Condensation of bacteriophage .vphi.W14 DNA of varying charge densities by trivalent counterions

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Abstract
Bacteriophage .vphi.W14 DNA carries the hypermodified, positively charged (2+) base .alpha.-putrescinylthymine (putThy) and consequently exhibits a decreased average linear charge density compared to the conventional B-form DNA helix. Noting that the unusual physical characteristics may contribute to the collapse properties of this DNA and may facilitate the exceptionally high density of packaging of its genome in .vphi.W14, total intensity light scattering was used to determine in vitro the critical concentrations of spermidine (Spd, 3+) required to induce the cooperative, monomolecular collapse of wild-type and mutant .vphi.W14 DNA samples and quasi-elastic light scattering to compare the dynamic characteristics of the compacted particles. The DNA samples carried various percentages of the modified base with average charge spacings ranging from 1.3-2.2 .ANG. in comparison to T4 phage DNA (1.7 .ANG.). The results are analyzed and discussed both from a general theoretical point of view according to the counterion condensation theory of Manning (1978) and from the more specialized aspect of DNA packaging in .vphi.W14. In accord with theory, DNA of lower charge density require a considerably higher critical counterion concentration (up to 118 .mu.M Spd), whereas the outside diameter of the toroidal condensates, which they form, varies only marginally. Specific ion effects were probed by substituting hexaamminecobalt(III) (Hc, 3+) for Spd. Hc appears to be more efficient than Spd: it induces the collapse of all DNA samples at only one-sixth the critical concentration of Spd, and its condensates are 30% smaller (1072-1142 .ANG. vs. 744-800 .ANG.) except for wild-type .vphi.W14 DNA, which forms Hc-collapsed particles indistinguishable from Spd-induced condensates. Collapse occurs, again with the exception of wild-type .vphi.W14 DNA, when .apprx. 89% of the charges on each DNA are neutralized by territorially bound Spd. The driving force for condensation clearly is a function of the charge density of the DNA and the charge distribution may be an important factor in determining the degree of neutralization at which collapse becomes possible. The sample with the lowest charge density, wild-type .vphi.W14 DNA, does not follow the trends set by the other members of the series. The possibility is discussed that lowering the charge density by covalent modification beyound a threshold may result in the compression of the DNA double helix; this allows more information to be carried on a genome, the packageable length of which is determined by the encapsidation mechanism.