Evolutionary variants of simian virus 40: Characterization of cloned complementing variants

Three evolutionary variants of simian virus 40 (SV40), cloned by complementation with temperature-sensitive SV40 mutants, have been mapped by restriction endonuclease analysis and by electron microscopic heteroduplex mapping. One of the variants contains a deletion of most of the “late” region of the genome, and the other two contain nonidentical deletions within the “early” region. Each of the variants has a distinctive duplication of a DNA segment which includes the initiation site for DNA replication. Based on the physical maps of these genomes, a recombinational mechanism is suggested for the generation of variants; once generated, encapsidatible molecules with duplications of the replication initiation site would have a selective growth advantage. One variant contains one of its two initiation sites close to the site where SV40-DNA replication normally terminates. In this variant, however, replication appears to terminate 180° from the initiation point. Therefore, unlike initiation, termination of SV40-DNA replication is probably not sequence specific.