Cations mediate B-DNA conformational heterogeneity

We demonstrate that DNA conformation is sensitive to cationic environment, Vile describe a high resolution (1.2 Angstrom) potassium form of CGCGAATTCG CG, determined from crystals grown in the presence of spermine and magnesiu m, along with potassium. The structure was refined with anisotropic displac ement-factors by SHELX-97 to an R-factor of 13.9%. A comparison of this str ucture with others, reveals that the conformation of CGCGAATTCGCG varies in direct response to cation type and position. The DNA conformation in the p resence of excess magnesium differs from the conformation in the presence o f excess spermine. Divalent cations near the minor groove sequester into th e lip, which is the region between opposing phosphate groups. Minor groove width is sensitive to, and can be predicted by, cation positions. Tt appear s that minor groove narrowing is facilitated by interactions of cations wit h opposing phosphate groups.