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.