Diffusion-enhanced resonance energy transfer shows that linker-DNA accessibility decreases during salt-induced chromatin condensation.

Accessibility of linker-DNA chromatin during salt-induced condensation of chicken erythrocytes chromatin was studied by diffusion-enhanced resonance energy transfer. A terbium complex was covalently bound to linker-DNA and fluorescein molecules bound to latex particles with diameters ranging from 14 to 2470 nm were used as acceptor. The accessibility of linker-DNA to molecules with a diameter superior to 14 nm diminished during condensation, but for an acceptor diameter of 14 nm or less, no accessibility variation was observed. It can be concluded that (1) linker-DNA is located inside the fiber when chromatin is in the condensed state, (2) chromatin condensation can prevent the approach to DNA due to steric hindrance, (3) salt-induced chromatin condensation is a gradual process, and (4) condensed chromatin models containing a central cavity are more likely.