Polyamines and acetylpolyamines increase the stability and alter the conformation of nucleosome core particles

The interactions of spermine (4+ charge at physiological pH), N1-acetylspermine(3+), spermidine(3+), N1- and N8-acetylsperimidine(2+), putrescine(2+), hexaamminecobalt(3+), and magnesium(2+) with nucleosome core particles have been examined by using thermal denaturation and circular dichroism. Tetra- and triamines were 2-3 times more effective than diamines at stabilizing core particles against thermal denaturation. Secondary effects were also observed, with acetylpolyamines slightly less effective than unmodified polyamines of equivalent charge. Hexaamminecobalt(3+) was less effective than the triamines, while magnesium had essentially no effect. This is surprising since magnesium is more effective than diamines at stabilizing naked DNA. All the cations tested altered the circular dichroism spectra of the core particles in the DNA region (284 nm). The peak at 284 nm was suppressed by tetra- and trivalent compounds to approximately twice the extent of divalent compounds. Magnesium appears to suppress the peak by a lesser extent than the diamines. This indicates that the DNA twist and/or folding is changed by these cations. A plateau of both thermal denaturation and circular dichroism effects was observed at cation concentrations where 30-40% of the total DNA negative charges could be neutralized by the added cations. We suggest that polyamine and histone acetylation function in concert to lower the stability and change the conformation of the nucleosome core, thus facilitating replication and transcription in vivo.