DNA-relaxing activity and endonuclease activity in Xenopus laevis oocytes.

Complex simian virus 40 DNA produced by a soluble cell-free extract derived from stage 6 oocytes of Xenopus laevis consists of fully relaxed circles (i.e., with no superhelical turns). An endonuclease and a DNA-relaxing protein, either or both of which could be responsible for the relaxation of the complex DNA, have been purified from the extract. The endonuclease(s) produces nicked circles (having a single-strand scission) and linear full-size molecules. The DNA-relaxing protein is in the nucleus, has a molecular weight of apporximately 70,000, and is able to remove both negative and positive superhelical turns.