Journal
BIOPHYSICAL JOURNAL
Volume 102, Issue 11, Pages 2575-2584Publisher
CELL PRESS
DOI: 10.1016/j.bpj.2012.04.024
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Funding
- National Institutes of Health [R15DK081191-01]
- Camille and Henry Dreyfus Foundation
- Ohio Board of Regents
- National Science Foundation [1004987]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [1004987] Funding Source: National Science Foundation
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G-quadruplex has demonstrated its biological functions in vivo. Although G-quadruplex in single-stranded DNA (ssDNA) has been well characterized, investigation of this species in double-stranded DNA (dsDNA) lags behind. Here we use chemical footprinting and laser-tweezers-based single-molecule approaches to demonstrate that a dsDNA fragment found in the insulin-linked polymorphic region (ILPR), 5'-(ACA GGGG TGT GGGG) 2 TGT, can fold into a G-quadruplex at pH 7.4 with 100 mM K+, and an i-motif at pH 5.5 with 100 mM Li+. Surprisingly, under a condition that favors the formation of both G-quadruplex and i-motif (pH 5.5, 100 mM K+), a unique determination of change in the free energy of unfolding (Delta G(unfold)) by laser-tweezers experiments provides compelling evidence that only one species is present in each dsDNA. Under this condition, molecules containing G-quadruplex are more stable than those with i-motif. These two species have mechanical stabilities (rupture force >= 17 pN) comparable to the stall force of RNA polymerases, which, from a mechanical perspective alone, could justify a regulatory mechanism for tetraplex structures in the expression of human insulin.
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