Pulling a single chromatin fiber reveals the forces that maintain its higher-order structure

摘要

Single chicken erythrocyte chromatin fibers were stretched and released at room temperature with force-measuring laser twee- zers. In low ionic strength, the stretch-release curves reveal a process of continuous deformation with little or no internucleo- somal attraction. A persistence length of 30 nm and a stretch modulus of ≈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 chroma- tin. In 40--150 mM NaCl, a distinctive condensation-decondensation transition appears between 5 and 6 pN, corresponding to an internucleosomal attraction energy of rs2.0 kcal/moI per nucleo- some. Thus, in physiological ionic strength the fibers possess a dynamic structure in which the fiber locally interconverting be- tween "open" and "closed" states because of thermal fluctuations.