Förster resonance energy transfer was used to monitor the dynamic conformations of mononucleosomes under different chromatin folding conditions to elucidate the role of the flexible N-terminal regions of H3 and H4 histones. The H3 tail was shown to partake in intranucleosomal interactions by restricting the DNA breathing motion and compacting the nucleosome. The H3 tail effects were mostly independent of the ionic strength and valency of the ions. The H4 tail was shown to not greatly affect the nucleosome conformation, but did slightly influence the relative population of the preferred conformation. The role of the H4 tail varied depending on the valency and ionic strength, suggesting that electrostatic forces play a primary role in H4 tail interactions. Interestingly, despite the H4 tail’s lack of influence, when H3 and H4 tails were simultaneously clipped, a more dramatic effect was seen than when only H3 or H4 tails were clipped. The combinatorial effect of H3 and H4 tail truncation suggests a potential mechanism by which various combinations of histone tail modifications can be used to control accessibility of DNA-binding proteins to nucleosomal DNA.
