Circular Herpesvirus sylvilagus DNA in spleen cells of experimentally infected cottontail rabbits

Circularization of the herpes simplex virus type 1 genome upon lytic infection

For many years, the generally accepted model for the replication of the double-stranded DNA genome of herpes simplex virus type 1 (HSV-1) incorporated initial circularization of linear molecules in the cell nucleus. Ensuing DNA synthesis resulted in the generation of head-to-tail concatemers which were subsequently cleaved into monomeric units and packaged into the nascent viral capsid. Recently, however, it has been proposed that circularization of HSV-1 genomes does not occur at the onset of lytic infection and moreover that this event is specifically inhibited by the HSV-1 transcriptional transactivator, ICP0 (S.A. Jackson and N.A. DeLuca, Proc. Natl. Acad. Sci. USA 100:7871-7876, 2003). To further investigate genome circularization, we have generated HSV-1 derivatives in which the viral a sequences, which contain the cleavage-packaging signals, have been replaced by a minimal packaging element located in the thymidine kinase gene. In contrast to wild-type HSV-1, fusion of the genomic termini of these viruses produces a novel fragment in circular or concatemeric DNA which can be detected by Southern blot hybridization. Utilizing these viruses, we demonstrate that fusion of the genomic termini occurred rapidly upon infection and in the presence of inhibitors of viral DNA or protein synthesis. We provide evidence indicating that the end joining represented circularization rather than concatemerization of input molecules and that circularized molecules functioned as templates for replication. Since the termini of these viruses lack direct repeats, our findings indicate that circularization can be mediated by direct end-to-end ligation of linear input genomes.

Partial characterization of two serologically related DNA-binding proteins specified by the cottontail rabbit herpesvirus CTHV

We have previously described two serologically related DNA-binding phosphoproteins of different apparent molecular mass (75 kDa and 35 kDa) produced in cottontail rabbit herpesvirus (CTHV)-infected cells. The 75 kDa protein appeared before the 35 kDa protein in the infectious cycle. Here, we extend the characterization of these proteins. Protease V8 fingerprints of methionine-labelled 35 kDa protein showed four major peptide products, three of which co-migrated with major peptides from digests of the 75 kDa protein. The fourth peptide, with an estimated mass of 10 kDa, reacted with an antiserum recognizing both proteins. In vitro translation of total or poly A-containing RNA isolated from infected cells at 24 h to 72 h post-infection produced only the 75 kDa protein as measured by immunoprecipitation with anti-75/35 kDa serum, suggesting that the 35 kDa protein is derived from the 75 kDa protein by proteolytic cleavage. Virus-specific RNA obtained by prehybridization to CTHV DNA also produced the 75 kDA protein, confirming its viral origin. The putative gene for the 75 kDa protein was localized to a region on the CTHV DNA restricted by PvuII.

Genome structure of cottontail rabbit herpesvirus

The genome structure of a herpesvirus isolated from primary cultures of kidney cells from the cottontail rabbit Sylvilagus floridanus was elucidated by using electron microscopy and restriction enzyme analysis. The genome, which was about 150 kilobase pairs long and which had an average G + C composition of 45%, consisted of two regions with unique base sequences (54 and 47 kilobase pairs) enclosed by reiterations of a 925-base-pair sequence with a variable copy number. The internal repeats were in opposite polarity with respect to the terminal repeats, and both unique regions underwent inversion. The nucleotide sequence of the repeat unit was determined, and virion DNA termini were precisely localized within this sequence. Elements showing homology with the cleavage-packaging signals common to other herpesviruses were detected. The data indicate that this virus is different from the previously described herpesvirus sylvilagus.

Arrangement of repetitive sequences in the genome of herpesvirus Sylvilagus

Herpesvirus sylvilagus is a lymphotropic (type gamma) herpesvirus of cottontail rabbits (Sylvilagus floridanus). Analysis of virion DNA of herpesvirus sylvilagus has revealed that the genome consists of one stretch of about 120 kilobase pairs of internal, unique DNA flanked by a variable number of 553-base-pair tandem repeats. The G + C content of the repetitive DNA is extremely high (83%), as determined by sequencing. The organization of the herpesvirus sylvilagus genome is, therefore, similar to that of the primate lymphotropic viruses herpesvirus saimiri and herpesvirus ateles.

Pathogenesis of Herpesvirus sylvilagus infection in cottontail rabbits

Experimental infection with Herpesvirus sylvilagus produces clinical and histopathologic changes in its natural host, the cottontail rabbit (Sylvilagus floridanus), similar to those observed in humans acutely infected with Epstein-Barr virus (EBV). Twenty-seven seronegative cottontail rabbits were infected with Herpesvirus sylvilagus and all developed antibodies within 10 days. Neutralizing antibody was detected as early as 7 days after infection. Virus was isolated from blood mononuclear cells, spleen, bone marrow, thymus, lymph nodes, kidneys, lung, and liver as early as 3 days after infection. Infected animals showed leucocytosis, monocytosis, and lymphocytosis with the appearance of atypical lymphocytes. Peripheral blood abnormalities peaked at 10-14 days after infection, and returned to normal by 28 days after infection, with the exception of atypical lymphocytosis that persisted in some animals for more than 2 years after experimental infection. More severe histopathologic changes were seen in virus-infected juvenile rabbits than adult rabbits; these changes included viral myocarditis, interstitial pneumonia, and lymphocytic myositis. Reactive hyperplasia and subsequent lymphocytic depletion of spleen and lymph nodes were reminiscent of that seen in virus-associated hemophagocytosis syndrome. Prominent lymphoid hyperplasia of many nonlymphoid organs, most notably the kidney and lungs, was observed. The development of these lymphoproliferative lesions and other lymphoid changes during H. sylvilagus infection suggest that this system may be a model to study similar lesions induced by EBV infection in humans.

Herpesvirus sylvilagus infects both B and T lymphocytes in vivo

Herpesvirus sylvilagus infection of cottontail rabbits (Sylvilagus floridanus) was studied as a model of herpesvirus-induced lymphoproliferative disorders. Leukocytosis, splenomegaly, proliferation of T cells and virus production by lymphocytes characterized this infectious mononucleosis-like disease. Approximately two copies of circular herpesvirus sylvilagus genomes per cell were detected in spleen cells at 2 weeks postinfection, and circular genomes could still be observed after 4 months. Circular viral genomes were found in both B and T lymphocytes. Small amounts of linear viral DNA (0.1 to 0.3 copies per cell) were also detected in both B and T cells. These results indicated that the virus did not replicate in the majority of lymphocytes in vivo. Herpesvirus sylvilagus infection in cottontail rabbits could be useful as a model for studying the complex virus-host relationships of lymphotropic herpesviruses and perhaps as an animal model for Epstein-Barr virus infection in humans.

Inversion and circularization of the varicella-zoster virus genome

The genome of varicella-zoster virus (VZV) is a linear, double-stranded molecule of DNA composed of a long (L) region covalently linked to a short (S) region. The S region is capable of inverting relative to a fixed orientation of the L region, giving rise to two equimolar populations. We have investigated other forms of the VZV genome which are present in infected cells and packaged into nucleocapsids. That a small proportion of nucleocapsid DNA molecules also possess inverted L regions has been verified by the identification of submolar restriction fragments corresponding to novel joints and novel ends generated by such an inversion. The presence of circular molecules has been investigated by agarose gel electrophoresis. Bands corresponding to circular forms were present in small amounts in both VZV-infected cell DNA and nucleocapsid DNA. Southern blot analysis verified that these bands contained VZV sequences. We therefore conclude that the VZV genome may occasionally contain an inverted L region or exist in a circular configuration.