Physiological concentration of magnesium ions induces a strong macroscopic curvature in GGGCCC-containing DNA

The bending propensity of non-A/T DNA sequence elements is well known, but helical phasing/gel mobility experiments fail to reveal an intensive macroscopic curvature if A/T tracts are not present in the sequence. Recent X-ray data prove on the other hand that a GGCC element is intrinsically curved toward the major groove, which seemingly contradicts the fact that macroscopic curvature at GGGCCC elements is hardly detectable with a conventional gel mobility assay. Here we show that GGGCCC containing DNA, with no A/T tracts in the sequence context, has a detectable, strong gel mobility anomaly only in the presence of divalent ions (10 mM Mg2+ or Ca2+, 1 mM Zn2+). Metal ions increase the gel mobility anomaly in A/T tracts as well, but the effect is substantially stronger for GGGCCC than for the rigid A/T tracts. Our data suggest that metal ions change the sequence-dependent dynamic features of DNA; on the other hand, there is no evidence of twist-mediated change of the planarity of curvature in the presence of metal ions. The results show that near-physiological concentrations of divalent cations (10 mM MgCl2) have a strong and differential effect on various sequence elements, so that the current picture of sequence-dependent DNA curvature is changed not only in a quantitative, but also in a qualitative sense.