Bombyx mori single repeat telomeric DMA sequence forms a G-quadruplex capped by base triads

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Abstract
A combined NMR-molecular dynamics approach has been applied to determine the solution structure of a truncated analogue of the Bombyx mori telomeric d(TTAGG) single repeat sequence in Na+ cation-containing aqueous solution. The two-fold symmetric four-stranded d(TAGG) quadruplex contains two adjacent G(syn).G(syn).G(anti).G(anti) G-tetrads sandwiched between novel (T.A).A triads with individual strands having both a parallel and antiparallel neighbour around the quadruplex. The (T.A).A triad represents the first experimental verification of a base triad alignment which constitutes a key postulate in the recently proposed model of triad-DNA. Further, the (T.A).A triad is generated by positioning an A residue through hydrogen bonding in the minor groove of a Watson-Crick T.A base pair and includes a T-A platform related to an A-A platform recently observed in the structure of the P4-P6 domain of the Tetrahymena self splicing group I ribozyme. The novel architecture of the truncated Bombyx mori quadruplex structure sets the stage for the design and potential identification of additional base tetrads and triads that could participate in pairing alignments of multi-stranded DNA structures during chromosome association and genetic recombination.