DNA-dependent phase separation by human SSB2 (NABP1/OBFC2A) protein points to adaptations to eukaryotic genome repair processes.

Single-stranded DNA binding proteins (SSBs) are ubiquitous across all domains of life and play essential roles via stabilizing and protecting single-stranded (ss) DNA as well as organizing multiprotein complexes during DNA replication, recombination, and repair. Two mammalian SSB paralogs (hSSB1 and hSSB2 in humans) were recently identified and shown to be involved in various genome maintenance processes. Following our recent discovery of the liquid-liquid phase separation (LLPS) propensity of Escherichia coli (Ec) SSB, here we show that hSSB2 also forms LLPS condensates under physiologically relevant ionic conditions. Similar to that seen for EcSSB, we demonstrate the essential contribution of hSSB2's C-terminal intrinsically disordered region (IDR) to condensate formation, and the selective enrichment of various genome metabolic proteins in hSSB2 condensates. However, in contrast to EcSSB-driven LLPS that is inhibited by ssDNA binding, hSSB2 phase separation requires single-stranded nucleic acid binding, and is especially facilitated by ssDNA. Our results reveal an evolutionarily conserved role for SSB-mediated LLPS in the spatiotemporal organization of genome maintenance complexes. At the same time, differential LLPS features of EcSSB and hSSB2 point to functional adaptations to prokaryotic versus eukaryotic genome metabolic contexts.

Bloom syndrome helicase contributes to germ line development and longevity in zebrafish.

RecQ helicases-also known as the "guardians of the genome"-play crucial roles in genome integrity maintenance through their involvement in various DNA metabolic pathways. Aside from being conserved from bacteria to vertebrates, their importance is also reflected in the fact that in humans impaired function of multiple RecQ helicase orthologs are known to cause severe sets of problems, including Bloom, Werner, or Rothmund-Thomson syndromes. Our aim was to create and characterize a zebrafish (Danio rerio) disease model for Bloom syndrome, a recessive autosomal disorder. In humans, this syndrome is characterized by short stature, skin rashes, reduced fertility, increased risk of carcinogenesis, and shortened life expectancy brought on by genomic instability. We show that zebrafish blm mutants recapitulate major hallmarks of the human disease, such as shortened lifespan and reduced fertility. Moreover, similarly to other factors involved in DNA repair, some functions of zebrafish Blm bear additional importance in germ line development, and consequently in sex differentiation. Unlike fanc genes and rad51, however, blm appears to affect its function independent of tp53. Therefore, our model will be a valuable tool for further understanding the developmental and molecular attributes of this rare disease, along with providing novel insights into the role of genome maintenance proteins in somatic DNA repair and fertility.

The toposiomerase IIIalpha-RMI1-RMI2 complex orients human Bloom's syndrome helicase for efficient disruption of D-loops.

Homologous recombination (HR) is a ubiquitous and efficient process that serves the repair of severe forms of DNA damage and the generation of genetic diversity during meiosis. HR can proceed via multiple pathways with different outcomes that may aid or impair genome stability and faithful inheritance, underscoring the importance of HR quality control. Human Bloom's syndrome (BLM, RecQ family) helicase plays central roles in HR pathway selection and quality control via unexplored molecular mechanisms. Here we show that BLM's multi-domain structural architecture supports a balance between stabilization and disruption of displacement loops (D-loops), early HR intermediates that are key targets for HR regulation. We find that this balance is markedly shifted toward efficient D-loop disruption by the presence of BLM's interaction partners Topoisomerase IIIα-RMI1-RMI2, which have been shown to be involved in multiple steps of HR-based DNA repair. Our results point to a mechanism whereby BLM can differentially process D-loops and support HR control depending on cellular regulatory mechanisms.

Evaluation and modulation of DNA lesion bypass in an SV40 large T antigen-based in vitro replication system.

DNA damage removal by nucleotide excision repair (NER) and replicative bypass via translesion synthesis (TLS) and template switch (TSw) are important in ensuring genome stability. In this study, we tested the applicability of an SV40 large T antigen-based replication system for the simultaneous examination of these damage tolerance processes. Using both Sanger and next-generation sequencing combined with lesion-specific qPCR and replication efficiency studies, we demonstrate that this system works well for studying NER and TLS, especially its one-polymerase branch, while it is less suited to investigations of homology-related repair processes, such as TSw. Cis-syn cyclobutane pyrimidine dimer photoproducts were replicated with equal efficiency to lesion-free plasmids in vitro, and the majority of TLS on this lesion could be inhibited by a peptide (PIR) specific for the polη-PCNA interaction interface. TLS on 6-4 pyrimidine-pyrimidone photoproduct proved to be inefficient and was slightly facilitated by PIR as well as by a recombinant ubiquitin-binding zinc finger domain of polη in HeLa extract, possibly by promoting polymerase exchange. Supplementation of the extract with recombinant PCNA variants indicated the dependence of TLS on PCNA ubiquitylation. In contrast to active TLS and NER, we found no evidence of successful TSw in cellular extracts. The established methods can promote in vitro investigations of replicative DNA damage bypass.

Phase separation by ssDNA binding protein controlled via protein-protein and protein-DNA interactions.

Bacterial single-stranded (ss)DNA-binding proteins (SSB) are essential for the replication and maintenance of the genome. SSBs share a conserved ssDNA-binding domain, a less conserved intrinsically disordered linker (IDL), and a highly conserved C-terminal peptide (CTP) motif that mediates a wide array of protein-protein interactions with DNA-metabolizing proteins. Here we show that the Escherichia coli SSB protein forms liquid-liquid phase-separated condensates in cellular-like conditions through multifaceted interactions involving all structural regions of the protein. SSB, ssDNA, and SSB-interacting molecules are highly concentrated within the condensates, whereas phase separation is overall regulated by the stoichiometry of SSB and ssDNA. Together with recent results on subcellular SSB localization patterns, our results point to a conserved mechanism by which bacterial cells store a pool of SSB and SSB-interacting proteins. Dynamic phase separation enables rapid mobilization of this protein pool to protect exposed ssDNA and repair genomic loci affected by DNA damage.

Mgs1 protein supports genome stability via recognition of G-quadruplex DNA structures.

The integrity of the genetic material is crucial for every organism. One intrinsic attack to genome stability is stalling of the replication fork which can result in DNA breakage. Several factors, such as DNA lesions or the formation of stable secondary structures (eg, G-quadruplexes) can lead to replication fork stalling. G-quadruplexes (G4s) are well-characterized stable secondary DNA structures that can form within specific single-stranded DNA sequence motifs and have been shown to block/pause the replication machinery. In most genomes several helicases have been described to regulate G4 unfolding to preserve genome integrity, however, different experiments raise the hypothesis that processing of G4s during DNA replication is more complex and requires additional, so far unknown, proteins. Here, we show that the Saccharomyces cerevisiae Mgs1 protein robustly binds to G4 structures in vitro and preferentially acts at regions with a strong potential to form G4 structures in vivo. Our results suggest that Mgs1 binds to G4-forming sites and has a role in the maintenance of genome integrity.

Homology sensing via non-linear amplification of sequence-dependent pausing by RecQ helicase.

RecQ helicases promote genomic stability through their unique ability to suppress illegitimate recombination and resolve recombination intermediates. These DNA structure-specific activities of RecQ helicases are mediated by the helicase-and-RNAseD like C-terminal (HRDC) domain, via unknown mechanisms. Here, employing single-molecule magnetic tweezers and rapid kinetic approaches we establish that the HRDC domain stabilizes intrinsic, sequence-dependent, pauses of the core helicase (lacking the HRDC) in a DNA geometry-dependent manner. We elucidate the core unwinding mechanism in which the unwinding rate depends on the stability of the duplex DNA leading to transient sequence-dependent pauses. We further demonstrate a non-linear amplification of these transient pauses by the controlled binding of the HRDC domain. The resulting DNA sequence- and geometry-dependent pausing may underlie a homology sensing mechanism that allows rapid disruption of unstable (illegitimate) and stabilization of stable (legitimate) DNA strand invasions, which suggests an intrinsic mechanism of recombination quality control by RecQ helicases.

Functional fine-tuning between bacterial DNA recombination initiation and quality control systems.

Homologous recombination (HR) is crucial for the error-free repair of DNA double-strand breaks (DSBs) and the restart of stalled replication. However, imprecise HR can lead to genome instability, highlighting the importance of HR quality control. After DSB formation, HR proceeds via DNA end resection and recombinase loading, whereas helicase-catalyzed disruption of a subset of subsequently formed DNA invasions is thought to be essential for maintaining HR accuracy via inhibiting illegitimate (non-allelic) recombination. Here we show that in vitro characterized mechanistic aberrations of E. coli RecBCD (resection and recombinase loading) RecQ (multifunctional DNA-restructuring helicase) mutant enzyme variants, on one hand, cumulatively deteriorate cell survival under certain conditions of genomic stress. On the other hand, we find that RecBCD and RecQ defects functionally compensate each other in terms of HR accuracy. The abnormally long resection and unproductive recombinase loading activities of a mutant RecBCD complex (harboring the D1080A substitution in RecB) cause enhanced illegitimate recombination. However, this compromised HR-accuracy phenotype is suppressed in double mutant strains harboring mutant RecQ variants with abnormally enhanced helicase and inefficient invasion disruptase activities. These results frame an in vivo context for the interplay of biochemical activities leading to illegitimate recombination, and underscore its long-range genome instability effects manifest in higher eukaryotes.

Human RAD51 rapidly forms intrinsically dynamic nucleoprotein filaments modulated by nucleotide binding state.

Formation of RAD51 filaments on single-stranded DNA is an essential event during homologous recombination, which is required for homology search, strand exchange and protection of replication forks. Formation of nucleoprotein filaments (NF) is required for development and genomic stability, and its failure is associated with developmental abnormalities and tumorigenesis. Here we describe the structure of the human RAD51 NFs and of its Walker box mutants using electron microscopy. Wild-type RAD51 filaments adopt an 'open' conformation when compared to a 'closed' structure formed by mutants, reflecting alterations in helical pitch. The kinetics of formation/disassembly of RAD51 filaments show rapid and high ssDNA coverage via low cooperativity binding of RAD51 units along the DNA. Subsequently, a series of isomerization or dissociation events mediated by nucleotide binding state creates intrinsically dynamic RAD51 NFs. Our findings highlight important a mechanistic divergence among recombinases from different organisms, in line with the diversity of biological mechanisms of HR initiation and quality control. These data reveal unexpected intrinsic dynamic properties of the RAD51 filament during assembly/disassembly, which may be important for the proper control of homologous recombination.

RecQ helicase triggers a binding mode change in the SSB-DNA complex to efficiently initiate DNA unwinding.

The single-stranded DNA binding protein (SSB) of Escherichia coli plays essential roles in maintaining genome integrity by sequestering ssDNA and mediating DNA processing pathways through interactions with DNA-processing enzymes. Despite its DNA-sequestering properties, SSB stimulates the DNA processing activities of some of its binding partners. One example is the genome maintenance protein RecQ helicase. Here, we determine the mechanistic details of the RecQ-SSB interaction using single-molecule magnetic tweezers and rapid kinetic experiments. Our results reveal that the SSB-RecQ interaction changes the binding mode of SSB, thereby allowing RecQ to gain access to ssDNA and facilitating DNA unwinding. Conversely, the interaction of RecQ with the SSB C-terminal tail increases the on-rate of RecQ-DNA binding and has a modest stimulatory effect on the unwinding rate of RecQ. We propose that this bidirectional communication promotes efficient DNA processing and explains how SSB stimulates rather than inhibits RecQ activity.

Shuttling along DNA and directed processing of D-loops by RecQ helicase support quality control of homologous recombination.

Cells must continuously repair inevitable DNA damage while avoiding the deleterious consequences of imprecise repair. Distinction between legitimate and illegitimate repair processes is thought to be achieved in part through differential recognition and processing of specific noncanonical DNA structures, although the mechanistic basis of discrimination remains poorly defined. Here, we show that Escherichia coli RecQ, a central DNA recombination and repair enzyme, exhibits differential processing of DNA substrates based on their geometry and structure. Through single-molecule and ensemble biophysical experiments, we elucidate how the conserved domain architecture of RecQ supports geometry-dependent shuttling and directed processing of recombination-intermediate [displacement loop (D-loop)] substrates. Our study shows that these activities together suppress illegitimate recombination in vivo, whereas unregulated duplex unwinding is detrimental for recombination precision. Based on these results, we propose a mechanism through which RecQ helicases achieve recombination precision and efficiency.

Recent adaptations of fluorescence techniques for the determination of mechanistic parameters of helicases and translocases.

Helicases and translocases are nucleic acid (NA)-based molecular motors that use the free energy liberated during the nucleoside triphosphate (NTP, usually ATP) hydrolysis cycle for unidirectional translocation along their NA (DNA, RNA or heteroduplex) substrates. Determination of the kinetic and thermodynamic parameters of their mechanoenzymatic cycle serves as a basis for the exploration of their physiological behavior and various cellular functions. Here we describe how recent adaptations of fluorescence-based solution kinetic methods can be used to determine practically all important mechanistic parameters of NA-based motor proteins. We outline practically useful analysis procedures for equilibrium, steady-state and transient kinetic data. This analysis can be used to quantitatively characterize the enzymatic steps of the NTP hydrolytic cycle, the binding site size, stoichiometry and energetics of protein-NA interactions, the rate and processivity of translocation along and unwinding of NA strands, and the mechanochemical coupling between these processes. The described methods yield insights into the functional role of the enzymes, and also greatly aid the design and interpretation of single-molecule experiments as well as the engineering of enzymatic properties for biotechnological applications.

The HRDC domain of E. coli RecQ helicase controls single-stranded DNA translocation and double-stranded DNA unwinding rates without affecting mechanoenzymatic coupling.

DNA-restructuring activities of RecQ-family helicases play key roles in genome maintenance. These activities, driven by two tandem RecA-like core domains, are thought to be controlled by accessory DNA-binding elements including the helicase-and-RnaseD-C-terminal (HRDC) domain. The HRDC domain of human Bloom's syndrome (BLM) helicase was shown to interact with the RecA core, raising the possibility that it may affect the coupling between ATP hydrolysis, translocation along single-stranded (ss)DNA and/or unwinding of double-stranded (ds)DNA. Here, we determined how these activities are affected by the abolition of the ssDNA interaction of the HRDC domain or the deletion of the entire domain in E. coli RecQ helicase. Our data show that the HRDC domain suppresses the rate of DNA-activated ATPase activity in parallel with those of ssDNA translocation and dsDNA unwinding, regardless of the ssDNA binding capability of this domain. The HRDC domain does not affect either the processivity of ssDNA translocation or the tight coupling between the ATPase, translocation, and unwinding activities. Thus, the mechanochemical coupling of E. coli RecQ appears to be independent of HRDC-ssDNA and HRDC-RecA core interactions, which may play roles in more specialized functions of the enzyme.

A nucleotide-dependent and HRDC domain-dependent structural transition in DNA-bound RecQ helicase.

The allosteric communication between the ATP- and DNA-binding sites of RecQ helicases enables efficient coupling of ATP hydrolysis to translocation along single-stranded DNA (ssDNA) and, in turn, the restructuring of multistranded DNA substrates during genome maintenance processes. In this study, we used the tryptophan fluorescence signal of Escherichia coli RecQ helicase to decipher the kinetic mechanism of the interaction of the enzyme with ssDNA. Rapid kinetic experiments revealed that ssDNA binding occurs in a two-step mechanism in which the initial binding step is followed by a structural transition of the DNA-bound helicase. We found that the nucleotide state of RecQ greatly influences the kinetics of the detected structural transition, which leads to a high affinity DNA-clamped state in the presence of the nucleotide analog ADP-AlF4. The DNA binding mechanism is largely independent of ssDNA length, indicating the independent binding of RecQ molecules to ssDNA and the lack of significant DNA end effects. The structural transition of DNA-bound RecQ was not detected when the ssDNA binding capability of the helicase-RNase D C-terminal domain was abolished or the domain was deleted. The results shed light on the nature of conformational changes leading to processive ssDNA translocation and multistranded DNA processing by RecQ helicases.

From keys to bulldozers: expanding roles for winged helix domains in nucleic-acid-binding proteins.

The winged helix domain (WHD) is a widespread nucleic-acid-binding protein structural element found in all kingdoms of life. Although the overall structure of the WHD is conserved, its functional properties and interaction profiles are extremely versatile. WHD-containing proteins can exploit nearly the full spectrum of nucleic acid structural features for recognition and even covalent modification or noncovalent rearrangement of target molecules. WHD functions range from sequence-recognizing keys in transcription factors and bulldozer-like strand-separating wedges in helicases to mediators of protein-protein interactions (PPIs). Further investigations are needed to understand the contribution of WHD structural dynamics to nucleic-acid-modifying enzymatic functions.

A subdomain interaction at the base of the lever allosterically tunes the mechanochemical mechanism of myosin 5a.

The motor domain of myosin is the core element performing mechanochemical energy transduction. This domain contains the actin and ATP binding sites and the base of the force-transducing lever. Coordinated subdomain movements within the motor are essential in linking the ATPase chemical cycle to translocation along actin filaments. A dynamic subdomain interface located at the base of the lever was previously shown to exert an allosteric influence on ATP hydrolysis in the non-processive myosin 2 motor. By solution kinetic, spectroscopic and ensemble and single-molecule motility experiments, we determined the role of a class-specific adaptation of this interface in the mechanochemical mechanism of myosin 5a, a processive intracellular transporter. We found that the introduction of a myosin 2-specific repulsive interaction into myosin 5a via the I67K mutation perturbs the strong-binding interaction of myosin 5a with actin, influences the mechanism of ATP binding and facilitates ATP hydrolysis. At the same time, the mutation abolishes the actin-induced activation of ADP release and, in turn, slows down processive motility, especially when myosin experiences mechanical drag exerted by the action of multiple motor molecules bound to the same actin filament. The results highlight that subtle structural adaptations of the common structural scaffold of the myosin motor enable specific allosteric tuning of motor activity shaped by widely differing physiological demands.

Complex activities of the human Bloom's syndrome helicase are encoded in a core region comprising the RecA and Zn-binding domains.

Bloom's syndrome DNA helicase (BLM), a member of the RecQ family, is a key player in homologous recombination (HR)-based error-free DNA repair processes. During HR, BLM exerts various biochemical activities including single-stranded (ss) DNA translocation, separation and annealing of complementary DNA strands, disruption of complex DNA structures (e.g. displacement loops) and contributes to quality control of HR via clearance of Rad51 nucleoprotein filaments. We performed a quantitative mechanistic analysis of truncated BLM constructs that are shorter than the previously identified minimal functional module. Surprisingly, we found that a BLM construct comprising only the two conserved RecA domains and the Zn(2+)-binding domain (residues 642-1077) can efficiently perform all mentioned HR-related activities. The results demonstrate that the Zn(2+)-binding domain is necessary for functional interaction with DNA. We show that the extensions of this core, including the winged-helix domain and the strand separation hairpin identified therein in other RecQ-family helicases, are not required for mechanochemical activity per se and may instead play modulatory roles and mediate protein-protein interactions.