Pulling a single chromatin fiber reveals the forces that maintain its higher-order structure.
Single chicken erythrocyte chromatin fibers were stretched and released at room temperature with force-measuring laser tweezers. In low ionic strength, the stretch-release curves reveal a process of continuous deformation with little or no internucleosomal attraction. A persistence length of 30 nm and a stretch modulus of approximately 5 pN is determined for the fibers. At forces of 20 pN and higher, the fibers are modified irreversibly, probably through the mechanical removal of the histone cores from native chromatin. In 40-150 mM NaCl, a distinctive condensation-decondensation transition appears between 5 and 6 pN, corresponding to an internucleosomal attraction energy of approximately 2.0 kcal/mol per nucleosome. Thus, in physiological ionic strength the fibers possess a dynamic structure in which the fiber locally interconverting between "open" and "closed" states because of thermal fluctuations.
Cui
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Y
Department of Molecular Biology, University of California, Berkeley, CA 94720, USA.
Bustamante
C
C
D000818:Animals; D002645:Chickens; D002843:Chromatin / Q000737:chemistry*; D004548:Elasticity; D004912:Erythrocytes / Q000737:chemistry; D006657:Histones / Q000737:chemistry; D009707:Nucleosomes / Q000737:chemistry; D009994:Osmolar Concentration; D011137:Polystyrenes; D012965:Sodium Chloride / Q000494:pharmacology; D013314:Stress, Mechanical; D013816:Thermodynamics
D002843:Chromatin; D006657:Histones; D009707:Nucleosomes; D011137:Polystyrenes; D012965:Sodium Chloride