FLOW OF STRUCTURAL INFORMATION BETWEEN 4 DNA CONFORMATIONAL LEVELS

Closed-circular supercoiled DNA molecules have been shown to form a ch olesteric assembly within bacteria as well as in vitro under physiolog ical DNA and salt concentrations. Circular dichroism and X-ray scatter ing studies indicate that the macroscopic structural properties of the chiral mesophase are directly and uniquely dictated by the supercoili ng parameters of the constituent molecules. Specifically, we find that the pitch of the DNA cholesteric phase derived from supercoiled DNA i s 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 structura l transitions, the tertiary superhelical conformation, and the quatern ary, supramolecular organization is accordingly pointed out. Since sec ondary conformational changes are both sequence and environment depend ent, alterations of cellular conditions may effectively modulate the p roperties of the packed DNA organization, through their effects on sec ondary structural transitions and hence on the superhelical parameters . On the basis of these results we suggest that liquid crystallinity r epresents an effectively regulated packaging mode of plectonemic, nucl eosome-free DNA molecules in living systems.