Eukaryotic DNA topoisomerase I reaction is topology dependent

The effects of supercoiling on the topoisomerization reaction by eukaryotic DNA topoisomerases I have been analyzed. The systems used were: DNA topoisomerase I from wheat germ, chicken erythrocyte and calf thymus on a 2.3 kb DNA fragment which encompasses the immunoglobulin x-light chain (Lx) promoter of the mouse plasmacytoma MPCll; S.cerevisiae DNA topoisomerase I on a 2.2 kb DNA fragment from the same organism which encompasses the regulatory and the coding region of the ADH II gene; wheat germ DNA topoisomerase I on the plasmid pUC18.It was found in every system that lack of torsional stress prevents topoisomerization of the substrate.A simple regulatory model of DNA topoisomerase I function, based on topological considerations, is presented. © 1988 IRL Press Limited.