The Good and Bad of RNA:DNA Hybrids in Double-Strand Break Repair

The Thermodynamic and Kinetic Properties of the dA-rU DNA-RNA Hybrid Base Pair Investigated via Molecular Dynamics Simulations

DNA-RNA hybrid duplexes play essential roles during the reverse transcription of RNA viruses and DNA replication. The opening and conformation changes of individual base pairs are critical to their biological functions. However, the microscopic mechanisms governing base pair closing and opening at the atomic level remain poorly understood. In this study, we investigated the thermodynamic and kinetic parameters of the dA-rU base pair in a DNA-RNA hybrid duplex using 4 μs all-atom molecular dynamics (MD) simulations at different temperatures. Our results showed that the thermodynamic parameters of the dA-rU base pair aligned with the predictions of the nearest-neighbor model and were close to those of the AU base pair in RNA. The temperature dependence of the average lifetimes of both the open and the closed states, as well as the transition path times, were obtained. The free-energy barrier for a single base pair opening and closing arises from an increase in enthalpy due to the disruption of the base-stacking interactions and hydrogen bonding, along with an entropy loss attributed to the accompanying restrictions, such as torsional angle constraints and solvent viscosity.

RNA polymerase III is required for the repair of DNA double-strand breaks by homologous recombination

End resection in homologous recombination (HR) and HR-mediated repair of DNA double-strand breaks (DSBs) removes several kilobases from 5' strands of DSBs, but 3' strands are exempted from degradation. The mechanism by which the 3' overhangs are protected has not been determined. Here, we established that the protection of 3' overhangs is achieved through the transient formation of RNA-DNA hybrids. The DNA strand in the hybrids is the 3' ssDNA overhang, while the RNA strand is newly synthesized. RNA polymerase III (RNAPIII) is responsible for synthesizing the RNA strand. Furthermore, RNAPIII is actively recruited to DSBs by the MRN complex. CtIP and MRN nuclease activity is required for initiating the RNAPIII-mediated RNA synthesis at DSBs. A reduced level of RNAPIII suppressed HR, and genetic loss > 30 bp increased at DSBs. Thus, RNAPIII is an essential HR factor, and the RNA-DNA hybrid is an essential repair intermediate for protecting the 3' overhangs in DSB repair.

The Mechanical Properties of RNA-DNA Hybrid Duplex Stretched by Magnetic Tweezers

RNA can anneal to its DNA template to generate an RNA-DNA hybrid (RDH) duplex and a displaced DNA strand, termed R-loop. RDH duplex occupies up to 5% of the mammalian genome and plays important roles in many biological processes. The functions of RDH duplex are affected by its mechanical properties, including the elasticity and the conformation transitions. The mechanical properties of RDH duplex, however, are still unclear. In this work, we studied the mechanical properties of RDH duplex using magnetic tweezers in comparison with those of DNA and RNA duplexes with the same sequences. We report that the contour length of RDH duplex is ∼0.30 nm/bp, and the stretching modulus of RDH duplex is ∼660 pN, neither of which is sensitive to NaCl concentration. The persistence length of RDH duplex depends on NaCl concentration, decreasing from ∼63 nm at 1 mM NaCl to ∼49 nm at 500 mM NaCl. Under high tension of ∼60 pN, the end-opened RDH duplex undergoes two distinct overstretching transitions; at high salt in which the basepairs are stable, it undergoes the nonhysteretic transition, leading to a basepaired elongated structure, whereas at low salt, it undergoes a hysteretic peeling transition, leading to the single-stranded DNA strand under force and the single-stranded RNA strand coils. The peeled RDH is difficult to reanneal back to the duplex conformation, which may be due to the secondary structures formed in the coiled single-stranded RNA strand. These results help us understand the full picture of the structures and mechanical properties of nucleic acid duplexes in solution and provide a baseline for studying the interaction of RDH with proteins at the single-molecule level.

DNA:RNA hybrids at telomeres - when it is better to be out of the (R) loop

R-loops (RLs) are three-stranded nucleic acid structures that contain a DNA:RNA hybrid and a displaced DNA strand. Genomic regions with GC skew and a G-rich transcript are particularly prone to form RLs. RLs play important physiological roles in cells; however, when present at abnormally high levels, they may threaten genome stability. The perfect GC skew of telomeric repeats and the discovery of telomeric repeat-containing RNA (TERRA), a long noncoding transcript that consists of the G-rich telomeric sequence, make telomeric sequences the perfect candidates for generating RLs. Indeed, in the past 5 years, telomere R-loops (TRLs) have been demonstrated in Saccharomyces cerevisiae, Trypanosoma brucei, and human cells. The presence of TRLs in normal human cells that transcribe low levels of TERRA, suggests a physiological role for these nucleic structures in telomere maintenance. Abnormally enhanced TERRA transcription, as found in several human pathological conditions, leads to high TRL levels and various cellular outcomes, depending on the recombinogenic capabilities of the cells. Study of TRLs in various organisms highlights the necessity for tight regulation of these structures, which can switch from beneficial to detrimental under different conditions. Here, we review the current state of knowledge on TRLs, describe several means by which TRLs are regulated, and discuss how findings from yeast are relevant to human pathological scenarios in which TRLs are deregulated.

Polymerase θ-helicase efficiently unwinds DNA and RNA-DNA hybrids

POLQ is a unique multifunctional replication and repair gene that encodes for a N-terminal superfamily 2 helicase and a C-terminal A-family polymerase. Although the function of the polymerase domain has been investigated, little is understood regarding the helicase domain. Multiple studies have reported that polymerase θ-helicase (Polθ-helicase) is unable to unwind DNA. However, it exhibits ATPase activity that is stimulated by single-stranded DNA, which presents a biochemical conundrum. In contrast to previous reports, we demonstrate that Polθ-helicase (residues 1-894) efficiently unwinds DNA with 3'-5' polarity, including DNA with 3' or 5' overhangs, blunt-ended DNA, and replication forks. Polθ-helicase also efficiently unwinds RNA-DNA hybrids and exhibits a preference for unwinding the lagging strand at replication forks, similar to related HELQ helicase. Finally, we find that Polθ-helicase can facilitate strand displacement synthesis by Polθ-polymerase, suggesting a plausible function for the helicase domain. Taken together, these findings indicate nucleic acid unwinding as a relevant activity for Polθ in replication repair.

Rad52 Inverse Strand Exchange Drives RNA-Templated DNA Double-Strand Break Repair

RNA can serve as a template for DNA double-strand break repair in yeast cells, and Rad52, a member of the homologous recombination pathway, emerged as an important player in this process. However, the exact mechanism of how Rad52 contributes to RNA-dependent DSB repair remained unknown. Here, we report an unanticipated activity of yeast and human Rad52: inverse strand exchange, in which Rad52 forms a complex with dsDNA and promotes strand exchange with homologous ssRNA or ssDNA. We show that in eukaryotes, inverse strand exchange between homologous dsDNA and RNA is a distinctive activity of Rad52; neither Rad51 recombinase nor the yeast Rad52 paralog Rad59 has this activity. In accord with our in vitro results, our experiments in budding yeast provide evidence that Rad52 inverse strand exchange plays an important role in RNA-templated DSB repair in vivo.