Polyamines stabilise DNA folds.

Abstract

THE DNA of bacterial cells, gently isolated in the presence of high concentrations of monovalent salts, remains folded in a particle resembling the bacterial nucleoid1,2. The double-helical DNA is condensed by folding1 and by supercoiling3 and the compact structure is dependent both on counterions in the solvent environment and RNA molecules bound to the DNA1–4. In the absence of these stabilising interactions the DNA unfolds and acquires the usual properties of extended double-helical DNA1,3. The concentrations of monovalent salts required during the isolation of folded DNA are significantly greater than those found in the cell. It is, however, believed that in vivo polyamines may be an important counterion neutralising DNA5. We have now found that moderate concentrations of the polyamine spermidine greatly stabilise the condensed DNA conformations in isolated nucleoids. In addition to broadening current understanding of the role of counterions in stabilising condensed DNA states, this finding will be of significance to studies in which the stability of the nucleoid in vitro is important.