Deletion of the origin of replication impairs the ability of polyomavirus DNA to transform cells and to form tandem insertions.

We examined the transforming properties of polyomavirus DNA molecules which can produce a functional large T-antigen but which are cis defective for viral DNA replication. The inability of these molecules to replicate results from the deletion of sequences comprising the viral replication origin. We found that even in the presence of a functional large T-antigen, transformation of rat cells by these viral DNAs was greatly reduced when compared with replication-competent parental DNA, and cells transformed by origin-minus mutants generally contained the integrated viral DNA in a nontandem arrangement. Therefore, polyomavirus large T-antigen promotes the establishment of transformation and tandem integration by interacting with the viral origin of DNA replication. This indicates that viral DNA synthesis is directly involved in these processes.