Excision of polyoma virus genomes from chromosomal DNA by homologous recombination

Abstract

Head-to-tail tandem duplications of DNA sequences have been observed to occur naturally in chromosomal DNA of mammalian cells^1,2 and head-to-tail tandem arrays of DNA sequences can be induced upon gene amplification^3,4. In these cases such sequences have not been detected as free or plasmid forms. After transformation by the two papovaviruses, polyoma and simian virus 40 (SV40), the viral DNA sequences integrated into the host chromosome are often found in head-to-tail tandem arrays^5–11. When either a partially permissive environment for viral DNA replication exists (polyoma–rat or SV40–human cells) or after fusion of non-permissive transformed cells with permissive cells (polyoma–mouse or SV40–monkey cells), free viral forms are induced provided that a functional viral replication protein and a competent viral origin of DNA replication are present¹². These results have led to the hypothesis that excision of integrated sequences occurs by viral DNA replication within the chromosomal DNA^6,13. We show here that free polyoma molecules can be excised from partial head-to-tail duplications of integrated viral sequences in the absence of a functional viral replication protein (large T antigen [T-Ag]) after fusion of transformed rat cells (partially permissive) to permissive mouse cells. The amount of free viral forms produced appears to be proportional to the extent of duplicated integrated viral sequences.