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

Abstract

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.

Polyelectrolyte Counterion Condensation Theory Explains Differential Scanning Calorimetry Studies of Salt-Induced Condensation of Chicken Erythrocyte Chromatin

“…Domain I is linker DNA, free of contact with histones and containing N I nucleotides. As it is located inside the condensed fiber (Zlatanova et al, 1994;Bartolome ´et al, 1994;Labarbe et al, 1996), contacts between DNA and non-histone proteins are unlikely; hence, the fraction of phosphate charges neutralized by the proteins is θ I ) 0. Domain II is the part of linker DNA, containing N II nucleotides, that is partially neutralized by the basic histone H1, resulting in a reduction of the negative charge of the phosphates θ II ) 0.58 (Clark & Kimura, 1990).…”

DNA of a circular minichromosome linearized by restriction enzymes or other reagents is resistant to further cleavage: an influence of chromatin topology on the accessibility of DNA

“…Studies of circular chromatin suggest how linker DNA could be exposed to probes; it has been proposed that nucleosomes are distorted ( 63–65 ) and/or re-orientated ( 71 , 73 ) and histone–DNA contacts loosened with transfer of some linker onto their surface ( 74 ). In contrast, in compact linear chromatin, linker DNA is poorly accessible ( 75 ); most evidence indicates that it lies in the interior in vitro ( 75–78 ) and in vivo ( 79 ), and its accessibility could be limited further by self-association and interdigitation ( 80 ) and hairpin formation ( 81 ) due to internucleosomal attractive forces.…”