The polymorphism in a DNA 16-mer (designated BZ-II) has been investigated by means of circular dichroism (CD) spectroscopy and chemical footprinting. CD spectra indicate that, in low salt, the oligomer is fully right-handed whereas, in high salt, it possesses a B-Z conformational junction: half of the duplex is right-handed while the other half is left-handed. Treatment of BZ-II with diethyl pyrocarbonate (DEPC) confirms the existence of a left-handed segment of the duplex in high salt: enhanced DEPC scission occurs at the G residues in the alternating CG sequence. The scission patterns of the upper and lower strands in BZ-II by the reactive chemical probe MPE-Fe(II), and the antitumor antibiotics dynemicin and Fe-(II)-bleomycin, are different under low salt conditions. The 3′-terminal region of both upper and lower strands and the middle region of the upper strand of BZ-II are preferential cleavage sites in low salt. This result suggests that the methylated cytosines or the alternating CG domain in the molecule perturbs the DNA structure. Under high salt conditions, the reactivity of the Z-DNA segment of BZ-II for MPE-Fe(II) and Fe(II)·bleomycin is dramatically enhanced, while it is reduced in the case of dynemicin. Excess propidium (PI) eliminates preferential cleavage by each of these chemical probes in high salt conditions. This is due in part to conversion of the BZ-DNA molecule into B-DNA, as is seen by a DEPC modification experiment. At low PI concentrations, the DEPC experiment suggests that the Z conformation remains intact; thus, at sufficiently low drug concentrations, we believe the cleavage data reveal a preferential binding mode of these drugs to the Z sequence itself that does not require concomitant Z → B isomerization.
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