title: Topoisomerase II, not topoisomerase I, is the proficient relaxase of nucleosomal DNA.
abstract: Eukaryotic topoisomerases I and II efficiently remove helical tension in naked DNA molecules. However, this activity has not been examined in nucleosomal DNA, their natural substrate. Here, we obtained yeast minichromosomes holding DNA under (+) helical tension, and incubated them with topoisomerases. We show that DNA supercoiling density can rise above +0.04 without displacement of the histones and that the typical nucleosome topology is restored upon DNA relaxation. However, in contrast to what is observed in naked DNA, topoisomerase II relaxes nucleosomal DNA much faster than topoisomerase I. The same effect occurs in cell extracts containing physiological dosages of topoisomeraseI and II. Apparently, the DNA strand-rotation mechanism of topoisomerase I does not efficiently relax chromatin, which imposes barriers for DNA twist diffusion. Conversely, the DNA cross-inversion mechanism of topoisomerase II is facilitated in chromatin, which favor the juxtaposition of DNA segments. We conclude that topoisomerase II is the main modulator of DNA topology in chromatin fibers. The nonessential topoisomerase I then assists DNA relaxation where chromatin structure impairs DNA juxtaposition but allows twist diffusion.
authors.affiliation: Institut de Biología Molecular de Barcelona, CSIC, Barcelona, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Salceda
authors.forename: Xavier
authors.initials: X
authors.lastname: Fernández
authors.forename: Joaquim
authors.initials: J
authors.lastname: Roca
mesh_terms: D002843:Chromatin / Q000737:chemistry / Q000378:metabolism; D004264:DNA Topoisomerases, Type I / Q000235:genetics / Q000378:metabolism*; D004250:DNA Topoisomerases, Type II / Q000235:genetics / Q000378:metabolism*; D004278:DNA, Superhelical / Q000737:chemistry / Q000378:metabolism; D009690:Nucleic Acid Conformation; D009707:Nucleosomes / Q000235:genetics* / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000201:enzymology / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D002843:Chromatin; D004278:DNA, Superhelical; D009707:Nucleosomes; D029701:Saccharomyces cerevisiae Proteins; D004264:DNA Topoisomerases, Type I; D004250:DNA Topoisomerases, Type II