CRYSTAL-STRUCTURE OF A DNA DECAMER SHOWING A NOVEL PSEUDO 4-WAY HELIX-HELIX JUNCTION

The crystal structure of the decanucleotide d(CGCAATTGCG)(2) has been solved by a combination of molecular replacement and heavy-atom proced ures and has been refined to an R factor of 20.2% at 2.7 Angstrom. It is not a fully base-paired duplex but has a central core of eight Wats on-Crick base pairs flanked by unpaired terminal guanosines and cytosi nes. These participate in hydrogen-bonding arrangements with adjacent decamer duplexes in the crystal lattice. The unpaired guanosines are b ound in the G+C regions of duplex minor grooves. The cytosines have re latively high mobility, even though they are constrained to be in one region where they are involved in base-paired triplets with G-C base p airs. The 5'-AATT sequence in the duplex region has a narrow minor gro ove, providing further confirmation of the sequence-dependent nature o f groove width.