Divalent zinc cations induce the formation of two distinct homoduplexes of a d(GA)20 DNA sequence.

Homopurine DNA sequences are highly structurally polymorphic. In particular, d(GA)n DNA sequences are known to be capable of forming intramolecular foldbacks, bimolecular homoduplexes, and tetrastranded complexes. Counterions play a determinant role on the equilibria between the different structural conformers of d(GA)n sequences. In this paper, the effect of divalent zinc cations on the structure of a d(GA)20 oligonucleotide has been analyzed by CD spectroscopy and polyacrylamide gel electrophoresis. Depending on the precise experimental conditions at which zinc is added, two distinct conformations of the d(GA)20 oligonucleotide are stabilized. At neutral pH in the absence of zinc, d(GA)20 is partially organized into intramolecular foldbacks and bimolecular homoduplexes [Casasnovas et al. (1993) J. Mol. Biol. 233, 671-681]. Under these conditions, addition of zinc results in the stabilization of the bimolecular homoduplex which is nonspecific for zinc since it is also stabilized by divalent magnesium cations, increasing ionic strength, or decreasing pH. Its CD spectrum is identical to that reported earlier for parallel-stranded d(GA)n homoduplexes [Rippe et al. (1992) EMBO J. 11, 3777-3786]. On the other hand, if zinc is added under conditions where the d(GA)20 oligonucleotide is exclusively single-stranded, a different bimolecular homoduplex appears which is only observed in the presence of zinc. The zinc-specific duplex melts cooperatively, and, in contrast to the nonspecific duplex, its thermostability is high. Transition from the nonspecific to the zinc-specific duplex is observed at high zinc concentrations or at high temperatures. The transition is cooperative. These results are discussed in the context of the specific cation effects on the formation of intramolecular R.R.Y triplexes at d(GA.TC)n DNA sequences.