Abstract
Two cloned evolutionary variants of simian virus 40 (SV40) containing substitutions of cellular DNA have been characterized by restriction endonuclease analysis, electron microscopic heteroduplex mapping, and DNA-DNA hybridization. Each variant genome is made up of a small, tandemly repeated segment of DNA consisting of cellular DNA and that portion of the SV40 genome containing the initiation signal for viral DNA replication. Cellular DNA sequences are different in the two variants, indicating that recombination between cell DNA and SV40-DNA can occur at more than one site. However, one end of the SV40 segment (0.68 SV40 map-units) is the same in each variant. The data suggest that substituted variants arise by integration of SV40-DNA into cellular DNA followed by excision of a small substituted genome which is subsequently amplified to a size compatible with encapsidation; the presence of multiple initiation signals in each molecule results in selective replication.
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