HIGH-RESOLUTION STRUCTURE OF A DNA HELIX-FORMING (C-CENTER-DOT-G)ASTERISK-G BASE TRIPLETS

TRIPLE helices result from interaction between single- and double-stra nded nucleic acids. Their formation is a possible mechanism for recomb ination of homologous gene sequences in nature and provides, inter ali a, a basis for artificial control of gene activity, Triple-helix motif s have been extensively studied by a variety of techniques, but few hi gh-resolution structural data are available, The only triplet structur es characterized so far by X-ray diffraction were in protein-DNA compl exes(1,2) studied at about 3 Angstrom resolution, We report here the X -ray analysis of a DNA nonamer, d(GCGAATTCG), to a resolution of 2.05 Angstrom, in which the extended crystal structure contains (C . G)G t riplets as a fragment of triple helix. The guanosine-containing chains are in a parallel orientation, This arrangement is a necessary featur e of models for homologous recombination which results ultimately in r eplacement of one length of DNA by another of similar sequence, The pr esent-structure agrees with many published predictions of tripler orga nization, and provides an accurate representation of an element that a llows sequence-specific association between single- and double-strande d nucleic acids.