We have used the recently developed MAC Mode Atomic Force Microscope (AFM) that operates in aqueous solution to mechanically stretch single chicken erythrocyte chromatin fibers. The fibers contained the full complement of histones, or, alternatively, were depleted of linker histones. The AFM was used to produce the so- called force curves, by monitoring the cantilever deflection (proportional to force) as the distance between the AFM tip and the sample was experimentally manipulated. To that end, the AFM tip was pushed into the chromatin sample and then withdrawn, to mechanically stretch the fiber that was physically adsorbed to the tip. Pulling of the chromatin fiber produced complex sawtooth-like patterns of peaks that were characterized by unexpectedly large forces, in the range of several hundred picoNewtons. The distribution of forces in the linker histone-containing and linker histone-depleted fibers was remarkably different, possibly indicating that linker histone binding significantly changes the mechanical properties of the chromatin fiber.
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