Cobalt hexammine induced tautomeric shift in Z-DNA: the structure of d(CGCGCA).d(TGCGCG) in

Part 1 of this thesis reviews the known dynamical processes of DNA, histories and nucleosomes. Part 2 presents research on salt-induced nucleosome dissociation. In elevated [NaCl], dissociation occurs within minutes to hours, to generate free DNA. Dissociation is reversible (or nearly so), and is enhanced by nucleosome dilution; thus it appears to be an equilibrium phenomenon. Upon exposure of nucleosome core particles to 0.35-0.75 M NaC1, a second reversible transition is observed, on a much faster time-scale (thesis, part 3). This fast transition results in a 15 % decrease in s g0. It also involves an increase in circular dichroism around 280 nm, amounting to 35-40 % of the difference in CD between free DNA and native core particles. The fast transition is interpreted in terms of a two-state model: the core particle is envisioned to move from the native state into a new state, where some portion of its DNA is now more like the solution "B " form. The CD increase (assayed in 0.65 M Nadi) is temperature independent over 5-35 C. This is interpreted (via the two-state model) to mean that the fast transition is entropy-driven. From a consideration of both these data and published studies, we suggest that the fast transition involves the release of two 25-30 by DNA "termini " from the histone core. Under this model, we calculate that 5-7 DNA/histone bonds are broken in the release of each terminus. From oligopeptide/DNA binding data, we calculate these bonds to be at least as strong as oligoarginine/DNA bonds; thus they probably have substantial non-electrostatic character. Part 4 of this thesis describes a test system with which to monitor the sliding of histone cores along DNA. Single-end-labelled 195 by Lytichinus variegatus 5S rDNA (+ flanking regions) serves as a DNA substrate upon which nucleosomes are reconstituted in a uniquely phased arrangement. A quantitative analysis of the loss of phasing expected from core sliding is presented.