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The ATP-dependent movement of simian virus 40 large tumor antigen on DNA can cause positive supercoiling of template DNA in the presence of bacterial DNA topoisomerase I, which specifically relaxes negative supercoils. With such a supercoiling assay, an activity capable of ATP-dependent, positive supercoiling of DNA in the presence of bacterial DNA topoisomerase I was found in 7S particles prepared from Xenopus laevis oocytes but not in purified transcription factor IIIA. The positive supercoils accumulated during the reaction are strained, as evidenced by their sensitivity to eukaryotic DNA topoisomerase I. Purification of this activity led to the identification of a DNA-dependent ATPase. Molar excess of transcription factor IIIA or spermidine was found to strongly stimulate the supercoiling reaction of the purified ATPase. These results suggest that this DNA-dependent ATPase can interact with and translocate along closed-circular duplex DNA, producing local alterations in the supercoiled state of the DNA template in a manner analogous to that of simian virus 40 large tumor antigen.

A protein factor from Xenopus oocytes with simian virus 40 large tumor antigen-like DNA supercoiling activity.