Origin of DNA replication in papovavirus chromatin is recognized by endogenous endonuclease.

Isolated simian virus 40 (SV40) and polyoma nucleoprotein complexes contain endonuclease that, under in vitro conditions, converts part (up to 30%) of the covalently closed superhelical DNA to full-length linear rods. The positions of the cleavage sites within the genomes of SV40 and polyoma were determined by digestion with various single-cut restriction endonucleases and subsequent agarose gel electrophoresis of the cleavage products. Both SV40 and polyoma covalently closed superhelical DNA were cleaved open at their respective origins of DNA replication (+/- 75 base pairs). The full-length linear DNA rods whose ends map adjacent to the origin of DNA replication could also be isolated by sodium dodecyl sulfate/phenol extraction both from SV40-infected permissive cells and from purified SV40 virions. These data reveal the presence of a unique structure of the papovavirus chromatin close to the initiation site of DNA replication.