(A) Superimposition of the DDD (PDB entry BDL084, in black) and DDDZ10 (gray) duplexes. (B) Superimposition of the DDD2+Z10 (black) and DDDZ10 (gray) duplexes. The view is into the major groove. The Mg2+ ion in the structure of the native DDD duplex is shown in a ball-and-stick representation. The helix axes based on the central hexamer duplexes are coincident (black line for DDD and DDD2+Z10 and gray line for DDDZ10). For both, the helix axes defined by base pairs G10 · C15, C11 · G14, and G12 · C13 are parallel with those of the central hexamers. The helix axes defined by base pairs C1 · G24, G2 · C23, and C3 · G22 in either DDD2+Z10 or DDD are bent as compared to that the DDDZ10 duplex.