DNA Toroids: Stages in Condensation

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Abstract
The effects of polylysine (PLL) and PLL−asialoorosomucoid (AsOR) on DNA condensation have been analyzed by AFM. Different types of condensed DNA structures were observed, which show a sequence of conformational changes as circular plasmid DNA molecules condense progressively. The structures range from circular molecules with the length of the plasmid DNA to small toroids and short rods with ∼1/6 to 1/8 the contour length of the uncondensed circular DNA. Single plasmid molecules of 6800 base pairs (bp) condense into single toroids of ∼110 nm diameter, measured center-to-center. The results are consistent with a model for DNA condensation in which circular DNA molecules fold several times into progressively shorter rods. Structures intermediate between toroids and rods suggest that at least some toroids may form by the opening up of rods as proposed by Dunlap et al. [(1997) Nucleic Acids Res. 25, 3095]. Toroids and rods formed at lysine:nucleotide ratios of 5:1 and 6:1. This high lysine:nucleotide ratio is discussed in relation to entropic considerations and the overcharging of macroions. PLL−AsOR is much more effective than PLL alone for condensing DNA, because several PLL molecules are attached to a single AsOR molecule, resulting in an increased cation density.