New, small circular DNA in transfected mammalian cells.

Circular DNA isolated by the Hirt procedure from transfected mammalian cells was examined by electron microscopy. Typically, the number of small (1- to 5-kilobase) DNA circles increased about fivefold even though DNA of larger size classes (5 to 15 kilobases) has been transferred. In one case, where extensive rearrangement of the transferred DNA was observed, the rearrangement products were cloned and analyzed. In most cases, however, no rearrangement could be detected, but the amount of small circular DNA was still increased. This effect was seen with two transfection procedures (erythrocyte ghost fusion and calcium phosphate precipitation) and with various combinations of transfecting DNA and recipient cell type. The origin of the new small circular DNA is discussed.