In vitro transcription by Xenopus oocytes RNA polymerase III requires a DNA topoisomerase II activity.

Extracts of Xenopus laevis oocytes are able to assemble minichromosomes in vitro when they are supplemented with ATP and Mg2+. We have followed the time course of in vitro DNA supercoiling and transcription of polyoma virus closed circular DNA. The transcriptional activity increased with the assembly time of chromatin in the presence of both ATP and Mg2+, but only residual activity was detected in the absence of either of them. We also found that polyoma DNA as well as 5S RNA gene transcription were carried out mainly by an RNA polymerase III. On the other hand, polyoma RNA and 5S RNA synthesis were inhibited by novobiocin in a way that suggested the requirement of a DNA topoisomerase II or DNA gyrase activity for the initiation of transcription.