Closed circular DNAs with tandem repeats of a sequence from polyoma virus

The R₁ restriction endonuclease from bacteria containing the RTF-1 plasmid cleaves polyoma virus DNA at a single site. Virus stocks which have been passaged through cells several times at moderate or high multiplicities contain closed circular DNAs which are not cleaved by the R₁ endonuclease. One such population of DNAs, is not infectious, and when sheared and denatured, reassociates at a rate approximately four times faster than polyoma DNA. The predominant DNA in the population is 10% smaller than polyoma DNA and contains a fraction of the polyoma genome tandemly repeated three to four times. There is no significant contamination of the covalently closed circular DNAs by repetitive host sequences. A model explaining how such trandemly repeated molecules could arise by abortive rounds of polyoma DNA replication is discussed.