Binding Features of Ethidium Bromide and Their Effects on Nuclease Susceptibility of Calf Thymus DNA in Presence of Spermine

These data prompted us to undertake present work testingthe effect of ethidium bromide on the susceptibility of calfthymus DNA for DNase 1 in the absence and presence ofspermine at the concentration where the DNA collapses,with the intent of developing probes to study the structuraltransition of DNA in its monomolecular condensation pro-cess induced by spermine. The present study demonstratesspermine effect on ethidium-dependent DNA susceptibilityto DNase 1 correlated with altered features of ethidium bind-ing to DNA. Experimental SectionMaterials. Calf thymus DNA (Type 1), RNA (purifiedfrom Baker’s yeast, Type XI), deoxyribonuclease 1 (DNase1, DN-100), ribonuclease A (RNase A, Type 1-AS), andspermine tetrahydrochloride were purchased from SigmaChemical Company and used without further purification.Ethidium bromide (Sigma) was recrystallized once withmethanol prior to use. The heat denatured DNA was pre-pared by heating the DNA (OD