Mg2+-induced transition to strongly cooperative binding of histone H1 to linear DNA

Theoretical analysis of gel electrophoretic data for interaction of lysine rich histone with supercoiled DNA

We demonstrate the possibility of using gel electrophoresis as a technique for the quantitative analysis of interaction of lysine rich histone with DNA. On the basis of theoretical framework for extended ligand binding to one-dimensional lattices such as DNA we have set up systems of equations which relate the ligand-to-DNA ratio to the observed gel migration distance of the complex. From the analysis of experimental data for gel electrophoresis of supercoiled DNA in the presence of lysine rich histones we have found that the observed variation of electrophoretic mobilities of the histone-DNA complexes at low histone-to-DNA ratios can be described by a non-cooperative binding behaviour. At this limit we have estimated the intrinsic binding constant to be of the order of 10(3) M-1.

Nature of protamine-DNA complexes. A special type of ligand binding co-operativity

The mode of protamine binding to DNA double helices has been analyzed for the example of clupein Z from herring and DNA samples from bacteriophages lambda and PM2 by measurements of light-scattering intensities, ultracentrifugation and kinetics. The light-scattering intensity of DNA increases co-operatively at a threshold clupein concentration suggesting co-operative binding of clupein to double helices. These data are first analyzed in terms of a model with a transition at a threshold degree of binding. The parameters resulting from this analysis appear to be reasonable, but are shown to be in contrast with data on the absolute degree of clupein binding to DNA obtained by centrifugation experiments. An analysis of the kinetics associated with clupein binding to DNA by measurements of the time-dependence of light-scattering intensities in the time range of seconds demonstrates directly that clupein-induced intermolecular interactions of DNA molecules are essential. The rate constants of DNA association increase co-operatively at threshold clupein concentrations, which correspond to those observed in the equilibrium titrations. Above the threshold, the rate constants arrive at a level that is almost constant, but shows some decrease with increasing clupein concentrations. These results are described by a model with a monomer and a dimer state of DNA, which bind ligands with different affinities according to an excluded-site binding scheme. When the ligand binding constant is larger for the dimer than for the monomer state, as should be expected, binding of ligands drives the DNA from the monomer to the dimer state, even if the dimerization equilibrium in the absence of ligands is far in favor of the monomer. The transition from the monomer to the dimer state proves to be strongly co-operative. When the ligand concentration is increased to higher values, the dimers may be converted back to monomers due to an increased extent of ligand binding to the monomer state. The model is consistent with the available experimental data. The analysis of the data by the model indicates the existence of a reaction unit much below the DNA chain length, corresponding to about 80 nucleotide residues. The present model describes ligand driven intermolecular association; an analogous model is applicable to ligand driven intramolecular association. In summary, the co-operativity of clupein binding to DNA double helices is not due to nearest neighbor interactions, but results from thermodynamic coupling of clupein binding with clupein-induced DNA association.

Histone H1-DNA interactions and their relation to chromatin structure and function

The belief that histone H1 interacts primarily with DNA in chromatin and much less with the protein component has led to numerous studies of artificial H1-DNA complexes. This review summarizes and discusses the data on different aspects of the interaction between the linker histone and naked DNA, including cooperativity of binding, preference for supercoiled DNA, selectivity with respect to base composition and nucleotide sequence, and effect of H1 binding on the conformation of the underlying DNA. The nature of the interaction, the structure of the complexes, and the role histone H1 exerts in chromatin are also discussed.