A COOPERATIVE CONFORMATIONAL CHANGE IN DUPLEX DNA INDUCED BY ZN2-IONS( AND OTHER DIVALENT METAL)

Zn2+ and some other divalent metal ions bind to duplex DNA at pHs abov e 8 and cause a conformational change. This new structure does not bin d ethidium, allowing the development of a rapid fluorescence assay. Al l duplex DNAs, regardless of sequence or G . C content, can form this structure. The rate of formation shows a strong dependence on temperat ure, pH, and Zn2+ concentration; at 20-degrees-C, 1 mM Zn2+, and pH 8. 6 the dismutation is half complete in 30 min. Addition of EDTA causes rapid reversion to 'B' DNA, showing that the new conformation retains two strands that are antiparallel. Unlike the ultraviolet or circular dichroism spectra, the nuclear magnetic resonance spectrum was informa tive since the imino protons of both A . T and G . C base pairs are lo st upon addition of a stoichiometric amount of Zn2+. The pitch of the helix was estimated from gel electrophoresis of circular DNAs in the p resence of Zn2+ and it contains at least 5% fewer base pairs per turn than 'B' DNA. The transformation is cooperative and shows hysteresis, suggesting that this is a distinct structure and not simply a minor va riant of 'B' DNA. It is proposed to call this new structure 'M' DNA be cause of the intimate involvement of metal ions.