PARALLEL AND ANTIPARALLEL (G-CENTER-DOT-GC)(2) TRIPLE-HELIX FRAGMENTSIN A CRYSTAL-STRUCTURE

Nucleic acid triplexes are formed by sequence-specific interactions be tween single-stranded polynucleotides and the double helix. These trip lexes are implicated in genetic recombination in vivo and have applica tion to areas that include genome analysis and antigene therapy. Despi te the importance of the triple helix, only limited high-resolution st ructural information is available. The x-ray crystal structure of the oligonucleotide d(GGCGAATTGG) is described; it was designed to contain the d(G . GC)(2) fragment and thus provide the basic repeat unit oi a DNA triple helix. Parameters derived from this crystal structure have made it possible to construct models of both parallel and antiparalle l triple helices.