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Title: Interplay between DNA sequence and negative superhelicity drives R-loop structures

R-loops are abundant three-stranded nucleic-acid structures that formin cisduring transcription. Experimental evidence suggests that R-loop formation is affected by DNA sequence and topology. However, the exact manner by which these factors interact to determine R-loop susceptibility is unclear. To investigate this, we developed a statistical mechanical equilibrium model of R-loop formation in superhelical DNA. In this model, the energy involved in forming an R-loop includes four terms—junctional and base-pairing energies and energies associated with superhelicity and with the torsional winding of the displaced DNA single strand around the RNA:DNA hybrid. This model shows that the significant energy barrier imposed by the formation of junctions can be overcome in two ways. First, base-pairing energy can favor RNA:DNA over DNA:DNA duplexes in favorable sequences. Second, R-loops, by absorbing negative superhelicity, partially or fully relax the rest of the DNA domain, thereby returning it to a lower energy state. In vitro transcription assays confirmed that R-loops cause plasmid relaxation and that negative superhelicity is required for R-loops to form, even in a favorable region. Single-molecule R-loop footprinting following in vitro transcription showed a strong agreement between theoretical predictions and experimental mapping of stable R-loop positions and further revealed the impact of DNA topology on the R-loop distribution landscape. Our results clarify the interplay between base sequence and DNA superhelicity in controlling R-loop stability. They also reveal R-loops as powerful and reversible topology sinks that cells may use to nonenzymatically relieve superhelical stress during transcription.

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NSF-PAR ID:
10087742
Author(s) / Creator(s):
; ; ; ; ;
Publisher / Repository:
Proceedings of the National Academy of Sciences
Date Published:
Journal Name:
Proceedings of the National Academy of Sciences
Volume:
116
Issue:
13
ISSN:
0027-8424
Page Range / eLocation ID:
p. 6260-6269
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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