Salt-dependent compaction of di- and trinucleosomes studied by small-angleneutron scattering

Using small-angle neutron scattering (SANS), we have measured the salt-depe ndent static structure factor of di- and trinucleosomes from chicken erythr ocytes and from COS-7 cells. We also determined the sedimentation coefficie nts of these dinucleosomes and dinucleosomes reconstituted on a 416-bp DNA containing two nucleosome positioning sequences of the 5S rDNA of Lytechinu s variegatus at low and high salt concentrations. The internucleosomal dist ance d was calculated by simulation as well as Fourier back-transformation of the SANS curves and by hydrodynamic simulation of sedimentation coeffici ents. Nucleosome dimers from chicken erythrocyte chromatin show a decrease in d from similar to 220 Angstrom at 5 mM NaCl to 150 Angstrom at 100 mM Na Cl. For dinucleosomes from COS-7 chromatin, d decreases from 180 Angstrom a t 5 mM to 140 Angstrom at 100 mM NaCl concentration. Our measurements on tr inucleosomes are compatible with a compaction through two different mechani sms. depending on the salt concentration. Between 0 and 20 mM NaCl, the int ernucleosomal distance between adjacent nucleosomes remains constant, where as the angle of the DNA strands entering and leaving the central nucleosome decreases. Above 20 mM NaCl, the adjacent nucleosomes approach each other, similar to the compaction of dinucleosomes. The internucleosomal distance of 140-150 Angstrom at 100 mM NaCl is in agreement with distances measured by scanning force microscopy and electron microscopy on long chromatin fila ments.