The isolation of simian virus 40 variants with specifically altered genomes

Serial passage of simian virus 40 (SV40) at high multiplicity of infection leads to the emergence of variants with deleted, substituted, and/or duplicated DNA. Individual variants have been cloned by selective complementation with temperature sensitive SV40 mutants, or nonselectively by coinfection of cells with wild-type helper virus. In each case, the presence of variants was detected by the appearance of discrete short viral genomes in infected cell lysates. Such short genomes, isolated by agarose gel electrophoresis, were shown to be specifically altered by comparing the electrophoretic pattern of their DNA fragments produced by Haemophilus influenzae restriction endonuclease with the pattern of fragments from parental DNA. In addition to defective variants, one infectious variant that had an additional segment of DNA within its genome was isolated.