Single molecule DNA compaction by purified histones

The compaction of single DNA molecules by purified histones is studied using magnetic tweezers. The compaction rate increases rapidly when the histone concentration is increased from 0.002 to 0.2 mmol/L, and saturates when the concentration is beyond 0.2 mmol/L. The time course of compaction is exponential at low histone concentrations. It becomes sigmoidal at high concentrations. Cooperativity between the histones bound to DNA is proposed to be responsible for the transition. The histones are loaded onto DNA randomly at low concentrations. They tend to bind DNA cooperatively at high concentrations because the structural torsions of DNA induced by the bound histones become overlapping so that the binding of one histone facilitates the binding of others. Under very large forces, the compacted histone-DNA complex can be disrupted in a discrete manner with a step size of ∼60 nm. But the histones cannot be completely stripped off DNA, as is revealed by the lowered B-S transition plateau of the histone-bound DNA.