DNA INTERNAL MOTIONS WITHIN NUCLEOSOMES DURING THE CELL-CYCLE AND AS A FUNCTION OF IONIC-STRENGTH

We have used P-31 NMR spectra to show that DNA internal motions are gr eatly hindered within oligonucleosomes. The fluctuations seem to be a function of both the cell cycle and the number of nucleosomes interlin ked. Namely, the resonance areas, directly related to unbound phosphat e, are consistently smaller in M-phase than in S-phase; at the same ti me, the resonance line width, inversely related to base plane, deoxyri bose, and phosphate internal motions, is consistently larger in mononu cleosomes than in oligonucleosomes. In all cases, the removal of chrom osomal proteins, by a progressive increase of ionic strength up to 2 M NaCl, increases the internal motion, as monitored by a decrease in li ne width toward that of free DNA. While for both oligo- and mononucleo somes in S-phase the decrease in line width is strictly correlated to a sharp increase in resonance area, in M-phase it is not, with the P-3 1 resonance area rather low even at 2.0 M NaCl extraction. Similarly, while S-phase P-31 line widths steadily grow from mono- to oligonucleo somes, in M-phase they do not. Moreover, the increase of the ionic str ength to 0.6 M NaCl, as compared to 0.35, 1.2, and 2 M NaCl, displays significant variations on P-31 line width and resonance area, independ ent of the cell cycle phase and the number of nucleosomes interlinked. These observations agree with earlier suggestions on the differential role of the various chromosomal protein subfractions, known to prefer entially dissociate at the different ionic strengths in question, in t he sealing of mononucleosomes and in the overall stability of polynucl eosomes.