The effects of monovalent (Na+, K+) and diva- lent (Mg2+, Ca2+, Mn2+) ions on the interaction between DNA and histone are studied using the molecular combing tech- nique. ?-DNA molecules and DNA-histone complexes incu- bated with metal cations (Na+, K+, Mg2+, Ca2+, Mn2+) are stretched on hydrophobic surfaces, and directly observed by fluorescence microscopy. The results indicate that when these cations are added into the DNA solution, the fluorescence intensities of the stained DNA are reduced differently. The monovalent cations (Na+, K+) inhibit binding of histone to DNA. The divalent cations (Mg2+, Ca2+, Mn2+) enhance sig- nificantly the binding of histone to DNA and the binding of the DNA-histone complex to the hydrophobic surface. Mn2+ also induces condensation and aggregation of the DNA- his- tone complex.
Molecular combing is a powerful and simple method for aligning DNA molecules onto a surface. Using this technique combined with fluorescence microscopy, DNA-histone complexes are stretched on a hydrophobic polymethyl methacrylate (PMMA) surface and observed directly. We have developed a new method to stretch single DNA-histone complexes, termed spin-stretching. The results show that the histones markedly enhance DNA binding to the PMMA surface. DNA winds around the histones and therefore decreases in length. The number of histones that bind to each DNA molecule is found to correlate with the histone concentration. The combed DNA-histone complexes are found to depend on two factors: the binding force on the surface and the centrifugal force at its local position. Na+ ions should compete with histones for binding to DNA; however, the observed competitive binding effect of Na+ ions at low concentrations was negligible.
LIU YuYingWANG PengYeDOU ShuoXingZHANG WeiWeiWANG XueJinSANG HongYi
