Distinct mouse DNA sequences enable establishment and persistence of plasmid DNA polymers in mouse cells.

Distinct elements isolated from mouse genomic DNA confer on plasmid DNA the ability to persist at high copy numbers in mouse L fibroblasts (1). Field inversion gel electrophoresis demonstrated that - in contrast to our previous assumption - the persisting plasmid DNA does not exist extrachromosomally but as clusters of tandem repeats integrated into genomic DNA. Digestion with restriction endonucleases that do not cut within the plasmid DNA results in fragments of 50-300 kb in length indicating reiteration of 10-50 plasmid DNA molecules. Restriction with several enzymes that cut once or twice within the plasmid sequences lead to fragment(s) indicative for head-to-tail tandem repeats. In situ hybridization revealed signals for a long homogeneously staining region (HSR) in one or two chromosomes per cell nucleus. Possibilities how these elements could act in the establishment and/or maintenance of the head-to-tail polymers of plasmid DNA in mouse cells are discussed.