Nucleosomes and oligonucleosomes were prepared by digestion of human pla- cental nuclei with staphlococccal nuclease and fractionated by gel filtration chromatography. The effect of increasing salt on the structure of nucleosomes

Nucleosomes and oligonucleosomes were prepared by digestion of human placental nuclei with staphlococccal nuclease and fractionated by gel filtration chromatography. The effect of increasing salt on the structure of nucleosomes was examined in the presence and absence of 10 mM MgCl-. Nucleosomes and oligonucleosomes are insoluble over a broad range of salt concentration. Nucleosomes are soluble in 2120 mM (NH.)?SO. containing 10 mM MgCU allowing analyses of changes in nucleosomal DNA by C7D. spectroscopy. Nucleosomes are insoluble in <. 120 mM (NHj-SO. containing 10 mM MgClas demonstrated by turbidity measurements. He conclude that the insolubility of nucleosomes accompanies salt-induced structural changes possibly due to individual particle condensation. As the salt concentration is increased the nucleosomes condense and then relax at higher salt concentrations. INTRODUCTION Eukaryotic chromatin exists as a regularly repeating structure that resembles beads on a s t r ing . The repeating subunits, nucleosomes, contain 140-150 base pairs and are connected by 50-60 base pairs of the continuing double-stranded DNA (1-11). The DNA in the nucleosome appears to be t i gh t l y folded around a protein core of 8 histone molecules ( 2 each of H2A, H2B, H3, and H4) (12-14). The Interaction between the histone core and DNA is not sequence specif ic (8,15-18). Some evidence has been obtained that DNA contained in nucleosomes is transcribed in vivo (19-20). One approach to examining the accessib i l i ty of nucleosomal DNA for t ranscr ipt ion involves the use of DNA-dependent RNA polymerase in v i t ro as a chemical probe of chromatin structure. Recently we have obtained evidence for in v i t ro t ranscr ibab i l i ty of nucleosomes (21). RNA polymerase requires both divalent (Mg or Mn ) and monovalent ( ( N H j + or K) cations for ac t i v i t y (22). Therefore, 1t 1s important to