RECOMBINATION-LIKE STRUCTURE OF D(CCGCGG)

We have solved the single crystal X-ray structure of the synthetic DIV A hexamer d(CCGCGG). The central alternating tetramer forms a Z-DNA du plex. The initial cytosine of each strand of the duplex swings out and forms a Watson-Crick base-pair with the terminal guanine of a symmetr y-related molecule. Thus, two symmetry-related DNA molecules form a tw in with intermolecular base-pairs at both ends. Such a twin is additio nally stabilized by a sodium ion located on a dyad axis between two DN A duplexes. The total structure has recombination-like features. It al so provides a model for B/Z junctions. The crystal used in this study belongs to space group C222(1) with a = 34.33 Angstrom, b = 44.04 Angs trom and c = 38.27 Angstrom. The structure was solved by molecular rep lacement using partial models, and refined by molecular dynamics simul ated annealing and positional treatment. The refinement has been concl uded with an R-factor of 18.5% for 2377 reflections with F greater tha n or equal to 2 sigma (F) in the resolution region 8.0 to 1.92 Angstro m. The asymmetric unit contains two strands of d(CCGCGG) and 38 water molecules.