Formation of branched DNA structures by Xenopus laevisoocyte extract

WE have reported that a soluble cell-free extract from stage 6 oocytes of Xenopus laeviscan act on supercoiled SV40 DNA to produce molecules with a single-strand scission, linear molecules of full length, shorter fragments and various forms of complex DNA1. The complex DNA consisted of various structures, such as figure-of-eight dimers, catenated dimers, circular dimers, catenated trimers, late Cairns' structures, complex multimers and circular monomers with tails. The few late Cairns' structures observed could have been due to replication, but it is possible that they and the other forms of complex DNA resulted from recombination among SV40 DNA molecules. We thought that fractionation of the extract should produce fractions lacking some of the activities necessary to produce complex DNA, so that intermediate structures would accumulate. We now report that some fractions of the extract produce a high percentage of branched DNA structures from SV40 DNA, and we describe the requirements and characteristics of this reaction. We also discuss the possibility that branched DNA is an intermediate in the formation of complex DNA.