Altered structure of the DNA duplex recognized by yeast transcription factor Reb1p

The Saccharomyces cerevisiae REB1 gene encodes a sequence-specific DNA binding protein that has been implicated in chromatin structure, transcription regulation and transcription termination. Previous work has shown that the DNA sequence recognized by Reb1p contains an adenosine residue that is unusually reactive toward chemical modification by dimethylsulfate and that methylation of this nucleoside increases the binding affinity of the Reb1p protein for its target. Prompted by these results, we determined the solution structure of the 13mer Reb1p DNA duplex recognition site d(GTCCGGGTAATGC).d(GCATTACCCGGAC) using 2D NMR, distance geometry and iterative 2D NOESY back-calculation structure refinement. The distance geometry-refined molecule demonstrated an unusual structure in the TAAT region of the sequence that was manifested in cross-strand base stacking, as indicated by unusually strong NOE interactions between H2 protons on three adjacent adenosine bases. This structure was compared to two published NMR studies of DNA duplexes containing the related sequence TAAC. The Reb1p DNA structure does not show the conformational mobility or the 'transient kink' at TpA steps characteristic of the related TAAT-containing sequences.