Physiological Levels ofSalt and Polyamines FavorWrithe and Limit Twist in DNA.

Quantitative analysis of single molecule experimentsshow thatadding either of two natural polyamines, spermine or spermidine, producedmore compact plectonemes in DNA in physiological concentrations ofmonovalent salt. They also promoted plectoneme formation at lowervalues of torsion in measurements of extension versus twist. Quantifyingchanges in the plectonemic DNA using some results from simple rodmodels suggested that exposure to polyamines reduced the radii andincreased the densities of plectonemes. Thus, polyamines may limitthe twist density by favoring writhe which maintains the B-form. Althoughpolymerases may significantly stretch the double helix, denature DNA,and produce twist instead of writhe, natural polyamines stabilizebase-pairing, limit twist to maintain the B-form, and promote supercoiling,which is conducive to replication and transcription and essentialfor DNA packaging. [ABSTRACT FROM AUTHOR]