FORMATION OF TRIPLE HELICES IN THE CRYSTAL STRUCTURE OF d(GGCCAATTGG). Alain Dautant¹, Bernard Gallois¹, Gilles Précigoux¹, Dominique Vlieghe², Luc Van Meervelt² & Olga Kennard³, ¹Unité de Biophysique structurale, CNRS, Université de Bordeaux, 33405 Talence, France, ²Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium, ³Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK

Although X-ray crystal structures of DNA triplets are already known, no high resolution structure could be established for triple helices. We present a way of obtaining high-resolution data for short triple helices, based on special choices of oligonucleotide lengths and sequences for crystallization. This crystal engineering technique is based on the previously determined structure of d(GCGAATTCG), where single triplets arose from interaction between double helices with overhanging bases at the 5'-ends of both strands(1,2). The crystal structure of d(GGCCAATTGG) was established to a resolution of 2 Å(3). This sequence forms a canonical B-DNA double helical octamer structure with at both 5'-sides two single stranded overhangs of two guanine nucleotides. These overhangs do interact with symmetrical equivalent structures to form short triple helices containing two successive (C-G)*G triplets. Surprisingly, two different kinds of triplexes are observed. The first one is formed by parallel interaction of the third strand with the second G-strand, and displays Hoogsteen-like hydrogen bond patterns. The second triplex is formed by antiparallel interaction between the third and the second strand, using reverse Hoogsteen-like hydrogen patterns to form triplexes. Both triplexes give us detailed information of parallel and antiparallel triplex formation. It is also a clear example of the asymmetric behavior of inherent symmetric sequences.

(1) Van Meervelt, L. et al. Nature 374, 742-744 (1995).

(2) Vlieghe D. et al., Acta Cryst D (accepted)

(3) Vlieghe D. et al., (to be published)