Chromatin structure is strictly regulated during the cell cycle. DNA viruses occasionally disturb the spatial organization of the host cell chromatin due to formation of the viral DNA replication compartment. To examine chromatin behavior in baculovirus-infected cells, we constructed recombinant plasmids expressing fluorescent protein-tagged histone H4 molecules and visualized the intracellular localization of chromatin by their transient expression in live infected cells. Similar to other DNA viruses, the baculovirus Bombyx mori nucleopolyhedrovirus induced marginal relocation of chromatin within the nuclei of BmN cells, simultaneously with expansion of the viral DNA replication compartment, the virogenic stroma (VS). In the late stage of infection, however, the peristromal region (PR), another virus-induced subnuclear compartment, was also excluded from the chromatin-localizing area. Provided that late-gene products such as PR proteins (e.g., envelope proteins of the occlusion-derived virus) were expressed, blockage of viral DNA synthesis failed to inhibit chromatin relocation, despite abrogation of VS expansion. Instead, chromatin became marginalized concomitantly with PR expansion, suggesting that the PR contributes directly to chromatin replacement. In addition, chromatin was excluded from relatively large subnuclear structures that were induced in uninfected cells by cotransfection with four baculovirus genes, ie1, lef3, p143, and hr. Omission of any of the four genes, however, failed to result in formation of the large structures or chromatin exclusion. This correlation between compartmentalization and chromatin exclusion suggests the possibility that a chromatin-exclusive property of viral molecules, at least in part, supports nuclear compartmentalization of virus-infected cells.Baculoviruses are arthropod-specific large, rod-shaped, enveloped viruses containing double-stranded circular DNA genomes of 100 to 180 kb. Similar to the case for most other DNA viruses, many steps in baculovirus replication proceed within the cell nucleus, and to facilitate their replication baculoviruses reorganize the nuclear architecture by the induction of subnuclear compartments. One such virus-induced compartment is the virogenic stroma (VS), in which progeny nucleocapsids are assembled. Previously, Okano et al. (17) demonstrated that DNA replication of the baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV) proceeds within a discrete subnuclear compartment in BmN cells. We have subsequently shown that the major capsid protein VP39 (reviewed in reference 6), as well as the viral DNA replication factors IE1 (a multifunctional transactivator [reviewed in reference 5]), LEF3 (a single-stranded DNA binding protein [8]), and P143 (a DNA helicase [13]), localizes to this compartment and that the compartment has a high DNA content (12). We therefore proposed that this DNA replication compartment is the VS, implying that the VS is the site for not only nucleocapsid assembly but also viral DNA replication (12). Previous observati...

Baculovirus DNA replication, transcription, and nucleocapsid assembly occur within a subnuclear structure called the virogenic stroma (VS) that consists of two subcompartments. Specific components of the VS sub-compartments have not been identified except for PP31, a DNA-binding protein that localizes specifically to the electron-dense region of VS. Here, we investigate the dynamic structure of VS using a GFP-tagged PP31 molecule (GFP-PP31). GFP-PP31 localizes to the VS throughout the course of infection. At later times post-infection, a PP31 reticulum distributed within VS was also apparent, indicating that VS sub-compartments compose a reticulate structure. Transient expression of PP31 with the viral proteins, IE1, LEF3, and P143, in uninfected cells resulted in the formation of a reticulate structure containing cellular chromatin and the spatial arrangements of the four proteins within the induced reticulum were the same as those within VS reticulum, suggesting that the two reticula are formed by a similar mechanism.