title: Micromanipulation studies of chromatin fibers in Xenopus egg extracts reveal ATP-dependent chromatin assembly dynamics.
abstract: We have studied assembly of chromatin using Xenopus egg extracts and single DNA molecules held at constant tension by using magnetic tweezers. In the absence of ATP, interphase extracts were able to assemble chromatin against DNA tensions of up to 3.5 piconewtons (pN). We observed force-induced disassembly and opening-closing fluctuations, indicating our experiments were in mechanochemical equilibrium. Roughly 50-nm (150-base pair) lengthening events dominated force-driven disassembly, suggesting that the assembled fibers are chiefly composed of nucleosomes. The ATP-depleted reaction was able to do mechanical work of 27 kcal/mol per 50 nm step, which provides an estimate of the free energy difference between core histone octamers on and off DNA. Addition of ATP led to highly dynamic behavior with time courses exhibiting processive runs of assembly and disassembly not observed in the ATP-depleted case. With ATP present, application of forces of 2 pN led to nearly complete fiber disassembly. Our study suggests that ATP hydrolysis plays a major role in nucleosome rearrangement and removal and that chromatin in vivo may be subject to highly dynamic assembly and disassembly processes that are modulated by DNA tension.
authors.affiliation: Department of Physics, University of Illinois at Chicago, Chicago, IL 60607-7059, USA.
authors.forename: Jie
authors.initials: J
authors.lastname: Yan
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Maresca
authors.forename: Dunja
authors.initials: D
authors.lastname: Skoko
authors.forename: Christian D
authors.initials: CD
authors.lastname: Adams
authors.forename: Botao
authors.initials: B
authors.lastname: Xiao
authors.forename: Morten O
authors.initials: MO
authors.lastname: Christensen
authors.forename: Rebecca
authors.initials: R
authors.lastname: Heald
authors.forename: John F
authors.initials: JF
authors.lastname: Marko
mesh_terms: D000255:Adenosine Triphosphate / Q000737:chemistry*; D000818:Animals; D002457:Cell Extracts / Q000737:chemistry; D002843:Chromatin / Q000737:chemistry*; D042002:Chromatin Assembly and Disassembly; D006868:Hydrolysis; D008280:Magnetics; D008846:Micromanipulation / Q000379:methods; D009707:Nucleosomes / Q000737:chemistry; D010063:Ovum; D014981:Xenopus
chemical_list: D002457:Cell Extracts; D002843:Chromatin; D009707:Nucleosomes; D000255:Adenosine Triphosphate