EXTENSION OF THE 4-STRANDED INTERCALATED CYTOSINE MOTIF BY ADENINE-CENTER-DOT-ADENINE BASE-PAIRING IN THE CRYSTAL-STRUCTURE OF D(CCCAAT)

The crystal structure of d(CCCAAT), refined at 2.0 Angstrom resolution , shows a four stranded molecule in which two parallel duplexes interc alate with opposite polarity using cytosine . protonated cytosine base pairs, The intercalation motif in this structure is extended by adeni ne adenine base pairs, Two topologically distinct broad grooves;are fo und in the lath-like central part of the molecule with the phosphate g roups on one side bent over towards each other, stabilized by bridging water molecules, At the 3' ends, two arrangements of intermolecular A . A . T base triplets are found, involving both asymmetric and symmet ric A . A base pairs joined to thymine residues by Watson-Crick and re verse Hoogsteen base pairing, respectively.