Flow of structural information between four DNA conformational levels.

Closed-circular supercoiled DNA molecules have been shown to form a cholesteric assembly within bacteria as well as in vitro under physiological DNA and salt concentrations. Circular dichroism and X-ray scattering studies indicate that the macroscopic structural properties of the chiral mesophase are directly and uniquely dictated by the supercoiling parameters of the constituent molecules. Specifically, we find that the pitch of the DNA cholesteric phase derived from supercoiled DNA is determined by the superhelical density, which, in turn, is modulated by secondary conformational changes. A direct interrelationship among four DNA structural levels, namely, DNA sequence, secondary structural transitions, the tertiary superhelical conformation, and the quaternary, supramolecular organization is accordingly pointed out. Since secondary conformational changes are both sequence and environment dependent, alterations of cellular conditions may effectively modulate the properties of the packed DNA organization, through their effects on secondary structural transitions and hence on the superhelical parameters. On the basis of these results we suggest that liquid crystallinity represents an effectively regulated packaging mode of plectonemic, nucleosome-free DNA molecules in living systems.