UNFOLDED STRUCTURE AND REACTIVITY OF NUCLEOSOME CORE DNA-HISTONE H2A,H2B COMPLEXES IN SOLUTION AS STUDIED BY SYNCHROTRON RADIATION X-RAY-SCATTERING

It has been previously found using different physicochemical technique s [Aragay, A., Diaz, P., & Daban, J.-R. (1988) J. Mol. Biol. 204, 141- 1541 that histones H2A,H2B in the absence of H3,H4 can associate with nucleosome core DNA (146 base pairs). Here we describe a synchrotron X -ray scattering study of core DNA-(H2A,H2B) complexes in solution. Our results obtained using different histone to DNA weight ratios and ion ic conditions ranging from very low ionic strength to 0.2 M NaCl show that histones H2A,H2B are unable to fold core DNA. Model calculations indicate that histones H2A,H2B produce very elongated structures even when the reconstituted complexes are prepared at physiological ionic s trength. In contrast, our scattering data indicate that the reconstitu ted complexes prepared at physiological salt concentration either with the four core histones or with histones H3,H4 without H2A,H2B are com pletely folded particles with a radius of gyration similar to that cor responding to the native nucleosome core (4.2 nm). Furthermore, our re sults show that the DNA of the extended complexes containing histones H2A, H2B becomes completely folded after the histone pair exchange rea ction that occurs spontaneously between preformed DNA-(H2A,H2B) and DN A-(H3,H4) complexes. These observations, together with our previous st udies, suggest that the open conformation of DNA-(H2A,H2B) complexes f acilitates the involvement of this structure as a transient intermedia te in the reaction of nucleosome formation at physiological ionic stre ngth.