Intrachromosomal recombination mediated by the polyomavirus large T antigen.

We used a spleen necrosis virus-based retroviral vector to introduce the polyomavirus replication origin into rat cells and developed a system to analyze homologous recombination events that do not reconstitute a selectable marker. Introduction of the gene coding for the polyomavirus large T antigen into the cell lines by DNA transfection promoted high-frequency recombination between the two retroviral LTRs, leading to amplification and excision of DNA sequences. To analyze homology requirements, we constructed cell lines carrying only the replication origin without exogenous repeats. Most of the cell lines sustained high-frequency recombination, presumably by undergoing homologous recombination between repetitive DNA lying in the vicinity of the integrated origin. Our results indicate that homologous recombination promoted by large T antigen does not require recombination hot spots in the viral genome other than the replication origin and they explain the cytotoxicity observed in some cell types when large T antigen is expressed in the presence of a functional origin.