REVERSIBLE OLIGONUCLEOSOME SELF-ASSOCIATION - DEPENDENCE ON DIVALENT-CATIONS AND CORE HISTONE TAIL DOMAINS

Regularly spaced nucleosomal arrays equilibrate between unfolded and h ighly folded conformations in <2 mM MgCl2, and self-associate above 2 mM MgCl2 [Schwarz, P. M., & Hansen, J. C. (1994) J. Biol. Chem. 269, 1 6284-16289]. Hen we use analytical and differential sedimentation tech niques to characterize the molecular mechanism and determinants of oli gonucleosome self-association. Divalent cations induce self-associatio n of intact nucleosomal arrays by binding to oligonucleosomal DNA and neutralizing its negative charge. Neither linker histones nor H2A/H2B dimers are required for Mg2+ dependent self-association. However, diva lent cations are unable to induce self-association of trypsinized nucl eosomal arrays lacking their N- and C-terminal core histone tail domai ns. This suggests that the H3/H4, tail domains directly mediate oligon ucleosome self-association through a non-Coulombic-based Self-associat ion occurs independently of whether the oligonucleosome monomers are f olded The first step in the self-association pathway is strongly coope rative and produces a soluble association intermediate that sediments similar to 10 times faster than the oligonucleosome monomers. The size of the oligonucleosome polymers increases rapidly as a consequence of small increases in the divalent cation concentration, eventually prod ucing polymeric species that sediment at much greater than 10 000 S. I mportantly, all steps in the self-association pathway are freely rever sible upon removal of the divalent cations. Taken together, these data indicate that short oligonucleosome fragments composed of only core h istone octamers and DNA possess all of the structural features require d to achieve chromosome-level DNA compaction. These findings provide a molecular basis for explaining many of the recently uncovered structu ral features of interphase and metaphase chromosomal fibers.