SUPERCOILING-REGULATED LIQUID-CRYSTALLINE PACKAGING OF TOPOLOGICALLY-CONSTRAINED, NUCLEOSOME-FREE DNA-MOLECULES

Electron microscopy and circular dichroism studies of cholesteric aggr egates derived from topologically-constrained DNA molecules indicate t hat the overall morphology and structural properties of these aggregat es are fundamentally different from those characterizing condensed str uctures of nonconstrained DNA species. Specifically, the cholesteric p itch and twist of all hitherto characterized lyotropic mesophases of b iopolymers-including those obtained from linear DNA-depend predominant ly upon environmental parameters such as the dielectric constant of th e solvent. In contrast, the properties of aggregates derived from clos ed circular supercoiled DNA are found to be solely and directly dictat ed by the superhelical density and handedness. On the basis of these r esults, as well as on the demonstrated ubiquity of liquid-crystalline DNA organizations in vivo, we suggest that supercoiling-regulated liqu id crystallinity represents an effective packaging mode of nucleosome- free, topologically-constrained DNA molecules in living systems.