A reversible conformational transition in chromatin

Abstract

THE DNA in the nucleosome is tightly wound in about two turns around a core protein octamer (reviewed in refs 1 and 2), and in this conformation the two strands of the double helix cannot be separated. Chromatin known to be transcriptionally active exhibits differences, both in the packing ratio of the DNA and in appearance under the electron microscope. At the same time, the periodicity revealed by nuclease digestion of nucleosomes is preserved, and histones remain attached to the DNA^2,3. This evidence strongly suggests that chromatin has the ability to undergo a conformational transition between a folded and an open state. Previous electron microscopic observations^4,5 have revealed that at very low ionic strengths nucleosomes undergo two transitions, one apparently involving a dissociation into ‘half-nucleosomes’, the other a transformation to a fully extended state. Transitions have also been deduced from hydro-dynamic studies⁶ and analysis of thermal denaturation profiles^7,8. All these methods are applied to dilute solutions and are confined to conditions in which the chromatin is soluble. On the other hand, methods usable at high concentrations have failed to produce evidence of an unfolded state at neutral pH, even when the concentrated chromatin solution was first dialysed against solutions of very low ionic strength. This seeming inconsistency can be understood, however, if one considers the concentration of the excess negative charges on the phosphate groups of the DNA in the condensed state, for as a consequence of the Donnan effect, one cannot reduce the sodium ion concentration below the level of charge equivalence without also lowering the pH. We now report the characterisation of a pH-dependent reversible transition of chromatin to an unfolded form.