Transcription factor TFIIIA stimulates DNA supercoiling promoted by a fractionated cell‐free extract from Xenopus laevis

An activity than can introduce negative supercoils into relaxed covalently closed DNA molecules has been isolated from a Xenopus laevis cell‐free extract (S‐150) and purified over 200‐fold. The exogenous addition of ATP, other ribonucleotides and deoxyribonucleotides, as well as nonhydrolyzable analogs, stimulate DNA supercoiling which may occur by a pathway involving multiple protein components. DNA supercoiling occurs in topological single steps and is inhibited by camptothecin and berenil, but not novobiocin or VM‐26, suggesting a catalytic role for topoisomerase I in the reaction. Transcription factor TFIIIA stimulates DNA supercoiling catalyzed by the isolated fraction at low factor to DNA ratios. Taken together, these results suggest that a isolated DNA supercoiling activity from the X. laevis S‐150 cell‐free extract can be stimulated by transcription factor TFIIIA.