Abstract
Regions of the genome are often held under torsional constraint. Nevertheless, the influence of such constraint on DNA–protein interactions during genome metabolism is still poorly understood. Here using a combined optical tweezers and fluorescence microscope, we quantify and explain how torsional constraint influences the structural stability of DNA under applied tension. We provide direct evidence that concomitant basepair melting and helical unwinding can occur in torsionally constrained DNA at forces >∼50 pN. This striking result indicates that local changes in linking number can be absorbed by the rest of the DNA duplex. We also present compelling new evidence that an overwound DNA structure (likely P-DNA) is created (alongside underwound structures) at forces >∼110 pN. These findings substantiate previous theoretical predictions and highlight a remarkable structural plasticity of torsionally constrained DNA. Such plasticity may be required in vivo to absorb local changes in linking number in DNA held under torsional constraint.
Highlights
Regions of the genome are often held under torsional constraint
We show that DNA under torsional constraint has a surprising plasticity, which may be exploited in vivo as a means to absorb local changes in Lk
We provide compelling new evidence that torsionally constrained DNA displays at least two structural transitions during the overstretching process
Summary
Regions of the genome are often held under torsional constraint. the influence of such constraint on DNA–protein interactions during genome metabolism is still poorly understood. We provide direct evidence that concomitant basepair melting and helical unwinding can occur in torsionally constrained DNA at forces 4B50 pN This striking result indicates that local changes in linking number can be absorbed by the rest of the DNA duplex. Free from torsional constraint, DNA displays a striking interplay between twist and stretch: applied forces up to 30 pN induce slight overwinding of the double helix, while increasing the force further from B35 to 65 pN results in underwinding of the double helix[12,13] The latter enables a small extension of the molecule of up to B10%. We show that DNA under torsional constraint has a surprising plasticity, which may be exploited in vivo as a means to absorb local changes in Lk
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