The role of histone H1 in compaction of nucleosomes. Sedimentation behaviour of oligonucleosomes in solution.

The dependence of sedimentation coefficients of oligonucleosomes on the number of nucleosomes in the chain in solutions of different ionic strength has been studied for oligonucleosomes both containing and lacking histone H1. The analysis of these dependencies has shown that oligonucleosomes with H1 at low concentration (I = 0.01 mol/l) may be described by the model of a short cylinder with an average length of the chain per nucleosome l0 = 11 nm where the neighbouring nucleosomes are in close contact. Oligonucleosomes without H1 at I = 0.01 mol/l can be described by the model of a worm-like chain with l0 = 27 nm. This suggests that when H1 is removed the linker DNA unfolds completely. In 0.15 M NaCl the oligonucleosome chain without H1 folds up to a compact configuration typical of oligonucleosomes with H1 at I = 0.01 mol/l. Thus, the linker DNA, with charges being screened, may fold due to interactions with core histones. Oligonucleosomes with H1 in 0.15 M NaCl form a supercoiled structure, whose stable conformation is accounted for by cooperative interactions of no less than five nucleosomes.