Two defective adenovirus-simian virus 40 hybrids which contain the entire SV40 genome (Ad2++HEY and Ad2++LEY)† † Abbreviations used: Ad, edenovirus; SV, simian virus. have been isolated. Upon infection of cells permissive for SV40 both hybrids give rise to infectious SV40 virions, but with markedly different efficiencies. In the case of Ad2++HEY nearly all cells infected with a hybrid particle yield SV40 progeny, whereas in the case of Ad2++LEY infectious SV40 is produced in only about one in 104 cells infected with hybrid particles. The structures of the DNA molecules in the Ad2++HEY and Ad2++LEY populations were examined using electron microscope heteroduplex methods. Both populations were found to be heterogeneous. Ad2++HEY contained three hybrids (HEY-I, HEY-II, and HEY-III) whose genomes differed only in their content of SV40 DNA (0.45 ± 0.02, 1.43 ± 0.04, and 2.39 ± 0.09 SV40 genomes, respectively). Ad2++LEY contained two hybrids (LEY-I and LEY-II), which also differed only in their content of SV40 DNA (0.03 ± 0.01 and 1.05 ± 0.01 SV40 genomes, respectively). In those hybrids which contained more than one complete SV40 genome (HEY-II, HEY-III, LEY-II) the excess SV40 DNA was shown to be organized as a tandem repetition. These data suggest that the various hybrid genomes within each population are interconvertible by recombination events, which insert or excise an SV40 genome. It is proposed that HEY-II and HEY-III yield infectious SV40 with higher efficiency than LEY-II because their SV40 DNA segments contain longer tandem repetitions; thus, the probability of an intramolecular recombination event which results in excision of an SV40 genome is greater.
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