Digitaria streak geminivirus replicative forms are abundant in S-phase nuclei of infected cells

We have analyzed the replication of Digitaria streak geminivirus (DSV) using flow cytometry. Nuclei from infected plants were sorted on the basis of their DNA content in the different phases of the cell cycle and viral DNA forms in them were analyzed. DSV replicative forms were much more abundant (up to 20 times in one experiment) in S-phase nuclei than in G0-G1 and G2 nuclei, while single-stranded viral DNA did not show such dramatic increase during any phase. DSV replicative forms constitute almost 90% of total viral DNA forms in S-phase nuclei. DSV replication seems to be synchronized with host DNA replication and might be primarily under control of host factors becoming available only at the G1/S boundary.     

Anorexia and lycanthropy ++: grandiosity and fall

Epstein-Barr virus (EBV) is linked to a spectrum of human diseases including epithelial and lymphoid malignancies in which it exists predominantly in a latent state. EBV is capable of establishing replicative infection at oropharyngeal and genital sites. Replicative EBV infection also occurs in oral hairy leukoplakia, in EBV associated lymphoproliferative disorders, and to a minor degree in nasopharyngeal carcinomas. Recent evidence also suggests that EBV replication, also, may be associated with AIDS related lymphomas and Hodgkin's disease. However it is widely believed that virus in circulating B-lymphocytes and in B-cell malignancies is stringently latent. We now show that by Southern blot analysis we can detect replicative forms of virion DNA in 14.5% (8 of 55) of EBV-positive Burkitt's lymphoma biopsies. This may be the explanation for the elevation of the titres of lytic cycle EBV antigens that is associated with presentation and relapse of EBV associated Burkitt's lymphoma.     

Very late DNA replication in the human cell cycle

G2 was defined originally as the temporal gap between the termination of DNA replication and the beginning of mitosis. In human cells, the G2 period was estimated to be 3-4 h. However, the absence of replicative DNA synthesis during this period designated G2 has never been shown conclusively. In this report, we show that, at some autosomal and X linked loci, programmed DNA replication continues within 90 min of mitosis. Furthermore, the major accumulation of cyclin B1, a cell-cycle marker that is usually ascribed to G2, overlaps extensively with very late DNA replication. We conclude that the G2 period is much shorter than previously thought and may, in some cells, be nonexistent.     

Competition for DNA binding sites between the short and long forms of E2 dimers underlies repression in bovine papillomavirus type 1 DNA replication control

Papillomaviruses establish a long-term latency in vivo by maintaining their genomes as nuclear plasmids in proliferating cells. Bovine papillomavirus type 1 encodes two proteins required for viral DNA replication: the helicase E1 and the positive regulator E2. The homodimeric E2 is known to cooperatively bind to DNA with E1 to form a preinitiation complex at the origin of DNA replication. The virus also codes for two short forms of E2 that can repress viral functions when overexpressed, and at least one copy of the repressor is required for stable plasmid maintenance in transformed cells. Employing a tetracycline-regulated system to control E1 and E2 production from integrated loci, we show that the short form of E2 negatively regulates DNA replication. We also found that the short form could repress replication in a cell-free replication system and that the repression requires the DNA binding domain of the protein. In contrast, heterodimers of the short and long forms were activators and, by footprint analysis, were shown to be as potent as homodimeric E2 in loading E1 to its cognate site. DNA binding studies show that when E1 levels are low and are dependent upon E2 for occupancy of the origin site, the repressor can block E1-DNA interactions. We conclude that DNA replication modulation results from competition between the different forms of E2 for DNA binding. Given that heterodimers are active and that the repressor form of E2 shows little cooperativity with E1 for DNA binding, this protein is a weak repressor.