Physical state of the latent herpes simplex virus genome in a mouse model system: evidence suggesting an episomal state.

Herpes simplex virus (HSV) can establish latent infections in tissues of the nervous system. We have examined the HSV-1 genome in both acutely and latently infected mice by CsCl buoyant density gradient centrifugation. Viral sequences, in gradient fractions, were detected by a spot blot technique using nick-translated HSV-1 cloned DNA as probe, and mouse chromosomal DNA was located by measuring optical density at 260 nm. Most HSV-1-specific DNA from both acutely and latently infected mouse brains was found to band at the buoyant density of virion DNA. However, some HSV-1-specific hybridization banded at the density of the mouse chromosomal DNA. Further rounds of CsCl density gradient centrifugation of this chromosomal DNA from acutely infected brains released most of the HSV-1-specific hybridizing material, suggesting that the association was due to trapping. In the case of the recycled chromosomal DNA from a latent infection, all the HSV-1-specific hybridization remained chromosomal DNA associated. However, the amount of hybridization was not significantly greater in quantity than the cross-hybridization between HSV-1 and chromosomal DNA from uninfected mice. AW-Ramos, an EBV-transformed cell line containing one integrated copy of the viral genome, was centrifuged under similar conditions and showed little if any shearing; the EBV DNA banded at the density of the host cell chromosomal DNA. We conclude that the majority of the latent HSV-1 DNA exists in an extrachromosomal state in the mouse model.