Dissociation of single-stranded DNA from nucleosomes following modification with acetic anhydride

Modification with acetic anhydride of nucleosomes from chicken erythrocytes at low ionic strength (less than 0.1 M NaCl) is accompanied by the formation of residual particles and the release of free DNA. This DNA has been identified as single-stranded by thermal denaturation, digestion with nuclease S1, and elution from hydroxyapatite. In contrast, if modification takes place at 0.6 M NaCl, the liberated DNA is mainly double-stranded. The release of the free energy stored in folded nucleosomal DNA, triggered by the weakening of lysine-DNA interactions which takes place upon modification, might be responsible for the observed denaturation of DNA at low ionic strength.