that are regulated coordinately during the lytic phase.