Structure and mechanism of helicases and nucleic acid translocases

Stabilization of the hexasome intermediate during histone exchange by yeast SWR1 complex

The yeast SWR1 complex catalyzes the exchange of histone H2A/H2B dimers in nucleosomes with Htz1/H2B dimers. We use cryoelectron microscopy to determine the structure of an enzyme-bound hexasome intermediate in the reaction pathway of histone exchange, in which an H2A/H2B dimer has been extracted from a nucleosome prior to the insertion of a dimer comprising Htz1/H2B. The structure reveals a key role for the Swc5 subunit in stabilizing the unwrapping of DNA from the histone core of the hexasome. By engineering a crosslink between an Htz1/H2B dimer and its chaperone protein Chz1, we show that this blocks histone exchange by SWR1 but allows the incoming chaperone-dimer complex to insert into the hexasome. We use this reagent to trap an SWR1/hexasome complex with an incoming Htz1/H2B dimer that shows how the reaction progresses to the next step. Taken together the structures reveal insights into the mechanism of histone exchange by SWR1 complex.

Immunoproteomic profile of Malus domestica in Mexican pediatric patients. Evidence of new allergen prospects

Background: Apple (Malus domestica) is a fruit commonly associated with allergic oral symptoms in the Mexican pediatric population; however, knowledge of its allergenic proteins is limited. This information is crucial as sensitization frequencies to specific allergens can vary among different populations. The main allergic symptomatology before apple ingestion derives from primary sensitizations induced by pollen, promoting cross-reactivity with the main allergenic protein of apple. Therefore, this study aims to identify new potential sensitizing proteins to apple using immunoproteomic techniques. Methods: We collected serum samples from 14 pediatric patients with confirmed immunoglobulin E (IgE)-mediated apple allergy and used these samples to assess immunoreactivity to apple protein extracts through 2D-western blot assays. The spots corresponding to the 2D-SDS-PAGE were analyzed using nanoLC-MS/MS. Results: We identified 11 non-redundant proteins, including Mal d 2 and Mal d 1, the latter showing a high frequency of sensitization (79%) in our patients, and being considered the main apple allergenic protein. The remaining identified proteins have not been previously described as apple allergens in the International Union of Immunological Societies databases. However, three of these may be categorized as pan-allergens. Conclusions: This study shows evidence that the repertoire of apple allergens in the Mexican population could differ from those reported internationally, highlighting the importance of studies in different countries to improve the certainty of allergy diagnosis and allow the implementation of precision medicine.

Essential and multifunctional mpox virus E5 helicase-primase in double and single hexamer

An outbreak of mpox virus in May 2022 has spread over 110 nonpandemic regions in the world, posing a great threat to global health. Mpox virus E5, a helicase-primase, plays an essential role in DNA replication, but the molecular mechanisms are elusive. Here, we report seven structures of mpox virus E5 in a double hexamer (DH) and six in single hexamer in different conformations, indicating a rotation mechanism for helicase and a coupling action for primase. The DH is formed through the interface of zinc-binding domains, and the central channel density indicates potential double-stranded DNA (dsDNA), which helps to identify dsDNA binding residues Arg249, Lys286, Lys315, and Lys317. Our work is important not only for understanding poxviral DNA replication but also for the development of novel therapeutics for serious poxviral infections including smallpox virus and mpox virus.

Genome-wide identification and analysis of DEAD-box RNA helicases in Gossypium hirsutum

DEAD-box RNA helicases, a prominent subfamily within the RNA helicase superfamily 2 (SF2), play crucial roles in the growth, development, and abiotic stress responses of plants. This study identifies 146 DEAD-box RNA helicase genes (GhDEADs) and categorizes them into four Clades (Clade A-D) through phylogenetic analysis. Promoter analysis reveals cis-acting elements linked to plant responses to light, methyl jasmonate (MeJA), abscisic acid (ABA), low temperature, and drought. RNA-seq data demonstrate that Clade C GhDEADs exhibit elevated and ubiquitous expression across different tissues, validating their connection to leaf development through real-time quantitative polymerase chain reaction (RT-qPCR) analysis. Notably, over half of GhDEADs display up-regulation in the leaves of virus-induced gene silencing (VIGS) plants of GhVIR-A/D (members of m6A methyltransferase complex, which regulate leaf morphogenesis). In conclusion, this study offers a comprehensive insight into GhDEADs, emphasizing their potential involvement in leaf development.

Enzymatic characterization and dominant sites of foot-and-mouth disease virus 2C protein

Foot-and-mouth disease virus (FMDV) 2C protein is a conserved non-structural protein and crucial for replication of the virus. In this study, FMDV 2C protein was prepared and the enzymatic activities were investigated in detail. The protein could digest ssDNA or ssRNA into a small fragment at about 10 nt, indicating that the protein has nuclease activity. But it did not show digestion to blunt-end dsDNA or dsRNA. The nuclease activity of 2C protein could be inhibited in 2 mM Zn2+ or Ca2+ while enhanced by Mg2+ or Mn2+. FMDV 2C protein exhibited unwinding activity to all the three kinds of dsDNA and dsRNA (5' protruded, 3' protruded, and blunt-end). The unwinding velocity to 5' protruded dsRNA was higher than to the blunt-end dsRNA. 2C protein only showed unwinding activity in high concentration of Mg2+, but no unwinding activity in physiological concentrations of Mg2+ and Ca2+, as well as in cell lysate. The 2C protein could catalyze two structured ssRNA to form double strand, thus it was proved to have RNA chaperone activity. The Mg2+ and ATP in different concentrations did not show promotion to the RNA chaperone activity. Finally, six mutant proteins (K116A, D160A, D170A, N207A, R226A, and F316A) were constructed and the enzymatic activities were analyzed. All the six mutations reduced the ATPase activity, D170A and F361A could inactivate the nuclease activity, while the N207A and F316A could inactivate the helicase activity. Our study provides a comprehensive understanding of the enzymatic activities of FMDV 2C protein.

DdmDE eliminates plasmid invasion by DNA-guided DNA targeting

Horizontal gene transfer is a key driver of bacterial evolution, but it also presents severe risks to bacteria by introducing invasive mobile genetic elements. To counter these threats, bacteria have developed various defense systems, including prokaryotic Argonautes (pAgos) and the DNA defense module DdmDE system. Through biochemical analysis, structural determination, and in vivo plasmid clearance assays, we elucidate the assembly and activation mechanisms of DdmDE, which eliminates small, multicopy plasmids. We demonstrate that DdmE, a pAgo-like protein, acts as a catalytically inactive, DNA-guided, DNA-targeting defense module. In the presence of guide DNA, DdmE targets plasmids and recruits a dimeric DdmD, which contains nuclease and helicase domains. Upon binding to DNA substrates, DdmD transitions from an autoinhibited dimer to an active monomer, which then translocates along and cleaves the plasmids. Together, our findings reveal the intricate mechanisms underlying DdmDE-mediated plasmid clearance, offering fundamental insights into bacterial defense systems against plasmid invasions.

The Role of SF1 and SF2 Helicases in Biotechnological Applications

Helicases, which utilize ATP hydrolysis to separate nucleic acid duplexes, play crucial roles in DNA and RNA replication, repair, recombination, and transcription. Categorized into the major groups superfamily 1 (SF1) and superfamily 2 (SF2), alongside four minor groups, these proteins exhibit a conserved catalytic core indicative of a shared evolutionary origin while displaying functional diversity through interactions with various substrates. This review summarizes the structures, functions and mechanisms of SF1 and SF2 helicases, with an emphasis on conserved ATPase sites and RecA-like domains essential for their enzymatic and nucleic acid binding capabilities. It highlights the unique 1B and 2B domains in SF1 helicases and their impact on enzymatic activity. The DNA unwinding process is detailed, covering substrate recognition, ATP hydrolysis, and conformational changes, while addressing debates over the active form of UvrD helicase and post-unwinding dissociation. More importantly, this review discusses the biotechnological potential of helicases in emerging technologies such as nanopore sequencing, protein sequencing, and isothermal amplification, focusing on their use in pathogen detection, biosensor enhancement, and cancer treatment. As understanding deepens, innovative applications in genome editing, DNA sequencing, and synthetic biology are anticipated.

DdmDE eliminates plasmid invasion by DNA-guided DNA targeting

Horizontal gene transfer is a key driver of bacterial evolution, but it also presents severe risks to bacteria by introducing invasive mobile genetic elements. To counter these threats, bacteria have developed various defense systems, including prokaryotic Argonautes (pAgo) and the D NA D efense M odule DdmDE system. Through biochemical analysis, structural determination, and in vivo plasmid clearance assays, we elucidate the assembly and activation mechanisms of DdmDE, which eliminates small, multicopy plasmids. We demonstrate that DdmE, a pAgo-like protein, acts as a catalytically inactive, DNA-guided, DNA-targeting defense module. In the presence of guide DNA, DdmE targets plasmids and recruits a dimeric DdmD, which contains nuclease and helicase domains. Upon binding to DNA substrates, DdmD transitions from an autoinhibited dimer to an active monomer, which then translocates along and cleaves the plasmids. Together, our findings reveal the intricate mechanisms underlying DdmDE-mediated plasmid clearance, offering fundamental insights into bacterial defense systems against plasmid invasions.

Short-range translocation by a restriction enzyme motor triggers diffusion along DNA

Cleavage of bacteriophage DNA by the Type III restriction-modification enzymes requires long-range interaction between DNA sites. This is facilitated by one-dimensional diffusion ('DNA sliding') initiated by ATP hydrolysis catalyzed by a superfamily 2 helicase-like ATPase. Here we combined ultrafast twist measurements based on plasmonic DNA origami nano-rotors with stopped-flow fluorescence and gel-based assays to examine the role(s) of ATP hydrolysis. Our data show that the helicase-like domain has multiple roles. First, this domain stabilizes initial DNA interactions alongside the methyltransferase subunits. Second, it causes environmental changes in the flipped adenine base following hydrolysis of the first ATP. Finally, it remodels nucleoprotein interactions via constrained translocation of a ∼ 5 to 22-bp double stranded DNA loop. Initiation of DNA sliding requires 8-15 bp of DNA downstream of the motor, corresponding to the site of nuclease domain binding. Our data unify previous contradictory communication models for Type III enzymes.

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.

Leucoverdazyls as Novel Potent Inhibitors of Enterovirus Replication

Enteroviruses (EV) are important pathogens causing human disease with various clinical manifestations. To date, treatment of enteroviral infections is mainly supportive since no vaccination or antiviral drugs are approved for their prevention or treatment. Here, we describe the antiviral properties and mechanisms of action of leucoverdazyls-novel heterocyclic compounds with antioxidant potential. The lead compound, 1a, demonstrated low cytotoxicity along with high antioxidant and virus-inhibiting activity. A viral strain resistant to 1a was selected, and the development of resistance was shown to be accompanied by mutation of virus-specific non-structural protein 2C. This resistant virus had lower fitness when grown in cell culture. Taken together, our results demonstrate high antiviral potential of leucoverdazyls as novel inhibitors of enterovirus replication and support previous evidence of an important role of 2C proteins in EV replication.

Activity and inhibition of the SARS-CoV-2 Omicron nsp13 R392C variant using RNA duplex unwinding assays

SARS-CoV-2 nsp13 helicase is an essential enzyme for viral replication and a promising target for antiviral drug development. This study compares the double-stranded RNA (dsRNA) unwinding activity of nsp13 and the Omicron nsp13R392C variant, which is predominant in currently circulating lineages. Using in vitro gel- and fluorescence-based assays, we found that both nsp13 and nsp13R392C have dsRNA unwinding activity with equivalent kinetics. Furthermore, the R392C mutation had no effect on the efficiency of the nsp13-specific helicase inhibitor SSYA10-001. We additionally confirmed the activity of several other helicase inhibitors against nsp13, including punicalagin that inhibited dsRNA unwinding at nanomolar concentrations. Overall, this study reveals the utility of using dsRNA unwinding assays to screen small molecules for antiviral activity against nsp13 and the Omicron nsp13R392C variant. Continual monitoring of newly emergent variants will be essential for considering resistance profiles of lead compounds as they are advanced towards next-generation therapeutic development.

DDX4 enhances antiviral activity of type I interferon by disrupting interaction of USP7/SOCS1 and promoting degradation of SOCS1

DEAD-box helicase (DDX) family members play differential roles in regulating innate antiviral immune response. However, the physiological roles played by DDX4 in antiviral innate immunity remain unclear. In this study, we unveiled that DDX4 acts as a positive regulatory molecule of Type-I interferon (IFN-I)-mediated antiviral activity. Our findings demonstrate that IFN-I upregulates DDX4 protein levels, and subsequently, overexpression of DDX4 enhances the IFN-I-mediated signaling pathway. This creates a positive feedback loop that amplifies the antiviral response. DDX4 was found to bind with deubiquitinase ubiquitin-specific protease 7 (USP7), leading to the disruption of the interaction between USP7 and suppressor of cytokine signaling 1 (SOCS1) and the subsequent degradation of SOCS1. This process enhances the antiviral function of IFN-I. Our findings provide new insights into the regulatory role of DDX4 in the IFN-I response.IMPORTANCEDDX4, identified as a putative RNA helicase that modulates RNA secondary structure through RNA binding, is primarily acknowledged for its role in regulating mRNA translation within the germline. Nevertheless, the extent of DDX4's involvement in the antiviral innate immune response remains largely unexplored. This study presents evidence of a previously unrecognized positive feedback loop between DDX4 and the antiviral response, suggesting that disruption of this loop may serve as a novel mechanism for viral evasion. Furthermore, our findings elucidate a positive regulatory mechanism by which the DDX4/USP7/SOCS1 axis mediates the antiviral activity of Type-I interferon, which provides new insight into strategies for improving the efficacy of IFN-based antiviral therapy.

High-resolution fleezers reveal duplex opening and stepwise assembly by an oligomer of the DEAD-box helicase Ded1p

DEAD-box RNA helicases are ubiquitous in all domains of life where they bind and remodel RNA and RNA-protein complexes. DEAD-box helicases unwind RNA duplexes by local opening of helical regions without directional movement through the duplexes and some of these enzymes, including Ded1p from Saccharomyces cerevisiae, oligomerize to effectively unwind RNA duplexes. Whether and how DEAD-box helicases coordinate oligomerization and unwinding is not known and it is unclear how many base pairs are actively opened. Using high-resolution optical tweezers and fluorescence, we reveal a highly dynamic and stochastic process of multiple Ded1p protomers assembling on and unwinding an RNA duplex. One Ded1p protomer binds to a duplex-adjacent ssRNA tail and promotes binding and subsequent unwinding of the duplex by additional Ded1p protomers in 4-6 bp steps. The data also reveal rapid duplex unwinding and rezipping linked with binding and dissociation of individual protomers and coordinated with the ATP hydrolysis cycle.

Hachiman is a genome integrity sensor

Hachiman is a broad-spectrum antiphage defense system of unknown function. We show here that Hachiman comprises a heterodimeric nuclease-helicase complex, HamAB. HamA, previously a protein of unknown function, is the effector nuclease. HamB is the sensor helicase. HamB constrains HamA activity during surveillance of intact dsDNA. When the HamAB complex detects DNA damage, HamB helicase activity liberates HamA, unleashing nuclease activity. Hachiman activation degrades all DNA in the cell, creating 'phantom' cells devoid of both phage and host DNA. We demonstrate Hachiman activation in the absence of phage by treatment with DNA-damaging agents, suggesting that Hachiman responds to aberrant DNA states. Phylogenetic similarities between the Hachiman helicase and eukaryotic enzymes suggest this bacterial immune system has been repurposed for diverse functions across all domains of life.

DDX41: exploring the roles of a versatile helicase

DDX41 is a DEAD-box helicase and is conserved across species. Mutations in DDX41 have been associated with myeloid neoplasms, including myelodysplastic syndrome and acute myeloid leukemia. Though its pathogenesis is not completely known, DDX41 has been shown to have many cellular roles, including in pre-mRNA splicing, innate immune sensing, ribosome biogenesis, translational regulation, and R-loop metabolism. In this review, we will summarize the latest understandings regarding the various roles of DDX41, as well as highlight challenges associated with drug development to target DDX41. Overall, understanding the molecular and cellular mechanisms of DDX41 could help develop novel therapeutic options for DDX41 mutation-related hematologic malignancies.

Research Progress of Group II Intron Splicing Factors in Land Plant Mitochondria

Mitochondria are important organelles that provide energy for the life of cells. Group II introns are usually found in the mitochondrial genes of land plants. Correct splicing of group II introns is critical to mitochondrial gene expression, mitochondrial biological function, and plant growth and development. Ancestral group II introns are self-splicing ribozymes that can catalyze their own removal from pre-RNAs, while group II introns in land plant mitochondria went through degenerations in RNA structures, and thus they lost the ability to self-splice. Instead, splicing of these introns in the mitochondria of land plants is promoted by nuclear- and mitochondrial-encoded proteins. Many proteins involved in mitochondrial group II intron splicing have been characterized in land plants to date. Here, we present a summary of research progress on mitochondrial group II intron splicing in land plants, with a major focus on protein splicing factors and their probable functions on the splicing of mitochondrial group II introns.

E. coli RecB Nuclease Domain Regulates RecBCD Helicase Activity but not Single Stranded DNA Translocase Activity

Much is still unknown about the mechanisms by which helicases unwind duplex DNA. Whereas structure-based models describe DNA unwinding as occurring by the ATPase motors mechanically pulling the DNA duplex across a wedge domain in the helicase, biochemical data show that processive DNA unwinding by E. coli RecBCD helicase can occur in the absence of ssDNA translocation by the canonical RecB and RecD motors. Here we show that DNA unwinding is not a simple consequence of ssDNA translocation by the motors. Using stopped-flow fluorescence approaches, we show that a RecB nuclease domain deletion variant (RecBΔNucCD) unwinds dsDNA at significantly slower rates than RecBCD, while the ssDNA translocation rate is unaffected. This effect is primarily due to the absence of the nuclease domain since a nuclease-dead mutant (RecBD1080ACD), which retains the nuclease domain, showed no change in ssDNA translocation or dsDNA unwinding rates relative to RecBCD on short DNA substrates (≤60 base pairs). Hence, ssDNA translocation is not rate-limiting for DNA unwinding. RecBΔNucCD also initiates unwinding much slower than RecBCD from a blunt-ended DNA. RecBΔNucCD also unwinds DNA ∼two-fold slower than RecBCD on long DNA (∼20 kilo base pair) in single molecule optical tweezer experiments, although the rates for RecBD1080ACD unwinding are intermediate between RecBCD and RecBΔNucCD. Surprisingly, significant pauses in DNA unwinding occur even in the absence of chi (crossover hotspot instigator) sites. We hypothesize that the nuclease domain influences the rate of DNA base pair melting, possibly allosterically and that RecBΔNucCD may mimic a post-chi state of RecBCD.

ZNFX1 promotes AMPK-mediated autophagy against Mycobacterium tuberculosis by stabilizing Prkaa2 mRNA

Tuberculosis has the highest mortality rate worldwide for a chronic infectious disease caused by a single pathogen. RNA-binding proteins (RBPs) are involved in autophagy - a key defense mechanism against Mycobacterium tuberculosis (M. tuberculosis) infection - by modulating RNA stability and forming intricate regulatory networks. However, the functions of host RBPs during M. tuberculosis infection remain relatively unexplored. Zinc finger NFX1-type containing 1 (ZNFX1), a conserved RBP critically involved in immune deficiency diseases and mycobacterial infections, is significantly upregulated in M. tuberculosis-infected macrophages. Here, we aimed to explore the immunoregulatory functions of ZNFX1 during M. tuberculosis infection. We observed that Znfx1 knockout markedly compromised the multifaceted immune responses mediated by macrophages. This compromise resulted in reduced phagocytosis, suppressed macrophage activation, increased M. tuberculosis burden, progressive lung tissue injury, and chronic inflammation in M. tuberculosis-infected mice. Mechanistic investigations revealed that the absence of ZNFX1 inhibited autophagy, consequently mediating immune suppression. ZNFX1 critically maintained AMPK-regulated autophagic flux by stabilizing protein kinase AMP-activated catalytic subunit alpha 2 mRNA, which encodes a key catalytic α subunit of AMPK, through its zinc finger region. This process contributed to M. tuberculosis growth suppression. These findings reveal a function of ZNFX1 in establishing anti-M. tuberculosis immune responses, enhancing our understanding of the roles of RBPs in tuberculosis immunity and providing a promising approach to bolster antituberculosis immunotherapy.

Structural insight into the assembly and working mechanism of helicase-primase D5 from Mpox virus

The Mpox pandemic, caused by the Mpox virus (or monkeypox virus, MPXV), has gained global attention. The D5 protein, a putative helicase-primase found in MPXV, plays a vital role in viral replication and genome uncoating. Here we determined multiple cryo-EM structures of full-length hexameric D5 in diverse states. These states were captured during ATP hydrolysis while moving along the single-stranded DNA (ssDNA) track. Through comprehensive structural analysis combined with the helicase activity system, we revealed that when the primase domain is truncated or the interaction between the primase and helicase domains is disrupted, the double-stranded DNA (dsDNA) unwinds into ssDNA, suggesting a critical regulatory role of the primase domain. Two transition states bound with ssDNA substrate during unwinding reveals that two ATP molecules were consumed to drive DNA moving forward two nucleotides. Collectively, our findings shed light on the molecular mechanism that links ATP hydrolysis to the DNA unwinding in poxviruses.

Werner helicase mediates the senescence and cell cycle of leukemia cells by regulating DNA repair pathways

Leukemia is a type of malignant hematological disease that is generally resistant to chemotherapy and has poor therapeutic outcomes. Werner (WRN) DNA helicase, an important member of the RecQ family of helicases, plays an important role in DNA repair and telomere stability maintenance. WRN gene dysfunction leads to premature aging and predisposes humans to various types of cancers. However, the biological function of WRN in cancer remains unknown. In this study, the expression of this RecQ family helicase was investigated in different types of leukemia cells, and the leukemia cell line K562 with high WRN expression was selected to construct a WRN knockdown cell line. The results showed that WRN knockdown inhibited leukemia occurrence and development by regulating the proliferation, cell cycle, differentiation, and aging of cells and other biological processes. The results of transcriptome sequencing revealed that WRN promoted the sensitivity of leukemia cells to the DNA damage inducer Etoposide by regulating cell cycle-related proteins, such as CDC2, cyclin B1, p16, and p21, as well as key proteins in DNA damage repair pathways, such as p53, RAD50, RAD51, and MER11. Our findings show that WRN helicase is a promising potential target for leukemia treatment, providing new ideas for the development of targeted drugs against leukemia.

Superfamily II helicases: the potential therapeutic target for cardiovascular diseases

Cardiovascular diseases (CVDs) still maintain high morbidity and mortality globally. Helicases, a unique class of enzymes, are extensively implicated in the processes of nucleic acid (NA) metabolism across various organisms. They play a pivotal role in gene expression, inflammatory response, lipid metabolism, and so forth. However, abnormal helicase expression has been associated with immune response, cancer, and intellectual disability in humans. Superfamily II (SFII) is one of the largest and most diverse of the helicase superfamilies. Increasing evidence has implicated SFⅡ helicases in the pathogenesis of multiple CVDs. In this review, we comprehensively review the regulation mechanism of SFⅡ helicases in CVDs including atherosclerosis, myocardial infarction, cardiomyopathies, and heart failure, which will contribute to the investigation of ideal therapeutic targets for CVDs.

G-quadruplexes of KSHV oriLyt play important roles in promoting lytic DNA replication

IMPORTANCE

Biological processes originating from the DNA and RNA can be regulated by the secondary structures present in the stretch of nucleic acids, and the G-quadruplexes are shown to regulate transcription, translation, and replication. In this study, we identified the presence of multiple G-quadruplex sites in the region (oriLyt) of Kaposi's sarcoma-associated herpesvirus (KSHV) DNA, which is essential for DNA replication during the lytic cycle. We demonstrated the roles of these G-quadruplexes through multiple biochemical and biophysical assays in controlling replication and efficient virus production. We demonstrated that KSHV achieves this by recruiting RecQ1 (helicase) at those G-quadruplex sites for efficient viral DNA replication. Analysis of the replicated DNA through nucleoside labeling and immunostaining showed a reduced initiation of DNA replication in cells with a pharmacologic stabilizer of G-quadruplexes. Overall, this study confirmed the role of the G-quadruplex in regulating viral DNA replication, which can be exploited for controlling viral DNA replication.

Differing structures and dynamics of two photolesions portray verification differences by the human XPD helicase

Ultraviolet light generates cyclobutane pyrimidine dimer (CPD) and pyrimidine 6-4 pyrimidone (6-4PP) photoproducts that cause skin malignancies if not repaired by nucleotide excision repair (NER). While the faster repair of the more distorting 6-4PPs is attributed mainly to more efficient recognition by XPC, the XPD lesion verification helicase may play a role, as it directly scans the damaged DNA strand. With extensive molecular dynamics simulations of XPD-bound single-strand DNA containing each lesion outside the entry pore of XPD, we elucidate strikingly different verification processes for these two lesions that have very different topologies. The open book-like CPD thymines are sterically blocked from pore entry and preferably entrapped by sensors that are outside the pore; however, the near-perpendicular 6-4PP thymines can enter, accompanied by a displacement of the Arch domain toward the lesion, which is thereby tightly accommodated within the pore. This trapped 6-4PP may inhibit XPD helicase activity to foster lesion verification by locking the Arch to other domains. Furthermore, the movement of the Arch domain, only in the case of 6-4PP, may trigger signaling to the XPG nuclease for subsequent lesion incision by fostering direct contact between the Arch domain and XPG, and thereby facilitating repair of 6-4PP.

The separation pin distinguishes the pro- and anti-recombinogenic functions of Saccharomyces cerevisiae Srs2

Srs2 is an Sf1a helicase that helps maintain genome stability in Saccharomyces cerevisiae through its ability to regulate homologous recombination. Srs2 downregulates HR by stripping Rad51 from single-stranded DNA, and Srs2 is also thought to promote synthesis-dependent strand annealing by unwinding D-loops. However, it has not been possible to evaluate the relative contributions of these two distinct activities to any aspect of recombination. Here, we used a structure-based approach to design an Srs2 separation-of-function mutant that can dismantle Rad51-ssDNA filaments but is incapable of disrupting D-loops, allowing us to assess the relative contributions of these pro- and anti-recombinogenic functions. We show that this separation-of-function mutant phenocopies wild-type SRS2 in vivo, suggesting that the ability of Srs2 to remove Rad51 from ssDNA is its primary role during HR.

HELQ as a DNA helicase: Its novel role in normal cell function and tumorigenesis (Review)

Helicase POLQ‑like (HELQ or Hel308), is a highly conserved, 3'‑5' superfamily II DNA helicase that contributes to diverse DNA processes, including DNA repair, unwinding, and strand annealing. HELQ deficiency leads to subfertility, due to its critical role in germ cell stability. In addition, the abnormal expression of HELQ has been observed in multiple tumors and a number of molecular pathways, including the nucleotide excision repair, checkpoint kinase 1‑DNA repair protein RAD51 homolog 1 and ATM/ATR pathways, have been shown to be involved in HELQ. In the present review, the structure and characteristics of HELQ, as well as its major functions in DNA processing, were described. Molecular mechanisms involving HELQ in the context of tumorigenesis were also described. It was deduced that HELQ biology warrants investigation, and that its critical roles in the regulation of various DNA processes and participation in tumorigenesis are clinically relevant.

Molecular wrench activity of DNA helicases: Keys to modulation of rapid kinetics in DNA repair

DNA helicase activity is essential for the vital DNA metabolic processes of recombination, replication, transcription, translation, and repair. Recently, an unexpected, rapid exponential ATP-stimulated DNA unwinding rate was observed from an Archaeoglobus fulgidus helicase (AfXPB) as compared to the slower conventional helicases from Sulfolobus tokodaii, StXPB1 and StXPB2. This unusual rapid activity suggests a "molecular wrench" mechanism arising from the torque applied by AfXPB on the duplex structure in transitioning from open to closed conformations. However, much remains to be understood. Here, we investigate the concentration dependence of DNA helicase binding and ATP-stimulated kinetics of StXPB2 and AfXPB, as well as their binding and activity in Bax1 complexes, via an electrochemical assay with redox-active DNA monolayers. StXPB2 ATP-stimulated activity is concentration-independent from 8 to 200 nM. Unexpectedly, AfXPB activity is concentration-dependent in this range, with exponential rate constants varying from seconds at concentrations greater than 20 nM to thousands of seconds at lower concentrations. At 20 nM, rapid exponential signal decay ensues, linearly reverses, and resumes with a slower exponential decay. This change in AfXPB activity as a function of its concentration is rationalized as the crossover between the fast molecular wrench and slower conventional helicase modes. AfXPB-Bax1 inhibits rapid activity, whereas the StXPB2-Bax1 complex induces rapid kinetics at higher concentrations. This activity is rationalized with the crystal structures of these complexes. These findings illuminate the different physical models governing molecular wrench activity for improved biological insight into a key factor in DNA repair.

Mutant forms of DDX3X with diminished catalysis form hollow condensates that exhibit sex-specific regulation

Mutations in the RNA helicase DDX3X, implicated in various cancers and neurodevelopmental disorders, often impair RNA unwinding and translation. However, the mechanisms underlying this impairment and the differential interactions of DDX3X mutants with wild-type (WT) X-linked DDX3X and Y-linked homolog DDX3Y remain elusive. This study reveals that specific DDX3X mutants more frequently found in disease form distinct hollow condensates in cells. Using a combined structural, biochemical, and single-molecule microscopy study, we show that reduced ATPase and RNA release activities contribute to condensate formation and the catalytic deficits result from inhibiting the catalytic cycle at multiple steps. Proteomic investigations further demonstrate that these hollow condensates sequester WT DDX3X/DDX3Y and other proteins crucial for diverse signaling pathways. WT DDX3X enhances the dynamics of heterogeneous mutant/WT hollow condensates more effectively than DDX3Y. These findings offer valuable insights into the catalytic defects of specific DDX3X mutants and their differential interactions with wild-type DDX3X and DDX3Y, potentially explaining sex biases in disease.

Conformational Changes and ATP Hydrolysis in Zika Helicase: The Molecular Basis of a Biomolecular Motor Unveiled by Multiscale Simulations

We computationally study the Zika NS3 helicase, a biological motor, using ATP hydrolysis energy for nucleic acid remodeling. Through molecular mechanics and hybrid quantum mechanics/molecular mechanics simulations, we explore the conformational landscape of motif V, a conserved loop connecting the active sites for ATP hydrolysis and nucleic acid binding. ATP hydrolysis, initiated by a meta-phosphate group formation, involves the nucleophilic attack of a water molecule activated by Glu286 proton abstraction. Motif V hydrogen bonds to this water via the Gly415 backbone NH group, assisting hydrolysis. Posthydrolysis, free energy is released when the inorganic phosphate moves away from the coordination shell of the magnesium ion, inducing a significant shift in the conformational landscape of motif V to establish a hydrogen bond between the Gly415 NH group and Glu285. According to our simulations, the Zika NS3 helicase acts as a ratchet biological motor with motif V transitions steered by Gly415's γ-phosphate sensing in the ATPase site.

Two novel phages PSPa and APPa inhibit planktonic, sessile and persister populations of Pseudomonas aeruginosa, and mitigate its virulence in Zebrafish model

The present study explores the avenue of phage therapy as an alternative antimicrobial therapeutic approach to counter multidrug-resistant (MDR) Pseudomonas aeruginosa infection. Our study investigated two novel virulent phages PSPa and APPa, specific to P. aeruginosa, in which in vitro evaluations were carried out to assess the therapeutic potential of phages. Both the identified phages exhibited host specificity by showing antagonistic activity of about 96.43% (27/28) and 92.85% (26/28) towards the 28 MDR clinical isolates of P. aeruginosa. The PSPa phage was found to have linear dsDNA with a sequence length of 66,368 bp and 92 ORFs, of which 32 were encoded for known functions of the phage life cycle and the remaining 60 were hypothetical functions. The APPa phage was found to have linear dsDNA with 59,591 bp of genome length and 79 ORFs, of which 15 were found to have known phage functions and the remaining 64 were found to be hypothetical proteins. Notably, the genome of both the phages lacks genes coding for tRNA, rRNA, and tmRNA. The phylogenetic analysis revealed that PSPa and APPa share > 95% sequence similarity with previously sequenced Pseudomonas viruses of their respective families. Further, the in vivo efficacy evaluation using the zebrafish model revealed that the treatment with PSPa and APPa has remarkably improved the survival rate of bacterial-infected zebrafish, reinforcing the anti-infective potential of the isolated phages PSPa and APPa against P. aeruginosa infection.

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.

E. coli RecBCD Nuclease Domain Regulates Helicase Activity but not Single Stranded DNA Translocase Activity

Much is still unknown about the mechanisms by which helicases unwind duplex DNA. Whereas structure-based models describe DNA unwinding as a consequence of mechanically pulling the DNA duplex across a wedge domain in the helicase by the single stranded (ss)DNA translocase activity of the ATPase motors, biochemical data indicate that processive DNA unwinding by the E. coli RecBCD helicase can occur in the absence of ssDNA translocation of the canonical RecB and RecD motors. Here, we present evidence that dsDNA unwinding is not a simple consequence of ssDNA translocation by the RecBCD motors. Using stopped-flow fluorescence approaches, we show that a RecB nuclease domain deletion variant (RecB ΔNuc CD) unwinds dsDNA at significantly slower rates than RecBCD, while the rate of ssDNA translocation is unaffected. This effect is primarily due to the absence of the nuclease domain and not the absence of the nuclease activity, since a nuclease-dead mutant (RecB D1080A CD), which retains the nuclease domain, showed no significant change in rates of ssDNA translocation or dsDNA unwinding relative to RecBCD on short DNA substrates (≤ 60 base pairs). This indicates that ssDNA translocation is not rate-limiting for DNA unwinding. RecB ΔNuc CD also initiates unwinding much slower than RecBCD from a blunt-ended DNA, although it binds with higher affinity than RecBCD. RecB ΔNuc CD also unwinds DNA ∼two-fold slower than RecBCD on long DNA (∼20 kilo base pair) in single molecule optical tweezer experiments, although the rates for RecB D1080A CD unwinding are intermediate between RecBCD and RecB ΔNuc CD. Surprisingly, significant pauses occur even in the absence of chi (crossover hotspot instigator) sites. We hypothesize that the nuclease domain influences the rate of DNA base pair melting, rather than DNA translocation, possibly allosterically. Since the rate of DNA unwinding by RecBCD also slows after it recognizes a chi sequence, RecB ΔNuc CD may mimic a post- chi state of RecBCD.

OsRuvBL1a DNA helicase boost salinity and drought tolerance in transgenic indica rice raised by in planta transformation

RuvBL, is a member of SF6 superfamily of helicases and is conserved among the various model systems. Recently, rice (Oryza sativa L.) homolog of RuvBL has been biochemically characterized for its ATPase and DNA helicase activities; however its involvement in stress has not been studied so far. Present investigation reports the detailed functional characterization of OsRuvBL under abiotic stresses through genetic engineering. An efficient Agrobacterium-mediated in planta transformation protocol was developed in indica rice to generate the transgenic lines and study was focused on optimization of factors to achieve maximum transformation efficiency. Overexpressing OsRuvBL1a transgenic lines showed enhanced tolerance under in vivo salinity stress as compared to WT plants. The physiological and biochemical analysis of the OsRuvBL1a transgenic lines showed better performance under salinity and drought stresses. Several stress responsive interacting partners of OsRuvBL1a were identified using Y2H system revealed to its role in stress tolerance. Functional mechanism for boosting stress tolerance by OsRuvBL1a has been proposed in this study. This integration of OsRuvBL1a gene in rice genome using in planta transformation method helped to achieve the abiotic stress resilient smart crop. This study is the first direct evidence to show the novel function of RuvBL in boosting abiotic stress tolerance in plants.

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.

Identification of potential inhibitors of Zika virus targeting NS3 helicase using molecular dynamics simulations and DFT studies

Despite the various research efforts towards the drug discovery program for Zika virus treatment, no antiviral drugs or vaccines have yet been discovered. The spread of the mosquito vector and ZIKV infection exposure is expected to accelerate globally due to continuing global travel. The NS3-Hel is a non-structural protein part and involved in different functions such as polyprotein processing, genome replication, etc. It makes an NS3-Hel protein an attractive target for designing novel drugs for ZIKV treatment. This investigation identifies the novel, potent ZIKV inhibitors by virtual screening and elucidates the binding pattern using molecular docking and molecular dynamics simulation studies. The molecular dynamics simulation results indicate dynamic stability between protein and ligand complexes, and the structures keep significantly unchanged at the binding site during the simulation period. All inhibitors found within the acceptable range having drug-likeness properties. The synthetic feasibility score suggests that all screened inhibitors can be easily synthesizable. Therefore, possible inhibitors obtained from this study can be considered a potential inhibitor for NS3 Hel, and further, it could be provided as a lead for drug development.

Molecular simulations of DEAH-box helicases reveal control of domain flexibility by ligands: RNA, ATP, ADP, and G-patch proteins

DEAH-box helicases use the energy from ATP hydrolysis to translocate along RNA strands. They are composed of tandem RecA-like domains and a C-terminal domain connected by flexible linkers, and the activity of several DEAH-box helicases is regulated by cofactors called G-patch proteins. We used all-atom molecular dynamics simulations of the helicases Prp43, Prp22, and DHX15 in various liganded states to investigate how RNA, ADP, ATP, or G-patch proteins influence their conformational dynamics. The simulations suggest that apo helicases are highly flexible, whereas binding of RNA renders the helicases more rigid. ATP and ADP control the stability of the RecA1-RecA2 interface, but they have only a smaller effect on domain flexibility in absence of a RecA1-RecA2 interface. Binding of a G-patch protein to DHX15 imposes a more structured conformational ensemble, characterized by more defined relative domain arrangements and by an increased conformational stability of the RNA tunnel. However, the effect of the G-patch protein on domain dynamics is far more subtle as compared to the effects of RNA or ATP/ADP. The simulations characterize DEAH-box helicase as dynamic machines whose conformational ensembles are strongly defined by the presence of RNA, ATP, or ADP and only fine-tuned by the presence of G-patch proteins.

Structural basis of the T4 bacteriophage primosome assembly and primer synthesis

The T4 bacteriophage gp41 helicase and gp61 primase assemble into a primosome to couple DNA unwinding with RNA primer synthesis for DNA replication. How the primosome is assembled and how the primer length is defined are unclear. Here we report a series of cryo-EM structures of T4 primosome assembly intermediates. We show that gp41 alone is an open spiral, and ssDNA binding triggers a large-scale scissor-like conformational change that drives the ring closure and activates the helicase. Helicase activation exposes a cryptic hydrophobic surface to recruit the gp61 primase. The primase binds the helicase in a bipartite mode in which the N-terminal Zn-binding domain and the C-terminal RNA polymerase domain each contain a helicase-interacting motif that bind to separate gp41 N-terminal hairpin dimers, leading to the assembly of one primase on the helicase hexamer. Our study reveals the T4 primosome assembly process and sheds light on the RNA primer synthesis mechanism.

Enrichment Culture but Not Metagenomic Sequencing Identified a Highly Prevalent Phage Infecting Lactiplantibacillus plantarum in Human Feces

Lactiplantibacillus plantarum (previously known as Lactobacillus plantarum) is increasingly used as a probiotic to treat human diseases, but its phages in the human gut remain unexplored. Here, we report its first gut phage, Gut-P1, which we systematically screened using metagenomic sequencing, virus-like particle (VLP) sequencing, and enrichment culture from 35 fecal samples. Gut-P1 is virulent, belongs to the Douglaswolinvirus genus, and is highly prevalent in the gut (~11% prevalence); it has a genome of 79,928 bp consisting of 125 protein coding genes and displaying low sequence similarities to public L. plantarum phages. Physiochemical characterization shows that it has a short latent period and adapts to broad ranges of temperatures and pHs. Furthermore, Gut-P1 strongly inhibits the growth of L. plantarum strains at a multiplicity of infection (MOI) of 1e-6. Together, these results indicate that Gut-P1 can greatly impede the application of L. plantarum in humans. Strikingly, Gut-P1 was identified only in the enrichment culture, not in our metagenomic or VLP sequencing data nor in any public human phage databases, indicating the inefficiency of bulk sequencing in recovering low-abundance but highly prevalent phages and pointing to the unexplored hidden diversity of the human gut virome despite recent large-scale sequencing and bioinformatics efforts. IMPORTANCE As Lactiplantibacillus plantarum (previously known as Lactobacillus plantarum) is increasingly used as a probiotic to treat human gut-related diseases, its bacteriophages may pose a certain threat to their further application and should be identified and characterized more often from the human intestine. Here, we isolated and identified the first gut L. plantarum phage that is prevalent in a Chinese population. This phage, Gut-P1, is virulent and can strongly inhibit the growth of multiple L. plantarum strains at low MOIs. Our results also show that bulk sequencing is inefficient at recovering low-abundance but highly prevalent phages such as Gut-P1, suggesting that the hidden diversity of human enteroviruses has not yet been explored. Our results call for innovative approaches to isolate and identify intestinal phages from the human gut and to rethink our current understanding of the enterovirus, particularly its underestimated diversity and overestimated individual specificity.

The ASC-1 complex promotes translation initiation by scanning ribosomes

Translation initiates when the eIF4F complex binds the 5' mRNA cap, followed by 5' untranslated region scanning for the start codon by scanning ribosomes. Here, we demonstrate that the ASC-1 complex (ASCC), which was previously shown to promote the dissociation of colliding 80S ribosomes, associates with scanning ribosomes to regulate translation initiation. Selective translation complex profiling (TCP-seq) analysis revealed that ASCC3, a helicase domain-containing subunit of ASCC, localizes predominantly to the 5' untranslated region of mRNAs. Ribo-seq, TCP-seq, and luciferase reporter analyses showed that ASCC3 knockdown impairs 43S preinitiation complex loading and scanning dynamics, thereby reducing translation efficiency. Whereas eIF4A, an RNA helicase in the eIF4F complex, is important for global translation, ASCC was found to regulate the scanning process for a specific subset of transcripts. Our results have thus revealed that ASCC is required not only for dissociation of colliding 80S ribosomes but also for efficient translation initiation by scanning ribosomes at a subset of transcripts.

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.

Helicase activities of Rad5 and Rrm3 genetically interact in the prevention of recombinogenic DNA lesions in Saccharomyces cerevisiae

The genome must be monitored to ensure its duplication is completed accurately to prevent genome instability. In Saccharomyces cerevisiae, the 5' to 3' DNA helicase Rrm3, a member of the conserved PIF1 family, facilitates replication fork progression through an unknown mechanism. Disruption of Rrm3 helicase activity leads to increased replication fork pausing throughout the yeast genome. Here, we show that Rrm3 contributes to replication stress tolerance in the absence of the fork reversal activity of Rad5, defined by its HIRAN domain and DNA helicase activity, but not in the absence of Rad5's ubiquitin ligase activity. The Rrm3 and Rad5 helicase activities also interact in the prevention of recombinogenic DNA lesions, and DNA lesions that accumulate in their absence need to be salvaged by a Rad59-dependent recombination pathway. Disruption of the structure-specific endonuclease Mus81 leads to accumulation of recombinogenic DNA lesions and chromosomal rearrangements in the absence of Rrm3, but not Rad5. Thus, at least two mechanisms exist to overcome fork stalling at replication barriers, defined by Rad5-mediated fork reversal and Mus81-mediated cleavage, and contribute to the maintenance of chromosome stability in the absence of Rrm3.

Single-molecule visualization of Pif1 helicase translocation on single-stranded DNA

Pif1 is a broadly conserved helicase that is essential for genome integrity and participates in numerous aspects of DNA metabolism, including telomere length regulation, Okazaki fragment maturation, replication fork progression through difficult-to-replicate sites, replication fork convergence, and break-induced replication. However, details of its translocation properties and the importance of amino acids residues implicated in DNA binding remain unclear. Here, we use total internal reflection fluorescence microscopy with single-molecule DNA curtain assays to directly observe the movement of fluorescently tagged Saccharomyces cerevisiae Pif1 on single-stranded DNA (ssDNA) substrates. We find that Pif1 binds tightly to ssDNA and translocates very rapidly (∼350 nucleotides per second) in the 5'→3' direction over relatively long distances (∼29,500 nucleotides). Surprisingly, we show the ssDNA-binding protein replication protein A inhibits Pif1 activity in both bulk biochemical and single-molecule measurements. However, we demonstrate Pif1 can strip replication protein A from ssDNA, allowing subsequent molecules of Pif1 to translocate unimpeded. We also assess the functional attributes of several Pif1 mutations predicted to impair contact with the ssDNA substrate. Taken together, our findings highlight the functional importance of these amino acid residues in coordinating the movement of Pif1 along ssDNA.

Inhibition of Replication Fork Formation and Progression: Targeting the Replication Initiation and Primosomal Proteins

Over 1.2 million deaths are attributed to multi-drug-resistant (MDR) bacteria each year. Persistence of MDR bacteria is primarily due to the molecular mechanisms that permit fast replication and rapid evolution. As many pathogens continue to build resistance genes, current antibiotic treatments are being rendered useless and the pool of reliable treatments for many MDR-associated diseases is thus shrinking at an alarming rate. In the development of novel antibiotics, DNA replication is still a largely underexplored target. This review summarises critical literature and synthesises our current understanding of DNA replication initiation in bacteria with a particular focus on the utility and applicability of essential initiation proteins as emerging drug targets. A critical evaluation of the specific methods available to examine and screen the most promising replication initiation proteins is provided.

Structural basis of the T4 bacteriophage primosome assembly and primer synthesis

The T4 bacteriophage gp41 helicase and gp61 primase assemble into a primosome complex to couple DNA unwinding with RNA primer synthesis for DNA replication. How a primosome is assembled and how the length of the RNA primer is defined in the T4 bacteriophage, or in any model system, are unclear. Here we report a series of cryo-EM structures of T4 primosome assembly intermediates at resolutions up to 2.7 Å. We show that the gp41 helicase is an open spiral in the absence of ssDNA, and ssDNA binding triggers a large-scale scissor-like conformational change that drives the open spiral to a closed ring that activates the helicase. We found that the activation of the gp41 helicase exposes a cryptic hydrophobic primase-binding surface allowing for the recruitment of the gp61 primase. The primase binds the gp41 helicase in a bipartite mode in which the N-terminal Zn-binding domain (ZBD) and the C-terminal RNA polymerase domain (RPD) each contain a helicase-interacting motif (HIM1 and HIM2, respectively) that bind to separate gp41 N-terminal hairpin dimers, leading to the assembly of one primase on the helicase hexamer. Based on two observed primosome conformations - one in a DNA-scanning mode and the other in a post RNA primer-synthesis mode - we suggest that the linker loop between the gp61 ZBD and RPD contributes to the T4 pentaribonucleotide primer. Our study reveals T4 primosome assembly process and sheds light on RNA primer synthesis mechanism.

Diverse Mechanisms of Helicase Loading during DNA Replication Initiation in Bacteria

Initiation of DNA replication is required for cell viability and passage of genetic information to the next generation. Studies in Escherichia coli and Bacillus subtilis have established ATPases associated with diverse cellular activities (AAA+) as essential proteins required for loading of the replicative helicase at replication origins. AAA+ ATPases DnaC in E. coli and DnaI in B. subtilis have long been considered the paradigm for helicase loading during replication in bacteria. Recently, it has become increasingly clear that most bacteria lack DnaC/DnaI homologs. Instead, most bacteria express a protein homologous to the newly described DciA (dnaC/dnaI antecedent) protein. DciA is not an ATPase, and yet it serves as a helicase operator, providing a function analogous to that of DnaC and DnaI across diverse bacterial species. The recent discovery of DciA and of other alternative mechanisms of helicase loading in bacteria has changed our understanding of DNA replication initiation. In this review, we highlight recent discoveries, detailing what is currently known about the replicative helicase loading process across bacterial species, and we discuss the critical questions that remain to be investigated.

Continuous millisecond conformational cycle of a DEAH box helicase reveals control of domain motions by atomic-scale transitions

Helicases are motor enzymes found in every living organism and viruses, where they maintain the stability of the genome and control against false recombination. The DEAH-box helicase Prp43 plays a crucial role in pre-mRNA splicing in unicellular organisms by translocating single-stranded RNA. The molecular mechanisms and conformational transitions of helicases are not understood at the atomic level. We present a complete conformational cycle of RNA translocation by Prp43 in atomic detail based on molecular dynamics simulations. To enable the sampling of such complex transition on the millisecond timescale, we combined two enhanced sampling techniques, namely simulated tempering and adaptive sampling guided by crystallographic data. During RNA translocation, the center-of-mass motions of the RecA-like domains followed the established inchworm model, whereas the domains crawled along the RNA in a caterpillar-like movement, suggesting an inchworm/caterpillar model. However, this crawling required a complex sequence of atomic-scale transitions involving the release of an arginine finger from the ATP pocket, stepping of the hook-loop and hook-turn motifs along the RNA backbone, and several others. These findings highlight that large-scale domain dynamics may be controlled by complex sequences of atomic-scale transitions.

Extended DNA threading through a dual-engine motor module of the activating signal co-integrator 1 complex

Activating signal co-integrator 1 complex (ASCC) subunit 3 (ASCC3) supports diverse genome maintenance and gene expression processes, and contains tandem Ski2-like NTPase/helicase cassettes crucial for these functions. Presently, the molecular mechanisms underlying ASCC3 helicase activity and regulation remain unresolved. We present cryogenic electron microscopy, DNA-protein cross-linking/mass spectrometry as well as in vitro and cellular functional analyses of the ASCC3-TRIP4 sub-module of ASCC. Unlike the related spliceosomal SNRNP200 RNA helicase, ASCC3 can thread substrates through both helicase cassettes. TRIP4 docks on ASCC3 via a zinc finger domain and stimulates the helicase by positioning an ASC-1 homology domain next to the C-terminal helicase cassette of ASCC3, likely supporting substrate engagement and assisting the DNA exit. TRIP4 binds ASCC3 mutually exclusively with the DNA/RNA dealkylase, ALKBH3, directing ASCC3 for specific processes. Our findings define ASCC3-TRIP4 as a tunable motor module of ASCC that encompasses two cooperating NTPase/helicase units functionally expanded by TRIP4.

A comparative transcriptome analysis of the head of 1 and 9 days old worker honeybees (Apis mellifera)

The role of bees in the environment, economic, biodiversity and pharmaceutical industries is due to its social behavior, which is oriented from the brain and hypopharyngeal gland that is the center of royal jelly (RJ) production. Limited studies have been performed on the head gene expression profile at the RJ production stage. The aim of this study was to compare the gene expressions in 9 and 1-day-old (DO) honeybee workers in order to achieve better understanding about head gene expression pattern. After sequencing of RNAs, transcriptome and their networks were compared. The head expression profile undergoes various changes. 1662 gene transcripts had differential expressions which 1125 and 537 were up and down regulated, respectively, in 9_DO compared with 1_DO honey bees. The day 1th had more significant role in the expression of genes related to RJ production as major RJ protein 1, 2, 3, 5, 6 and 9 encoding genes, but their maximum secretion occurred at day 9th. All process related to hypopharyngeal glands activities as CYP450 gene, fatty acid synthase gene, vitamin B6 metabolism and some of genes involved in fatty acid elongation and degradation process had an upward trend from 1_DO and were age-dependent. By increasing the age, the activity of pathways related to immune system increased for keeping the health of bees against the chemical compound. The expression of aromatic amino acid genes involved in Phenylalanine, tyrosine and tryptophan biosynthesis pathway are essential for early stage of life. In 9_DO honeybees, the energy supplying, reducing stress, protein production and export pathways have a crucial role for support the body development and the social duties. It can be stated that the activity of honeybee head is focused on energy supply instead of storage, while actively trying to improve the level of cell dynamics for increasing the immunity and reducing stress. Results of current study identified key genes of certain behaviors of honeybee workers. Deeper considering of some pathways will be evaluated in future studies.

Cryo-EM structure of the RuvAB-Holliday junction intermediate complex from Pseudomonas aeruginosa

Holliday junction (HJ) is a four-way structured DNA intermediate in homologous recombination. In bacteria, the HJ-specific binding protein RuvA and the motor protein RuvB together form the RuvAB complex to catalyze HJ branch migration. Pseudomonas aeruginosa (P. aeruginosa, Pa) is a ubiquitous opportunistic bacterial pathogen that can cause serious infection in a variety of host species, including vertebrate animals, insects and plants. Here, we describe the cryo-Electron Microscopy (cryo-EM) structure of the RuvAB-HJ intermediate complex from P. aeruginosa. The structure shows that two RuvA tetramers sandwich HJ at the junction center and disrupt base pairs at the branch points of RuvB-free HJ arms. Eight RuvB subunits are recruited by the RuvA octameric core and form two open-rings to encircle two opposite HJ arms. Each RuvB subunit individually binds a RuvA domain III. The four RuvB subunits within the ring display distinct subdomain conformations, and two of them engage the central DNA duplex at both strands with their C-terminal β-hairpins. Together with the biochemical analyses, our structure implicates a potential mechanism of RuvB motor assembly onto HJ DNA.

A broadly distributed predicted helicase/nuclease confers phage resistance via abortive infection

There is strong selection for the evolution of systems that protect bacterial populations from viral attack. We report a single phage defense protein, Hna, that provides protection against diverse phages in Sinorhizobium meliloti, a nitrogen-fixing alpha-proteobacterium. Homologs of Hna are distributed widely across bacterial lineages, and a homologous protein from Escherichia coli also confers phage defense. Hna contains superfamily II helicase motifs at its N terminus and a nuclease motif at its C terminus, with mutagenesis of these motifs inactivating viral defense. Hna variably impacts phage DNA replication but consistently triggers an abortive infection response in which infected cells carrying the system die but do not release phage progeny. A similar host cell response is triggered in cells containing Hna upon expression of a phage-encoded single-stranded DNA binding protein (SSB), independent of phage infection. Thus, we conclude that Hna limits phage spread by initiating abortive infection in response to a phage protein.