The mechanism of nucleosome assembly onto oligomers of the sea urchin 5 S DNA positioning sequence

We have used a model system composed of tandem repeats of Lytechinus variegatus 5 S rDNA (Simpson, R. T., Thoma, F., and Brubaker, J. M. (1985) Cell 42, 799-808) reconstituted into chromatin with chicken erythrocyte core histones to investigate the mechanism of chromatin assembly. Nucleosomes are assembled onto the DNA template by mixing histone octamers and DNA in 2 M NaCl followed by stepwise dialysis into very low ionic strength buffer over a 24-h period. By 1.0 M NaCl, a defined intermediate composed of arrays of H3.H4 tetramers has formed, as shown by analytical and preparative ultracentrifugation. Digestion with methidium propyl EDTA.Fe(II) indicates that these tetramers are spaced at 207 base pair intervals, i.e. one/repeat length of the DNA positioning sequence. In 0.8 M NaCl, some H2A.H2B has become associated with the H3.H4 tetramers and DNA. Surprisingly, under these conditions DNA is protected from methidium propyl EDTA.Fe(II) digestion almost as well as in the complete nucleosome, even though these structures are quite deficient in H2A.H2B. By 0.6 M NaCl, nucleosome assembly is complete, and the MPE digestion pattern is indistinguishable from that observed for oligonucleosomes at very low ionic strength. Below 0.6 M NaCl, the oligonucleosomes are involved in various salt-dependent conformational equilibria: at approximately 0.6 M, a 15% reduction in S20,w that mimics a conformational change observed previously with nucleosome core particles; at and above 0.1 M, folding into a more compact structure(s); at and above 0.1 M NaCl, a reaction involving varying amounts of dissociation of histone octamers from a small fraction of the DNA templates. In low ionic strength buffer (less than 1 mM NaCl), oligonucleosomes are present as fully loaded templates in the extended beads-on-a-string structure.