Mechanistic basis of 5'-3' translocation in SF1B helicases

Two residues in the DNA binding site of Pif1 helicase are essential for nuclear functions but dispensable for mitochondrial respiratory growth

Pif1 helicase functions in both the nucleus and mitochondria. Pif1 tightly couples ATP hydrolysis, single-stranded DNA translocation, and duplex DNA unwinding. We investigated two Pif1 variants (F723A and T464A) that have each lost one site of interaction of the protein with the DNA substrate. Both variants exhibit minor reductions in affinity for DNA and ATP hydrolysis but have impaired DNA unwinding activity. However, these variants translocate on single-stranded DNA faster than the wildtype enzyme and can slide on the DNA substrate in an ATP-independent manner. This suggests they have lost their grip on the DNA, interfering with coupling ATP hydrolysis to translocation and unwinding. Yeast expressing these variants have increased gross chromosomal rearrangements, increased telomere length, and can overcome the lethality of dna2Δ, similar to phenotypes of yeast lacking Pif1. However, unlike pif1Δ mutants, they are viable on glycerol containing media and maintain similar mitochondrial DNA copy numbers as Pif1 wildtype. Overall, our data indicate that a tight grip of the trailing edge of the Pif1 enzyme on the DNA couples ATP hydrolysis to DNA translocation and DNA unwinding. This tight grip appears to be essential for the Pif1 nuclear functions tested but is dispensable for mitochondrial respiratory growth.

Iron‑sulfur clusters in viral proteins: Exploring their elusive nature, roles and new avenues for targeting infections

Viruses have evolved complex mechanisms to exploit host factors for replication and assembly. In response, host cells have developed strategies to block viruses, engaging in a continuous co-evolutionary battle. This dynamic interaction often revolves around the competition for essential resources necessary for both host cell and virus replication. Notably, iron, required for the biosynthesis of several cofactors, including iron‑sulfur (FeS) clusters, represents a critical element in the ongoing competition for resources between infectious agents and host. Although several recent studies have identified FeS cofactors at the core of virus replication machineries, our understanding of their specific roles and the cellular processes responsible for their incorporation into viral proteins remains limited. This review aims to consolidate our current knowledge of viral components that have been characterized as FeS proteins and elucidate how viruses harness these versatile cofactors to their benefit. Its objective is also to propose that viruses may depend on incorporation of FeS cofactors more extensively than is currently known. This has the potential to revolutionize our understanding of viral replication, thereby carrying significant implications for the development of strategies to target infections.

Rare SNP in the HELB gene interferes with RPA interaction and cellular function of HELB

HELB is a human helicase involved in initiation of DNA replication, the replication stress response, and regulation of double-strand DNA break repair. rs75770066 is a rare SNP in the HELB gene that affects age at natural menopause. rs75770066 results in a D506G substitution in an acidic patch within the 1A domain of the helicase that is known to interact with RPA. We found that this amino acid change dramatically impairs the cellular function of HELB. D506G-HELB exhibits impaired interaction with RPA, which likely results in the effects of rs75770066 as this reduces recruitment of HELB to sites of DNA damage. Reduced recruitment of D506G-HELB to double-strand DNA breaks and the concomitant increase in homologous recombination likely alters the levels of meiotic recombination, which affects the viability of gametes. Because menopause occurs when oocyte levels drop below a minimum threshold, altered repair of meiotic double-stranded DNA breaks has the potential to directly affect the age at natural menopause.

Sen1 architecture: RNA-DNA hybrid resolution, autoregulation, and insights into SETX inactivation in AOA2

The senataxin (SETX, Sen1 in yeasts) RNA-DNA hybrid resolving helicase regulates multiple nuclear transactions, including DNA replication, transcription, and DNA repair, but the molecular basis for Sen1 activities is ill defined. Here, Sen1 cryoelectron microscopy (cryo-EM) reconstructions reveal an elongated inchworm-like architecture. Sen1 is composed of an amino terminal helical repeat Sen1 N-terminal (Sen1N) regulatory domain that is flexibly linked to its C-terminal SF1B helicase motor core (Sen1Hel) via an intrinsically disordered tether. In an autoinhibited state, the Sen1Sen1N domain regulates substrate engagement by promoting occlusion of the RNA substrate-binding cleft. The X-ray structure of an activated Sen1Hel engaging single-stranded RNA and ADP-SO4 shows that the enzyme encircles RNA and implicates a single-nucleotide power stroke in the Sen1 RNA translocation mechanism. Together, our data unveil dynamic protein-protein and protein-RNA interfaces underpinning helicase regulation and inactivation of human SETX activity by RNA-binding-deficient mutants in ataxia with oculomotor apraxia 2 neurodegenerative disease.

Diketo acid inhibitors of nsp13 of SARS-CoV-2 block viral replication

For RNA viruses, RNA helicases have long been recognized to play critical roles during virus replication cycles, facilitating proper folding and replication of viral RNAs, therefore representing an ideal target for drug discovery. SARS-CoV-2 helicase, the non-structural protein 13 (nsp13) is a highly conserved protein among all known coronaviruses, and, at the moment, is one of the most explored viral targets to identify new possible antiviral agents. In the present study, we present six diketo acids (DKAs) as nsp13 inhibitors able to block both SARS-CoV-2 nsp13 enzymatic functions. Among them four compounds were able to inhibit viral replication in the low micromolar range, being active also on other human coronaviruses such as HCoV229E and MERS CoV. The experimental investigation of the binding mode revealed ATP-non-competitive kinetics of inhibition, not affected by substrate-displacement effect, suggesting an allosteric binding mode that was further supported by molecular modelling calculations predicting the binding into an allosteric conserved site located in the RecA2 domain.

Unwinding mechanism of SARS-CoV helicase (nsp13) in the presence of Ca2+, elucidated by biochemical and single-molecular studies

The recent outbreak of COVID-19 has created a serious health crisis with fatFal infectious viral diseases, such as Severe Acute Respiratory Syndrome (SARS). The nsp13, a helicase of coronaviruses is an essential element for viral replication that unwinds secondary structures of DNA and RNA, and is thus considered a major therapeutic target for treatment. The replication of coronaviruses and other retroviruses occurs in the cytoplasm of infected cells, in association with viral replication organelles, called virus-induced cytosolic double-membrane vesicles (DMVs). In addition, an increase in cytosolic Ca2+ concentration accelerates viral replication. However, the molecular mechanism of nsp13 in the presence of Ca2+ is not well understood. In this study, we applied biochemical methods and single-molecule techniques to demonstrate how nsp13 achieves its unwinding activity while performing ATP hydrolysis in the presence of Ca2+. Our study found that nsp13 could efficiently unwind double stranded (ds) DNA under physiological concentration of Ca2+ of cytosolic DMVs. These findings provide new insights into the properties of nsp13 in the range of calcium in cytosolic DMVs.

Type IV-A CRISPR-Csf complex: Assembly, dsDNA targeting, and CasDinG recruitment

Type IV CRISPR-Cas systems, which are primarily found on plasmids and exhibit a strong plasmid-targeting preference, are the only one of the six known CRISPR-Cas types for which the mechanistic details of their function remain unknown. Here, we provide high-resolution functional snapshots of type IV-A Csf complexes before and after target dsDNA binding, either in the absence or presence of CasDinG, revealing the mechanisms underlying Csf^crRNA complex assembly, "DWN" PAM-dependent dsDNA targeting, R-loop formation, and CasDinG recruitment. Furthermore, we establish that CasDinG, a signature DinG family helicase, harbors ssDNA-stimulated ATPase activity and ATP-dependent 5'-3' DNA helicase activity. In addition, we show that CasDinG unwinds the non-target strand (NTS) and target strand (TS) of target dsDNA from the Csf^crRNA complex. These molecular details advance our mechanistic understanding of type IV-A CRISPR-Csf function and should enable Csf complexes to be harnessed as genome-engineering tools for biotechnological applications.

Insights into the Dynamics and Helicase Activity of RecD2 of Deinococcus radiodurans during DNA Repair: A Single-Molecule Perspective

While the double helix is the most stable conformation of DNA inside cells, its transient unwinding and subsequent partial separation of the two complementary strands yields an intermediate single-stranded DNA (ssDNA). The ssDNA is involved in all major DNA transactions such as replication, transcription, recombination, and repair. The process of DNA unwinding and translocation is shouldered by helicases that transduce the chemical energy derived from nucleotide triphosphate (NTP) hydrolysis to mechanical energy and utilize it to destabilize hydrogen bonds between complementary base pairs. Consequently, a comprehensive understanding of the molecular mechanisms of these enzymes is essential. In the last few decades, a combination of single-molecule techniques (force-based manipulation and visualization) have been employed to study helicase action. These approaches have allowed researchers to study the single helicase-DNA complex in real-time and the free energy landscape of their interaction together with the detection of conformational intermediates and molecular heterogeneity during the course of helicase action. Furthermore, the unique ability of these techniques to resolve helicase motion at nanometer and millisecond spatial and temporal resolutions, respectively, provided surprising insights into their mechanism of action. This perspective outlines the contribution of single-molecule methods in deciphering molecular details of helicase activities. It also exemplifies how each technique was or can be used to study the helicase action of RecD2 in recombination DNA repair.

A single-molecule approach to unravel the molecular mechanism of the action of Deinococcus radiodurans RecD2 and its interaction with SSB and RecA in DNA repair

Helicases are ATP-driven molecular machines that directionally remodel nucleic acid polymers in all three domains of life. They are responsible for resolving double-stranded DNA (dsDNA) into single-strands, which is essential for DNA replication, nucleotide excision repair, and homologous recombination. RecD2 from Deinococcus radiodurans (DrRecD2) has important contributions to the organism's unusually high tolerance to gamma radiation and hydrogen peroxide. Although the results from X-ray Crystallography studies have revealed the structural characteristics of the protein, direct experimental evidence regarding the dynamics of the DNA unwinding process by DrRecD2 in the context of other accessory proteins is yet to be found. In this study, we have probed the exact binding event and processivity of DrRecD2 at single-molecule resolution using Protein-induced fluorescence enhancement (smPIFE) and Forster resonance energy transfer (smFRET). We have found that the protein prefers to bind at the 5' terminal end of the single-stranded DNA (ssDNA) by Drift and has helicase activity even in absence of ATP. However, a faster and iterative mode of DNA unwinding was evident in presence of ATP. The rate of translocation of the protein was found to be slower on dsDNA compared to ssDNA. We also showed that DrRecD2 is recruited at the binding site by the single-strand binding protein (SSB) and during the unwinding, it can displace RecA from ssDNA.

Alignment of helicases on single-stranded DNA increases activity

Helicases function in most biological processes that utilize RNA or DNA nucleic acids including replication, recombination, repair, transcription, splicing, and translation. They are motor proteins that bind ATP and then catalyze hydrolysis to release energy which is transduced for conformational changes. Different conformations correspond to different steps in a process that results in movement of the enzyme along the nucleic acid track in a unidirectional manner. Some helicases such as DEAD-box helicases do not translocate, but these enzymes transduce chemical energy from ATP hydrolysis to unwind secondary structure in DNA or RNA. Some helicases function as monomers while others assemble into defined structures, either dimers or higher order oligomers. Dda helicase from bacteriophage T4 and NS3 helicase domain from the hepatitis C virus are examples of monomeric helicases. These helicases can bind to single-stranded DNA in a manner that appears like train engines on a track. When monomeric helicases align on DNA, the activity of the enzymes increases. Helicase activity can include the rate of duplex unwinding and the total number of base pairs melted during a single binding event or processivity. Dda and NS3h are considered as having low processivity, unwinding fewer than 50 base pairs per binding event. Here, we report fusing two molecules of NS3h molecules together through genetically linking the C-terminus of one molecule to the N-terminus of a second NS3h molecule. We observed increased processivity relative to NS3h possibly arising from the increased probability that at least one of the helicases will completely unwind the DNA prior to dissociation. The dimeric enzyme also binds DNA more like the full-length NS3 helicase. Finally, the dimer can displace streptavidin from biotin-labeled oligonucleotide, whereas monomeric NS3h cannot.

Human HELB is a processive motor protein that catalyzes RPA clearance from single-stranded DNA

Human DNA helicase B (HELB) is a poorly characterized helicase suggested to play both positive and negative regulatory roles in DNA replication and recombination. In this work, we used bulk and single-molecule approaches to characterize the biochemical activities of HELB protein with a particular focus on its interactions with Replication Protein A (RPA) and RPA–single-stranded DNA (ssDNA) filaments. HELB is a monomeric protein that binds tightly to ssDNA with a site size of ∼20 nucleotides. It couples ATP hydrolysis to translocation along ssDNA in the 5′ to 3′ direction accompanied by the formation of DNA loops. HELB also displays classical helicase activity, but this is very weak in the absence of an assisting force. HELB binds specifically to human RPA, which enhances its ATPase and ssDNA translocase activities but inhibits DNA unwinding. Direct observation of HELB on RPA nucleoprotein filaments shows that translocating HELB concomitantly clears RPA from ssDNA. This activity, which can allow other proteins access to ssDNA intermediates despite their shielding by RPA, may underpin the diverse roles of HELB in cellular DNA transactions.

Natural Compounds Inhibit SARS-CoV-2 nsp13 Unwinding and ATPase Enzyme Activities

SARS-CoV-2 infection is still spreading worldwide, and new antiviral therapies are an urgent need to complement the approved vaccine preparations. SARS-CoV-2 nps13 helicase is a validated drug target participating in the viral replication complex and possessing two associated activities: RNA unwinding and 5′-triphosphatase. In the search of SARS-CoV-2 direct antiviral agents, we established biochemical assays for both SARS-CoV-2 nps13-associated enzyme activities and screened both in silico and in vitro a small in-house library of natural compounds. Myricetin, quercetin, kaempferol, and flavanone were found to inhibit the SARS-CoV-2 nps13 unwinding activity at nanomolar concentrations, while licoflavone C was shown to block both SARS-CoV-2 nps13 activities at micromolar concentrations. Mode of action studies showed that all compounds are nsp13 noncompetitive inhibitors versus ATP, while computational studies suggested that they can bind both nucleotide and 5′-RNA nsp13 binding sites, with licoflavone C showing a unique pattern of interaction with nsp13 amino acid residues. Overall, we report for the first time natural flavonoids as selective inhibitors of SARS-CoV-2 nps13 helicase with low micromolar activity.

Developmental energetics: Energy expenditure, budgets and metabolism during animal embryogenesis

Developing embryos are metabolically active, open systems that constantly exchange matter and energy with their environment. They function out of thermodynamic equilibrium and continuously use metabolic pathways to obtain energy from maternal nutrients, in order to fulfill the energetic requirements of growth and development. While an increasing number of studies highlight the role of metabolism in different developmental contexts, the physicochemical basis of embryogenesis, or how cellular processes use energy and matter to act together and transform a zygote into an adult organism, remains unknown. As we obtain a better understanding of metabolism, and benefit from current technology development, it is a promising time to revisit the energetic cost of development and how energetic principles may govern embryogenesis. Here, we review recent advances in methodology to measure and infer energetic parameters in developing embryos. We highlight a potential common pattern in embryonic energy expenditure and metabolic strategy across animal embryogenesis, and discuss challenges and open questions in developmental energetics.

The RecD2 helicase balances RecA activities

DNA helicases of the RecD2 family are ubiquitous. Bacillus subtilis RecD2 in association with the single-stranded binding protein SsbA may contribute to replication fork progression, but its detailed action remains unknown. In this work, we explore the role of RecD2 during DNA replication and its interaction with the RecA recombinase. RecD2 inhibits replication restart, but this effect is not observed in the absence of SsbA. RecD2 slightly affects replication elongation. RecA inhibits leading and lagging strand synthesis, and RecD2, which physically interacts with RecA, counteracts this negative effect. In vivo results show that recD2 inactivation promotes RecA–ssDNA accumulation at low mitomycin C levels, and that RecA threads persist for a longer time after induction of DNA damage. In vitro, RecD2 modulates RecA-mediated DNA strand-exchange and catalyzes branch migration. These findings contribute to our understanding of how RecD2 may contribute to overcome a replicative stress, removing RecA from the ssDNA and, thus, it may act as a negative modulator of RecA filament growth.

Ensemble cryo-EM reveals conformational states of the nsp13 helicase in the SARS-CoV-2 helicase replication-transcription complex

The SARS-CoV-2 nonstructural proteins coordinate genome replication and gene expression. Structural analyses revealed the basis for coupling of the essential nsp13 helicase with the RNA-dependent RNA polymerase (RdRp) where the holo-RdRp and RNA substrate (the replication-transcription complex or RTC) associated with two copies of nsp13 (nsp13₂-RTC). One copy of nsp13 interacts with the template-RNA in an opposing polarity to the RdRp and is envisaged to drive the RdRp backward on the RNA template (backtracking), prompting questions as to how the RdRp can efficiently synthesize RNA in the presence of nsp13. Here we use cryogenic-electron microscopy and molecular dynamics simulations to analyze the nsp13₂-RTC, revealing four distinct conformational states of the helicases. The results indicate a mechanism for the nsp13₂-RTC to turn backtracking on and off, using an allosteric mechanism to switch between RNA synthesis or backtracking in response to stimuli at the RdRp active site.

A structural feature of Dda helicase which enhances displacement of streptavidin and trp repressor from DNA

Helicases are molecular motors with many activities. They use the energy from ATP hydrolysis to unwind double-stranded nucleic acids while translocating on the single-stranded DNA. In addition to unwinding, many helicases are able to remove proteins from nucleic acids. Bacteriophage T4 Dda is able to displace a variety of DNA binding proteins and streptavidin bound to biotinylated oligonucleotides. We have identified a subdomain of Dda that when deleted, results in a protein variant that has nearly wild type activity for unwinding double-stranded DNA but exhibits greatly reduced streptavidin displacement activity. Interestingly, this domain has little effect on displacement of either gp32 or BamHI bound to DNA but does affect displacement of trp repressor from DNA. With this variant, we have identified residues which enhance displacement of some proteins from DNA.

Monitoring helicase-catalyzed unwinding of multiple duplexes simultaneously

Helicases catalyze the unwinding of duplex nucleic acids to aid a variety of cellular processes. Although helicases unwind duplex DNA in the same direction that they translocate on single-stranded DNA, forked duplexes provide opportunities to monitor unwinding by helicase monomers bound to each arm of the fork. The activity of the helicase bound to the displaced strand can be discerned alongside the helicase bound to the translocase strand using a forked substrate with accessible duplexes on both strands labeled with different fluorophores. In order to quantify the effect of protein-protein interactions on the activity of multiple monomers of the Bacteroides fragilis Pif1 helicase bound to separate strands of a forked DNA junction, an ensemble gel-based assay for monitoring simultaneous duplex unwinding was developed (Su et al., 2019). Here, the use of that assay is described for measuring the total product formation and rate constants of product formation of multiple duplexes on a single nucleic acid substrate. Use of this assay may aid characterization of protein-protein interactions between multiple helicase monomers at forked nucleic acid junctions and can assist with the characterization of helicase action on the displaced strand of forked duplexes.

Genome-wide chromosomal association of Upf1 is linked to Pol II transcription in Schizosaccharomyces pombe

Although the RNA helicase Upf1 has hitherto been examined mostly in relation to its cytoplasmic role in nonsense mediated mRNA decay (NMD), here we report high-throughput ChIP data indicating genome-wide association of Upf1 with active genes in Schizosaccharomyces pombe. This association is RNase sensitive, correlates with Pol II transcription and mRNA expression levels. Changes in Pol II occupancy were detected in a Upf1 deficient (upf1Δ) strain, prevalently at genes showing a high Upf1 relative to Pol II association in wild-type. Additionally, an increased Ser2 Pol II signal was detected at all highly transcribed genes examined by ChIP-qPCR. Furthermore, upf1Δ cells are hypersensitive to the transcription elongation inhibitor 6-azauracil. A significant proportion of the genes associated with Upf1 in wild-type conditions are also mis-regulated in upf1Δ. These data envisage that by operating on the nascent transcript, Upf1 might influence Pol II phosphorylation and transcription.

The large bat Helitron DNA transposase forms a compact monomeric assembly that buries and protects its covalently bound 5'-transposon end

Helitrons are widespread eukaryotic DNA transposons that have significantly contributed to genome variability and evolution, in part because of their distinctive, replicative rolling-circle mechanism, which often mobilizes adjacent genes. Although most eukaryotic transposases form oligomers and use RNase H-like domains to break and rejoin double-stranded DNA (dsDNA), Helitron transposases contain a single-stranded DNA (ssDNA)-specific HUH endonuclease domain. Here, we report the cryo-electron microscopy structure of a Helitron transposase bound to the 5'-transposon end, providing insight into its multidomain architecture and function. The monomeric transposase forms a tightly packed assembly that buries the covalently attached cleaved end, protecting it until the second end becomes available. The structure reveals unexpected architectural similarity to TraI, a bacterial relaxase that also catalyzes ssDNA movement. The HUH active site suggests how two juxtaposed tyrosines, a feature of many replication initiators that use HUH nucleases, couple the conformational shift of an α-helix to control strand cleavage and ligation reactions.

Srs2 and Pif1 as Model Systems for Understanding Sf1a and Sf1b Helicase Structure and Function

Helicases are enzymes that convert the chemical energy stored in ATP into mechanical work, allowing them to move along and manipulate nucleic acids. The helicase superfamily 1 (Sf1) is one of the largest subgroups of helicases and they are required for a range of cellular activities across all domains of life. Sf1 helicases can be further subdivided into two classes called the Sf1a and Sf1b helicases, which move in opposite directions on nucleic acids. The results of this movement can range from the separation of strands within duplex nucleic acids to the physical remodeling or removal of nucleoprotein complexes. Here, we describe the characteristics of the Sf1a helicase Srs2 and the Sf1b helicase Pif1, both from the model organism Saccharomyces cerevisiae, focusing on the roles that they play in homologous recombination, a DNA repair pathway that is necessary for maintaining genome integrity.

Structure, mechanism and crystallographic fragment screening of the SARS-CoV-2 NSP13 helicase

There is currently a lack of effective drugs to treat people infected with SARS-CoV-2, the cause of the global COVID-19 pandemic. The SARS-CoV-2 Non-structural protein 13 (NSP13) has been identified as a target for anti-virals due to its high sequence conservation and essential role in viral replication. Structural analysis reveals two "druggable" pockets on NSP13 that are among the most conserved sites in the entire SARS-CoV-2 proteome. Here we present crystal structures of SARS-CoV-2 NSP13 solved in the APO form and in the presence of both phosphate and a non-hydrolysable ATP analog. Comparisons of these structures reveal details of conformational changes that provide insights into the helicase mechanism and possible modes of inhibition. To identify starting points for drug development we have performed a crystallographic fragment screen against NSP13. The screen reveals 65 fragment hits across 52 datasets opening the way to structure guided development of novel antiviral agents.

Structure, mechanism and crystallographic fragment screening of the SARS-CoV-2 NSP13 helicase

There is currently a lack of effective drugs to treat people infected with SARS-CoV-2, the cause of the global COVID-19 pandemic. The SARS-CoV-2 Non-structural protein 13 (NSP13) has been identified as a target for anti-virals due to its high sequence conservation and essential role in viral replication. Structural analysis reveals two "druggable" pockets on NSP13 that are among the most conserved sites in the entire SARS-CoV-2 proteome. Here we present crystal structures of SARS-CoV-2 NSP13 solved in the APO form and in the presence of both phosphate and a non-hydrolysable ATP analog. Comparisons of these structures reveal details of conformational changes that provide insights into the helicase mechanism and possible modes of inhibition. To identify starting points for drug development we have performed a crystallographic fragment screen against NSP13. The screen reveals 65 fragment hits across 52 datasets opening the way to structure guided development of novel antiviral agents.

Structural study of the function of Candida Albicans Pif1

Pif1 helicases, conserved in eukaryotes, are involved in maintaining genome stability in both the nucleus and mitochondria. Here, we report the crystal structure of a truncated Candida Albicans Pif1 (CaPif1^368-883) in complex with ssDNA and an ATP analog. Our results show that the Q-motif is responsible for identifying adenine bases, and CaPif1 preferentially utilizes ATP/dATP during dsDNA unwinding. Although CaPif1 shares structural similarities with Saccharomyces cerevisiae Pif1, CaPif1 can contact the thymidine bases of DNA by hydrogen bonds, whereas ScPif1 cannot. More importantly, the crosslinking and mutant experiments have demonstrated that the conformational change of domain 2B is necessary for CaPif1 to unwind dsDNA. These findings contribute to further the understanding of the unwinding mechanism of Pif1.

The three-component helicase/primase complex of herpes simplex virus-1

Herpes simplex virus type 1 (HSV-1) is one of the nine herpesviruses that infect humans. HSV-1 encodes seven proteins to replicate its genome in the hijacked human cell. Among these are the herpes virus DNA helicase and primase that are essential components of its replication machinery. In the HSV-1 replisome, the helicase-primase complex is composed of three components including UL5 (helicase), UL52 (primase) and UL8 (non-catalytic subunit). UL5 and UL52 subunits are functionally interdependent, and the UL8 component is required for the coordination of UL5 and UL52 activities proceeding in opposite directions with respect to the viral replication fork. Anti-viral compounds currently under development target the functions of UL5 and UL52. Here, we review the structural and functional properties of the UL5/UL8/UL52 complex and highlight the gaps in knowledge to be filled to facilitate molecular characterization of the structure and function of the helicase-primase complex for development of alternative anti-viral treatments.

Structural and functional studies of SF1B Pif1 from Thermus oshimai reveal dimerization-induced helicase inhibition

Pif1 is an SF1B helicase that is evolutionarily conserved from bacteria to humans and plays multiple roles in maintaining genome stability in both nucleus and mitochondria. Though highly conserved, Pif1 family harbors a large mechanistic diversity. Here, we report crystal structures of Thermus oshimai Pif1 (ToPif1) alone and complexed with partial duplex or single-stranded DNA. In the apo state and in complex with a partial duplex DNA, ToPif1 is monomeric with its domain 2B/loop3 adopting a closed and an open conformation, respectively. When complexed with a single-stranded DNA, ToPif1 forms a stable dimer with domain 2B/loop3 shifting to a more open conformation. Single-molecule and biochemical assays show that domain 2B/loop3 switches repetitively between the closed and open conformations when a ToPif1 monomer unwinds DNA and, in contrast with other typical dimeric SF1A helicases, dimerization has an inhibitory effect on its helicase activity. This mechanism is not general for all Pif1 helicases but illustrates the diversity of regulation mechanisms among different helicases. It also raises the possibility that although dimerization results in activation for SF1A helicases, it may lead to inhibition for some of the other uncharacterized SF1B helicases, an interesting subject warranting further studies.

Novel insights into the function of an N-terminal region of DENV2 NS4B for the optimal helicase activity of NS3

Dengue virus NS3 is a prototypical DEx(H/D) helicase that binds and hydrolyzes NTP to translocate along and unwind double-stranded nucleic acids. NS3 and NS4B are essential components of the flavivirus replication complex. Evidences showed that NS4B interacted with NS3 and modulated the helicase activity of NS3. Despite important insights into structural, mechanistic, and cellular aspects of the NS3 function, there is still a gap in understanding how it coordinates the helicase activities within the replicase complex for efficient replication. Here, using the DENV2 as a model, we redefined the critical region of NS4B required for NS3 function by pull-down and MST assays. The FRET-based unwinding assay showed that NS3 would accelerate unwinding duplex nucleic acids in the presence of NS4B (51-83). The simulated NS3-NS4B complex models based on the rigid-body docking delineated the potential interaction sites located in the conserved motif within the core domain of NS3. Mutations in motif I (I190A) and motif III (P319L) of NS3 interfered with the unwinding activity stimulated by NS4B. Upon binding to the NS3 helicase, NS4B assisted NS3 to dissociate from single-stranded nucleic acid and enabled NS3 helicase to keep high activity at high ATP concentrations. These results suggest that NS4B probably serves as an essential cofactor for NS3 to coordinate the ATP cycles and nucleic acid binding during viral genome replication.

Biophysics of Chromatin Remodeling

As primary carriers of epigenetic information and gatekeepers of genomic DNA, nucleosomes are essential for proper growth and development of all eukaryotic cells. Although they are intrinsically dynamic, nucleosomes are actively reorganized by ATP-dependent chromatin remodelers. Chromatin remodelers contain helicase-like ATPase motor domains that can translocate along DNA, and a long-standing question in the field is how this activity is used to reposition or slide nucleosomes. In addition to ratcheting along DNA like their helicase ancestors, remodeler ATPases appear to dictate specific alternating geometries of the DNA duplex, providing an unexpected means for moving DNA past the histone core. Emerging evidence supports twist-based mechanisms for ATP-driven repositioning of nucleosomes along DNA. In this review, we discuss core experimental findings and ideas that have shaped the view of how nucleosome sliding may be achieved.

Structural biology of SARS-CoV-2 and implications for therapeutic development

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is an unprecedented global health crisis. However, therapeutic options for treatment are still very limited. The development of drugs that target vital proteins in the viral life cycle is a feasible approach for treating COVID-19. Belonging to the subfamily Orthocoronavirinae with the largest RNA genome, SARS-CoV-2 encodes a total of 29 proteins. These non-structural, structural and accessory proteins participate in entry into host cells, genome replication and transcription, and viral assembly and release. SARS-CoV-2 proteins can individually perform essential physiological roles, be components of the viral replication machinery or interact with numerous host cellular factors. In this Review, we delineate the structural features of SARS-CoV-2 from the whole viral particle to the individual viral proteins and discuss their functions as well as their potential as targets for therapeutic interventions.

Structural Basis for Helicase-Polymerase Coupling in the SARS-CoV-2 Replication-Transcription Complex

SARS-CoV-2 is the causative agent of the 2019-2020 pandemic. The SARS-CoV-2 genome is replicated and transcribed by the RNA-dependent RNA polymerase holoenzyme (subunits nsp7/nsp8₂/nsp12) along with a cast of accessory factors. One of these factors is the nsp13 helicase. Both the holo-RdRp and nsp13 are essential for viral replication and are targets for treating the disease COVID-19. Here we present cryoelectron microscopic structures of the SARS-CoV-2 holo-RdRp with an RNA template product in complex with two molecules of the nsp13 helicase. The Nidovirales order-specific N-terminal domains of each nsp13 interact with the N-terminal extension of each copy of nsp8. One nsp13 also contacts the nsp12 thumb. The structure places the nucleic acid-binding ATPase domains of the helicase directly in front of the replicating-transcribing holo-RdRp, constraining models for nsp13 function. We also observe ADP-Mg²⁺ bound in the nsp12 N-terminal nidovirus RdRp-associated nucleotidyltransferase domain, detailing a new pocket for anti-viral therapy development.

Lysine acetylation regulates the activity of nuclear Pif1

In Saccharomyces cerevisiae, the Pif1 helicase functions in both nuclear and mitochondrial DNA replication and repair processes, preferentially unwinding RNA:DNA hybrids and resolving G-quadruplex structures. We sought to determine how the various activities of Pif1 are regulated in vivo Here, we report lysine acetylation of nuclear Pif1 and demonstrate that it influences both Pif1's cellular roles and core biochemical activities. Using Pif1 overexpression toxicity assays, we determined that the acetyltransferase NuA4 and deacetylase Rpd3 are primarily responsible for the dynamic acetylation of nuclear Pif1. MS analysis revealed that Pif1 was modified in several domains throughout the protein's sequence on the N terminus (Lys-118 and Lys-129), helicase domain (Lys-525, Lys-639, and Lys-725), and C terminus (Lys-800). Acetylation of Pif1 exacerbated its overexpression toxicity phenotype, which was alleviated upon deletion of its N terminus. Biochemical assays demonstrated that acetylation of Pif1 stimulated its helicase, ATPase, and DNA-binding activities, whereas maintaining its substrate preferences. Limited proteolysis assays indicate that acetylation of Pif1 induces a conformational change that may account for its altered enzymatic properties. We propose that acetylation is involved in regulating of Pif1 activities, influencing a multitude of DNA transactions vital to the maintenance of genome integrity.

Single-molecule probing the duplex and G4 unwinding patterns of a RecD family helicase

RecD family helicases play an important role in prokaryotic genome stability and serve as the structural models for studying superfamily 1B (SF1B) helicases. However, RecD-catalyzed duplex DNA unwinding behavior and the underlying mechanism are still elusive. RecD family helicases share a common proto-helicase with eukaryotic Pif1 family helicases, which are well known for their outstanding G-quadruplex (G4) unwinding ability. However, there are still controversial points as to whether and how RecD helicases unfold G4 structures. Here, single-molecule fluorescence resonance energy transfer (smFRET) and magnetic tweezers (MT) were used to study Deinococcus radiodurans RecD2 (DrRecD2)-mediated duplex DNA unwinding and resolution of G4 structures. A symmetric, repetitive unwinding phenomenon was observed on duplex DNA, revealed from the strand switch and translocation of one monomer. Furthermore, we found that DrRecD2 was able to unwind both parallel and antiparallel G4 structures without obvious topological preferences. Surprisingly, the unwinding properties of RecD on duplex and G4 DNA are different from those of Pif1. The findings provide an example, in which the patterns of two molecules derived from a common ancestor deviate during evolution, and they are of significance for understanding the unwinding mechanism and function of SF1B helicases.

Structural basis for helicase-polymerase coupling in the SARS-CoV-2 replication-transcription complex

SARS-CoV-2 is the causative agent of the 2019-2020 pandemic. The SARS-CoV-2 genome is replicated-transcribed by the RNA-dependent RNA polymerase holoenzyme (subunits nsp7/nsp82/nsp12) along with a cast of accessory factors. One of these factors is the nsp13 helicase. Both the holo-RdRp and nsp13 are essential for viral replication and are targets for treating the disease COVID-19. Here we present cryo-electron microscopic structures of the SARS-CoV-2 holo-RdRp with an RNA template-product in complex with two molecules of the nsp13 helicase. The Nidovirus-order-specific N-terminal domains of each nsp13 interact with the N-terminal extension of each copy of nsp8. One nsp13 also contacts the nsp12-thumb. The structure places the nucleic acid-binding ATPase domains of the helicase directly in front of the replicating-transcribing holo-RdRp, constraining models for nsp13 function. We also observe ADP-Mg2+ bound in the nsp12 N-terminal nidovirus RdRp-associated nucleotidyltransferase domain, detailing a new pocket for anti-viral therapeutic development.

A high ATP concentration enhances the cooperative translocation of the SARS coronavirus helicase nsP13 in the unwinding of duplex RNA

Severe acute respiratory syndrome coronavirus nonstructural protein 13 (SCV nsP13), a superfamily 1 helicase, plays a central role in viral RNA replication through the unwinding of duplex RNA and DNA with a 5' single-stranded tail in a 5' to 3' direction. Despite its putative role in viral RNA replication, nsP13 readily unwinds duplex DNA by cooperative translocation. Herein, nsP13 exhibited different characteristics in duplex RNA unwinding than that in duplex DNA. nsP13 showed very poor processivity on duplex RNA compared with that on duplex DNA. More importantly, nsP13 inefficiently unwinds duplex RNA by increasing the 5'-ss tail length. As the concentration of nsP13 increased, the amount of unwound duplex DNA increased and that of unwound duplex RNA decreased. The accumulation of duplex RNA/nsP13 complexes increased as the concentration of nsP13 increased. An increased ATP concentration in the unwinding of duplex RNA relieved the decrease in duplex RNA unwinding. Thus, nsP13 has a strong affinity for duplex RNA as a substrate for the unwinding reaction, which requires increased ATPs to processively unwind duplex RNA. Our results suggest that duplex RNA is a preferred substrate for the helicase activity of nsP13 than duplex DNA at high ATP concentrations.

History of DNA Helicases

Since the discovery of the DNA double helix, there has been a fascination in understanding the molecular mechanisms and cellular processes that account for: (i) the transmission of genetic information from one generation to the next and (ii) the remarkable stability of the genome. Nucleic acid biologists have endeavored to unravel the mysteries of DNA not only to understand the processes of DNA replication, repair, recombination, and transcription but to also characterize the underlying basis of genetic diseases characterized by chromosomal instability. Perhaps unexpectedly at first, DNA helicases have arisen as a key class of enzymes to study in this latter capacity. From the first discovery of ATP-dependent DNA unwinding enzymes in the mid 1970's to the burgeoning of helicase-dependent pathways found to be prevalent in all kingdoms of life, the story of scientific discovery in helicase research is rich and informative. Over four decades after their discovery, we take this opportunity to provide a history of DNA helicases. No doubt, many chapters are left to be written. Nonetheless, at this juncture we are privileged to share our perspective on the DNA helicase field - where it has been, its current state, and where it is headed.

Delicate structural coordination of the Severe Acute Respiratory Syndrome coronavirus Nsp13 upon ATP hydrolysis

To date, an effective therapeutic treatment that confers strong attenuation toward coronaviruses (CoVs) remains elusive. Of all the potential drug targets, the helicase of CoVs is considered to be one of the most important. Here, we first present the structure of the full-length Nsp13 helicase of SARS-CoV (SARS-Nsp13) and investigate the structural coordination of its five domains and how these contribute to its translocation and unwinding activity. A translocation model is proposed for the Upf1-like helicase members according to three different structural conditions in solution characterized through H/D exchange assay, including substrate state (SARS-Nsp13-dsDNA bound with AMPPNP), transition state (bound with ADP-AlF₄⁻) and product state (bound with ADP). We observed that the β19–β20 loop on the 1A domain is involved in unwinding process directly. Furthermore, we have shown that the RNA dependent RNA polymerase (RdRp), SARS-Nsp12, can enhance the helicase activity of SARS-Nsp13 through interacting with it directly. The interacting regions were identified and can be considered common across CoVs, which provides new insights into the Replication and Transcription Complex (RTC) of CoVs.

Viral SPP1 DNA is infectious in naturally competent Bacillus subtilis cells: inter- and intramolecular recombination pathways

A proteolyzed bacteriophage (phage) might release its DNA into the environment. Here, we define the recombination functions required to resurrect an infective lytic phage from inactive environmental viral DNA in naturally competent Bacillus subtilis cells. Using phage SPP1 DNA, a model that accounts for the obtained data is proposed (i) the DNA uptake apparatus takes up environmental SPP1 DNA, fragments it, and incorporates into the cytosol different linear single-stranded (ss) DNA molecules shorter than genome-length; (ii) the SsbA-DprA mediator loads RecA onto any fragmented linear SPP1 ssDNA, but negative modulators (RecX and RecU) promote a net RecA disassembly from these ssDNAs not homologous to the host genome; (iii) single strand annealing (SSA) proteins, DprA and RecO, anneal the SsbA- or SsbB-coated complementary strands, yielding tailed SPP1 duplex intermediates; (iv) RecA polymerized on these tailed intermediates invades a homologous region in another incomplete molecule, and in concert with RecD2 helicase, reconstitutes a complete linear phage genome with redundant regions at the ends of the molecule; and (v) DprA, RecO or viral G35P SSA, may catalyze the annealing of these terminally redundant regions, alone or with the help of an exonuclease, to produce a circular unit-length duplex viral genome ready to initiate replication.

Different mechanisms for translocation by monomeric and hexameric helicases

Helicases are ATP-dependent motor proteins that translocate along single-stranded or double-stranded nucleic acids to alter base-pairing structures or molecular interactions. Helicases can be divided to monomeric and hexameric types, each with distinct ternary structures, nucleic acid-binding modes, and translocation mechanisms. It is well established that monomeric helicases translocate by the inchworm mechanism. Recent structures of different superfamilies of hexameric helicases reveal that they use a hand-over hand mechanism for translocation. Structures of bacteriophage T7 replisome illustrate how helicase and polymerase cooperatively catalyze DNA unwinding. In this review, we survey structures of monomeric and hexameric helicases and compare different mechanisms for translocation.

Viral Satellites Exploit Phage Proteins to Escape Degradation of the Bacterial Host Chromosome

Phage defense systems are often found on mobile genetic elements (MGEs), where they constitutively defend against invaders or are induced to respond to new assaults. Phage satellites, one type of MGE, are induced during phage infection to promote their own transmission, reducing phage production and protecting their hosts in the process. One such satellite in Vibrio cholerae, phage-inducible chromosomal island-like element (PLE), sabotages the lytic phage ICP1, which triggers PLE excision from the bacterial chromosome, replication, and transduction to neighboring cells. Analysis of patient stool samples from different geographic regions revealed that ICP1 has evolved to possess one of two syntenic loci encoding an SF1B-type helicase, either of which PLE exploits to drive replication. Further, loss of PLE mobilization limits anti-phage activity because of phage-mediated degradation of the bacterial genome. Our work provides insight into the unique challenges facing parasites of lytic phages and underscores the adaptions of satellites to their ever-evolving target phage.

Insights into the structural and mechanistic basis of multifunctional S. cerevisiae Pif1p helicase

The Saccharomyces cerevisiae Pif1 protein (ScPif1p) is the prototypical member of the Pif1 family of DNA helicases. ScPif1p is involved in the maintenance of mitochondrial, ribosomal and telomeric DNA and suppresses genome instability at G-quadruplex motifs. Here, we report the crystal structures of a truncated ScPif1p (ScPif1p^237−780) in complex with different ssDNAs. Our results have revealed that a yeast-specific insertion domain protruding from the 2B domain folds as a bundle bearing an α-helix, α16. The α16 helix regulates the helicase activities of ScPif1p through interactions with the previously identified loop3. Furthermore, a biologically relevant dimeric structure has been identified, which can be further specifically stabilized by G-quadruplex DNA. Basing on structural analyses and mutational studies with DNA binding and unwinding assays, a potential G-quadruplex DNA binding site in ScPif1p monomers is suggested. Our results also show that ScPif1p uses the Q-motif to preferentially hydrolyze ATP, and a G-rich tract is preferentially recognized by more residues, consistent with previous biochemical observations. These findings provide a structural and mechanistic basis for understanding the multifunctional ScPif1p.

Structures and operating principles of the replisome

Visualization in atomic detail of the replisome that performs concerted leading- and lagging-DNA strand synthesis at a replication fork has not been reported. Using bacteriophage T7 as a model system, we determined cryo-electron microscopy structures up to 3.2-angstroms resolution of helicase translocating along DNA and of helicase-polymerase-primase complexes engaging in synthesis of both DNA strands. Each domain of the spiral-shaped hexameric helicase translocates sequentially hand-over-hand along a single-stranded DNA coil, akin to the way AAA+ ATPases (adenosine triphosphatases) unfold peptides. Two lagging-strand polymerases are attached to the primase, ready for Okazaki fragment synthesis in tandem. A β hairpin from the leading-strand polymerase separates two parental DNA strands into a T-shaped fork, thus enabling the closely coupled helicase to advance perpendicular to the downstream DNA duplex. These structures reveal the molecular organization and operating principles of a replisome.

DNA translocation mechanism of an XPD family helicase

The XPD family of helicases, that includes human disease-related FANCJ, DDX11 and RTEL1, are Superfamily two helicases that contain an iron-sulphur cluster domain, translocate on ssDNA in a 5’−3’ direction and play important roles in genome stability. Consequently, mutations in several of these family members in eukaryotes cause human diseases. Family members in bacteria, such as the DinG helicase from Escherichia coli, are also involved in DNA repair. Here we present crystal structures of complexes of DinG bound to single-stranded DNA (ssDNA) in the presence and absence of an ATP analogue (ADP•BeF₃), that suggest a mechanism for 5’−3’ translocation along the ssDNA substrate. This proposed mechanism has implications for how those enzymes of the XPD family that recognise bulky DNA lesions might stall at these as the first step in initiating DNA repair. Biochemical data reveal roles for conserved residues that are mutated in human diseases.

Thermostable DNA helicase improves the sensitivity of digital PCR

DNA/RNA helicases, which catalyze the unwinding of duplex nucleic acids using the energy of ATP hydrolysis, contribute to various biological functions involving DNA or RNA. Euryarchaeota-specific helicase Tk-EshA (superfamily 2) from the hyperthermophilic archaeon Thermococcus kodakarensis has been used to decrease generation of mis-amplified products (noise DNAs) during PCR. In this study, we focused on another type (superfamily 1B) of helicase, Tk-Upf1 (TK0178) from T. kodakarensis, and compared its effectiveness in PCR and digital PCR with that of Tk-EshA. For this purpose, we obtained Tk-Upf1 as a recombinant protein and assessed its enzymatic characteristics. Among various double-stranded DNA (dsDNA) substrates (forked, 5' overhung, 3' overhung, and blunt-ended duplex), Tk-Upf1 had the highest unwinding activity toward 5' overhung DNAs. Noise DNAs were also eliminated in the presence of Tk-Upf1 at concentrations 10-fold lower than those required to yield a comparable reduction with Tk-EshA. When a 5' or 3' overhung mis-annealed primer was included as a competitive primer along with specific primers, noise DNAs derived from the mis-annealed primer were eliminated in the presence of Tk-Upf1. In digital PCR, addition of Tk-EshA or Tk-Upf1 increased fluorescent intensities and improved separation between common and risk allele clusters, indicating that both helicases functioned as signal enhancers. In comparison with Tk-EshA, a smaller amount of Tk-Upf1 was required to improve the performance of digital PCR.

Modulation of Escherichia coli UvrD Single-Stranded DNA Translocation by DNA Base Composition

Escherichia coli UvrD is an SF1A DNA helicase/translocase that functions in chromosomal DNA repair and replication of some plasmids. UvrD can also displace proteins such as RecA from DNA in its capacity as an anti-recombinase. Central to all of these activities is its ATP-driven 3'-5' single-stranded (ss) DNA translocation activity. Previous ensemble transient kinetic studies have estimated the average translocation rate of a UvrD monomer on ssDNA composed solely of deoxythymidylates. Here we show that the rate of UvrD monomer translocation along ssDNA is influenced by DNA base composition, with UvrD having the fastest rate along polypyrimidines although decreasing nearly twofold on ssDNA containing equal amounts of the four bases. Experiments with DNA containing abasic sites and polyethylene glycol spacers show that the ssDNA base also influences translocation processivity. These results indicate that changes in base composition and backbone insertions influence the translocation rates, with increased ssDNA base stacking correlated with decreased translocation rates, supporting the proposal that base-stacking interactions are involved in the translocation mechanism.

Functional link between DEAH/RHA helicase Prp43 activation and ATP base binding

The DEAH box helicase Prp43 is a bifunctional enzyme from the DEAH/RHA helicase family required both for the maturation of ribosomes and for lariat intron release during splicing. It interacts with G-patch domain containing proteins which activate the enzymatic activity of Prp43 in vitro by an unknown mechanism. In this work, we show that the activation by G-patch domains is linked to the unique nucleotide binding mode of this helicase family. The base of the ATP molecule is stacked between two residues, R159 of the RecA1 domain (R-motif) and F357 of the RecA2 domain (F-motif). Using Prp43 F357A mutants or pyrimidine nucleotides, we show that the lack of stacking of the nucleotide base to the F-motif decouples the NTPase and helicase activities of Prp43. In contrast the R159A mutant (R-motif) showed reduced ATPase and helicase activities. We show that the Prp43 R-motif mutant induces the same phenotype as the absence of the G-patch protein Gno1, strongly suggesting that the processing defects observed in the absence of Gno1 result from a failure to activate the Prp43 helicase. Overall we propose that the stacking between the R- and F-motifs and the nucleotide base is important for the activity and regulation of this helicase family.

Nucleosome-Chd1 structure and implications for chromatin remodelling

Chromatin remodelling factors change nucleosome positioning and facilitate DNA transcription, replication, and repair1. The conserved remodelling factor Chd12 can shift nucleosomes and induce a regular nucleosome spacing3–5. Chd1 is required for RNA polymerase II passage through nucleosomes6 and for cellular pluripotency7. Chd1 contains the DNA-binding domains SANT and SLIDE, a bilobal motor domain that hydrolyses adenosine triphosphate (ATP), and a regulatory double chromodomain. Here we report the cryo-electron microscopy (cryo-EM) structure of Chd1 from the yeast S. cerevisiae bound to a nucleosome at a resolution of 4.8 Å. Chd1 detaches two turns of DNA from the histone octamer and binds between the two DNA gyres in a state poised for catalysis. The SANT and SLIDE domains contact detached DNA around superhelical location (SHL) -7 of the first DNA gyre. The ATPase motor binds the second DNA gyre at SHL +2 and is anchored to the N-terminal tail of histone H4 as in a recent nucleosome-Snf2 ATPase structure8. Comparison with published results9 reveals that the double chromodomain swings towards nucleosomal DNA at SHL +1, resulting in ATPase closure. The ATPase can then promote translocation of DNA towards the nucleosome dyad, thereby loosening the first DNA gyre and remodelling the nucleosome. Translocation may involve ratcheting of the two lobes of the ATPase, which is trapped in a pre- or post-translocated state in the absence8 or presence, respectively, of transition state-mimicking compounds.

Structure and function of Pif1 helicase

Pif1 family helicases have multiple roles in the maintenance of nuclear and mitochondrial DNA in eukaryotes. Saccharomyces cerevisiae Pif1 is involved in replication through barriers to replication, such as G-quadruplexes and protein blocks, and reduces genetic instability at these sites. Another Pif1 family helicase in S. cerevisiae, Rrm3, assists in fork progression through replication fork barriers at the rDNA locus and tRNA genes. ScPif1 (Saccharomyces cerevisiae Pif1) also negatively regulates telomerase, facilitates Okazaki fragment processing, and acts with polymerase δ in break-induced repair. Recent crystal structures of bacterial Pif1 helicases and the helicase domain of human PIF1 combined with several biochemical and biological studies on the activities of Pif1 helicases have increased our understanding of the function of these proteins. This review article focuses on these structures and the mechanism(s) proposed for Pif1's various activities on DNA.

Cryo-EM Structure of a Relaxase Reveals the Molecular Basis of DNA Unwinding during Bacterial Conjugation

Relaxases play essential roles in conjugation, the main process by which bacteria exchange genetic material, notably antibiotic resistance genes. They are bifunctional enzymes containing a trans-esterase activity, which is responsible for nicking the DNA strand to be transferred and for covalent attachment to the resulting 5'-phosphate end, and a helicase activity, which is responsible for unwinding the DNA while it is being transported to a recipient cell. Here we show that these two activities are carried out by two conformers that can both load simultaneously on the origin of transfer DNA. We solve the structure of one of these conformers by cryo electron microscopy to near-atomic resolution, elucidating the molecular basis of helicase function by relaxases and revealing insights into the mechanistic events taking place in the cell prior to substrate transport during conjugation.

Interdomain Communication of the Chd1 Chromatin Remodeler across the DNA Gyres of the Nucleosome

Chromatin remodelers use a helicase-like ATPase motor to reposition and reorganize nucleosomes along genomic DNA. Yet, how the ATPase motor communicates with other remodeler domains in the context of the nucleosome has so far been elusive. Here, we report for the Chd1 remodeler a unique organization of domains on the nucleosome that reveals direct domain-domain communication. Site-specific cross-linking shows that the chromodomains and ATPase motor bind to adjacent SHL1 and SHL2 sites, respectively, on nucleosomal DNA and pack against the DNA-binding domain on DNA exiting the nucleosome. This domain arrangement spans the two DNA gyres of the nucleosome and bridges both ends of a wrapped, ∼90-bp nucleosomal loop of DNA, suggesting a means for nucleosome assembly. This architecture illustrates how Chd1 senses DNA outside the nucleosome core and provides a basis for nucleosome spacing and directional sliding away from transcription factor barriers.