Nucleic Acids Research, 1986, Vol. 14, No. 12 5081-5097
© 1986
Articles |
Conformational lability of poly(dG-m5dC):poly(dG-m5dC)
Department of Chemistry, Tennessee State University Nashville, TN 37203, USA
Received February 28, 1986. Revised May 19, 1986. Accepted May 19, 1986.
The remarkable ccnformational lability of poly(dG-m5dC): poly(dG-m5dC) is demonstrated by the observation of an acid-mediated confonrational hysteresis. An acid-mediated Z conformation that exists in solutions containing lew sodium concentrations that would nomally favor the B conformation is described in this report. This Z conformation is reached by an acid-base titration of a B-poly(dG-m5dC) :poly(dG-m5dC) solution which is not far from the B-Z transition midpoiont. The resulting Z conformation is thermally very stable, with direct melting into single strands at approximately 100°C. In contrast, the B form DNA, initially in solutions of the same ionic strength but without exposure to acidic pH, exhibits a biphasic melting profile, with conversion into the Z form (with high cooperativity) prior to an eventual denaturation into single strands at around 100°C. Cooling experiments reveal that such biphasic transitions are quite reversible. The transition midpoint for the thermally poised B to Z transformation depends strongly on the NaCl concentration and varies with sample batch. The acid-mediated Z form binds ethidium more weakly than its B counterpart, and the ethidium induced Z to B conversion occurs in a step-wise (non-allosteric) fashion without the requirement of a threshold concentration. The acid-mediated as well as the thermally poised Z conformations are reversed by the addition of EDTA, suggesting the involvement of trace amounts of multivalent metal ions.