Abstract
A human telomere ends in a single-stranded 3′ tail, composed of repeats of T2AG3. G-quadruplexes (GQs) formed from four consecutive repeats have been shown to possess high-structural and mechanical diversity. In principle, a GQ can form from any four repeats that are not necessarily consecutive. To understand the dynamics of GQs with positional multiplicity, we studied five and six repeats human telomeric sequence using a combination of single molecule FRET and optical tweezers. Our results suggest preferential formation of GQs at the 3′ end both in K+ and Na+ solutions, with minor populations of 5′-GQ or long-loop GQs. A vectorial folding assay which mimics the directional nature of telomere extension showed that the 3′ preference holds even when folding is allowed to begin from the 5′ side. In 100 mM K+, the unassociated T2AG3 segment has a streamlining effect in that one or two mechanically distinct species was observed at a single position instead of six or more observed without an unassociated repeat. We did not observe such streamlining effect in 100 mM Na+. Location of GQ and reduction in conformational diversity in the presence of extra repeats have implications in telomerase inhibition, T-loop formation and telomere end protection.
Highlights
As early as 1962, guanosine moieties were known to self-assemble into tetrameric structures via Hoogsteen hydrogen bonds [1]
Formation of secondary structures was first examined through single molecule fluorescence resonance energy transfer (smFRET) as a function of K+ concentration (Fig. S1b)
In the context of longer telomeric sequences harboring more than four T2AG3 repeats, as we studied here using five repeats and six repeats using fluorescence-force spectroscopy, GQ polymorphism in tandem with their positional multiplicity along the DNA add to the complexity
Summary
As early as 1962, guanosine moieties were known to self-assemble into tetrameric structures via Hoogsteen hydrogen bonds [1]. Planar association between four guanines by eight hydrogen bonds in the presence of a central coordinating monovalent cation (e.g. Na+ and K+) generates a G-quartet or a G-tetrad [2,3]. Stacking of two or more G-quartets creates a very stable DNA secondary structure called the G-Quadruplex (GQ) [4]. Complete sequencing of human genome has identified about 300,000 putative sequences that can fold into GQs [5]. Expansion of GQforming motifs have been implicated in pathogenicity associated with several human neurological disorders [6]. GQ-forming sequences have been reported in various viral genomes including human immuno-deficiency virus, Epstein-Barr virus and human papillomavirus [7]
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