Giardia Telomeric Sequence d(TAGGG) 4 Forms Two Intramolecular G-Quadruplexes in K + Solution: Effect of Loop Length and Sequence on the Folding Topology

Recently, it has been shown that in K(+) solution the human telomeric sequence dTAGGG(TTAGGG)(3) forms a (3 + 1) intramolecular G-quadruplex, while the Bombyx mori telomeric sequence dTAGG(TTAGG)(3), which differs from the human counterpart only by one G deletion in each repeat, forms a chair-type intramolecular G-quadruplex, indicating an effect of G-tract length on the folding topology of G-quadruplexes. To explore the effect of loop length and sequence on the folding topology of G-quadruplexes, here we examine the structure of the four-repeat Giardia telomeric sequence dTAGGG(TAGGG)(3), which differs from the human counterpart only by one T deletion within the non-G linker in each repeat. We show by NMR that this sequence forms two different intramolecular G-quadruplexes in K(+) solution. The first one is a novel basket-type antiparallel-stranded G-quadruplex containing two G-tetrads, a G x (A-G) triad, and two A x T base pairs; the three loops are consecutively edgewise-diagonal-edgewise. The second one is a propeller-type parallel-stranded G-quadruplex involving three G-tetrads; the three loops are all double-chain-reversal. Recurrence of several structural elements in the observed structures suggests a "cut and paste" principle for the design and prediction of G-quadruplex topologies, for which different elements could be extracted from one G-quadruplex and inserted into another.