The ring-shaped sliding clamp proliferating cell nuclear antigen (PCNA) enables DNA polymerases to perform processive DNA synthesis during replication and repair. The loading of PCNA onto DNA is catalyzed by the ATPase clamp-loader replication factor C (RFC). Using a single-molecule platform to visualize the dynamic interplay between PCNA and RFC on DNA, we unexpectedly discovered that RFC continues to associate with PCNA after loading, contrary to the conventional view. Functionally, this clamp-loader/clamp (CLC) complex is required for processive DNA synthesis by polymerase ẟ (Polẟ), as the PCNA-Polẟ assembly is inherently unstable. This architectural role of RFC is dependent on the BRCA1 C-terminal homology (BRCT) domain of Rfc1, and mutation of its DNA-binding residues causes sensitivity to genotoxic stress in vivo. We further showed that flap endonuclease I (FEN1) can also stabilize the PCNA-Polẟ interaction and mediate robust synthesis. Overall, our work revealed that, beyond their canonical enzymatic functions, PCNA-binding proteins harbor non-catalytic functions important for DNA replication and genome maintenance.

DNA polymerase clamp loaders are AAA + ATPases that load sliding clamps on DNA for high-speed replication. Using a platform for high-throughput mutagenesis of replication proteins in T4 bacteriophage, we carried out saturation mutagenesis of the AAA + ATPase module of the T4 clamp loader bearing a mutation, Gln 118→Asn (Q118N), that reduces fitness. We identified residues for which different mutations improve the fitness of the Q118N variant but are neutral in the wild-type background. These conditionally neutral "rescue hotspots" overlap with those identified earlier in another defective variant (D110C). These rescue hotspots localize to regions where the sequence is not optimal for the structure, as determined by energetic frustration analysis. We designed new sequences for three of these regions, using the protein-design algorithm ProteinMPNN. In two helical regions, several designed sequences increased the fitness of both wild-type and mutant proteins, likely due to enhanced stability. An inter-domain hinge in the AAA + module changes conformation during activation, and designs for the hinge lead to loss of fitness in the wild-type background. However, when using the active conformation as the template, designs for the hinge increase the fitness of defective variants. In contrast, designs templated on the inactive conformation lead to loss of fitness, suggesting that a proper conformational balance is crucial. Thus, adaptive capacity in the clamp loader resides in a network of conditionally neutral sites that enable functional tuning through shifts in stability and conformational equilibria.

DNA sliding clamps are essential for processive DNA synthesis in all domains of life and are loaded by ATP-dependent clamp loaders that recognize recessed 3' ends. How clamp loaders function at nicks and small single-stranded DNA (ssDNA) gaps-common DNA repair intermediates-remains unclear. Here, we show that the bacterial E. coli DnaX clamp loader uses a mechanism distinct from its eukaryotic counterpart. Whereas eukaryotic replication factor C (RFC) unwinds DNA at the recessed 3' end and stabilizes the 5'-dsDNA (double-stranded DNA) at a shoulder site, the bacterial DnaX-complex neither unwinds DNA nor stably binds the 5'-dsDNA in vitro. Instead, cryo-EM structures reveal that the β-clamp contains a conserved external DNA-binding site that bends gapped DNA by ∼150°, promoting insertion of 3'-dsDNA into the clamp. This DNA-bending mechanism enables efficient β-clamp loading at nicks and small gaps and reveals a distinct bacterial strategy likely important for DNA repair.

DNA sliding clamps, including PCNA (proliferating cell nuclear antigen) and the 9-1-1 (RAD9-RAD1-HUS1 in humans) complex, are ring-shaped protein complexes that encircle DNA and serve as central interaction platforms in DNA replication, repair, and checkpoint signaling. While clamp loading at canonical primer-template junctions by AAA+ (ATPases associated with diverse cellular activities) clamp loaders is well established, how clamps are loaded onto physiologically relevant but geometrically constrained DNA intermediates, such as nicks and single-stranded gaps, has remained unclear. Recent cryo-electron microscopy studies reveal that clamp loaders have evolved distinct strategies to overcome these constraints and to specialize for different genomic contexts. At gapped DNA, the eukaryotic clamp loader RFC (replication factor C) engages both 3'- and 5'-recessed DNA ends and can locally unwind DNA, enabling PCNA loading across a wide range of gap sizes. In contrast, the bacterial DnaX clamp loader lacks a 5'-DNA-binding site and does not unwind DNA, instead loading the β-clamp at small gaps (<6 nt) by sharply bending DNA. The checkpoint clamp loader Rad24-RFC (RAD17-RFC in humans) similarly lacks DNA unwinding activity, restricting loading of 9-1-1 clamp to larger gaps (≥6 nt). In a distinct specialization, Ctf18-RFC interacts with the leading-strand DNA polymerase ε, positioning it as a dedicated loader for leading-strand synthesis, whereas Elg1-RFC (ATAD5-RFC in humans) excludes DNA from its chamber and functions as a PCNA unloader. Together, these mechanisms illustrate how clamp loaders are diversified to accommodate DNA structure and replisome context, ensuring coordinated control of genome replication and maintenance.

DNA sliding clamps are used by the replication machines of all organisms, from bacteria to human. These clamps do not self-assemble onto DNA but require a multiprotein ATPase clamp loader that recognizes a DNA 3′-recessed end to open and close sliding clamps onto it. We find here that the mechanism of clamp loading onto small ssDNA gaps is profoundly different between bacteria and eukaryotes. The eukaryotic RFC clamp loader unwinds dsDNA from the 3′-recessed end to widen the ssDNA gap and also binds the 5′-recessed DNA end at an external shoulder site. In contrast, the bacterial DnaX-complex neither unwinds DNA nor stably binds DNA at the shoulder site in vitro. Instead, the bacterial β-clamp has evolved an external DNA binding site that enables the DnaX-complex to sharply bend a gapped DNA by 150° to insert it into the clamp. Small DNA gaps are frequently associated with DNA damage, and the clamps loaded at these sites likely function in DNA repair. ### Competing Interest Statement The authors have declared no competing interest. National Institute of General Medical Sciences, https://ror.org/04q48ey07, GM115809, GM131754 Howard Hughes Medical Institute, https://ror.org/006w34k90 Van Andel Institute, https://ror.org/00wm07d60

Smc5/6 is an essential genome maintenance complex that interacts with double-stranded (ds) DNA, single-stranded (ss) DNA, and ss-dsDNA junctions. DNA association underlies Smc5/6’s functions in managing intermediates generated during genome replication and repair. However, the mechanisms of this activity are not fully understood. Here, we report a single-molecule study examining Smc5/6 association with a dsDNA substrate containing a ssDNA gap with defined 3’ and 5’ junctions. We found that Smc5/6 associates with both 3’ and 5’ junctions but prefers the 3’ junction in the presence of the ssDNA-binding complex RPA. Further, Smc5/6’s junction association frequency and dwell time are regulated by two non-SMC subcomplexes and DNA binding residues of Smc6. Moreover, Smc5/6 prefers binding to junction sites free of the sliding clamp PCNA over those occupied with it. These results suggest that Smc5/6 utilizes its multiple structural modules to associate with junction sites in coordination with other genome maintenance factors. Smc5/6 association with DNA junctions can support genomic functions. Here, the authors show that Smc5/6 junction polarity preferences, targeting, and dwell times are determined by its structural modules as well as the RPA and PCNA genomic factors.

DNA replication fidelity depends on the integrity of the replication fork to prevent DNA damage and preserve genome stability. Disruptions to this process can trigger replication stress, leading to the accumulation of single-strand DNA (ssDNA) and double-strand breaks (DSBs), which drive mutagenesis and ultimately contribute to disease. While the roles of core histones and their post-translational modifications in this context have been more well-studied, far less is known about how linker histones regulate the replication stress response. Here, we demonstrate that the S-phase-phosphorylated form of the linker histone H1 (pH1) plays a key role in DNA damage repair at collapsed replication forks, both in vitro and in cells. Using phosphomimic and phosphonull H1 mutants, we show that phosphorylation enhances H1 assembly with ssDNA. Utilizing intein chemistry for the site-specific incorporation of a photocrosslinker to the C-terminus of H1, we map the direct interactors of H1. We identify phosphorylated H1 at replication forks, where it engages replication machinery and DNA damage response factors, including Histone PARylation Factor 1 (HPF1). We further demonstrate that ssDNA induces pH1-HPF1 interactions that promote liquid-like assemblies, correlating with reduced DNA damage and histone PARylation. Consistent with this role, reduction of total H1 increases cellular sensitivity to DNA damage, a phenotype that is partially rescued by reintroduction of H1.4. Together, these findings establish pH1 as a new regulator of DNA damage repair at collapsed replication forks through the controlled sequestration of repair factors.

Abstract Adenosine triphosphate (ATP) hydrolysis is the main cellular source of energy used to drive biochemical reactions that are otherwise energetically unfavourable. The chemical energy stored in phosphoanhydride bonds is released upon hydrolysis of ATP to ADP and is used to drive mechanical work and conformational change. DNA replication is a canonical process in which the multi-enzyme replisome is thought to rely on ATP hydrolysis for its function. Here we show, through single-molecule visualisation of DNA replication by the Escherichia coli replisome, that the replicative DnaB helicase does not rely on hydrolysis of ATP in the context of the elongating replisome. Even in the presence of physiologically-relevant concentrations of ATP, dTTP is hydrolysed preferably. Finally, we show that the replicative helicases from S. cerevisiae, D. melanogaster, and Homo sapiens can also use dTTP to unwind DNA. Our observations suggest that replicative helicases across domains of life are ‘flex-fuel’ helicases.

The eukaryotic DNA damage and replication stress checkpoint is initiated by activation of the apical kinase complex ATR-ATRIP on RPA-coated ssDNA. In Saccharomyces cerevisiae, the Mec1-Ddc2 (hATR-ATRIP) activator and checkpoint mediator Dpb11 (hTopBP1) is recruited to the 9-1-1 checkpoint clamp (another Mec1-Ddc2 activator) at 5' ss-dsDNA junctions. It remains unclear how Mec1-Ddc2 encounters its activators on damaged DNA due to their differential DNA binding preferences. Using real-time single-molecule imaging, we show that Dpb11 binds to ssDNA directly and localizes to ss-dsDNA junctions in an RPA-dependent manner. Furthermore, Dpb11 recruits Mec1-Ddc2 to ss-dsDNA junctions. Single-molecule force spectroscopy was used to demonstrate that Dpb11 forms bridges on ssDNA, both alone and in the presence of RPA, reducing the end-to-end distance of gapped DNA. These data support a model in which Dpb11 facilitates Mec1-Ddc2 colocalization with its activators directly by recruiting Mec1-Ddc2 to gap junctions and indirectly by decreasing the effective gap length.

The eukaryotic leading-strand DNA polymerase ε (Polε) is a dual-function enzyme with a proofreading 3′-5′ exonuclease ( exo ) site located 40 Å from the DNA synthesizing pol site. Errors in Polε proofreading can cause various mutations, including C-to-G transversions, the most prevalent mutation in cancers and genetic diseases. Polε interacts with all three subunits of the PCNA ring to assemble a functional holoenzyme. Despite previous studies on proofreading of several Pol’s, how Polε—or any Pol complexed with its sliding clamp—proofreads a mismatch generated in situ has been unknown. We show here by cryo-EM that a template/primer DNA substrate with a preexisting mismatch cannot enter the exo site of Polε–PCNA holoenzyme, but a mismatch generated in situ in the pol site yields three bona fide proofreading intermediates of Polε–PCNA holoenzyme. These intermediates reveal how the mismatch is dislodged from the pol site, how the DNA unwinds six base pairs, and how the unpaired primer 3′-end is inserted into the exo site for cleavage. These results unexpectedly demonstrate that PCNA imposes strong steric constraints that extend unwinding and direct the trajectory of mismatched DNA and that this trajectory is dramatically different than for Polε in the absence of PCNA. These findings suggest a physiologically relevant proofreading mechanism for the human Polε holoenzyme.

The ability of proteins involved in eukaryotic DNA replication to overcome obstacles - such as protein and DNA 'roadblocks' - is critical for ensuring faithful genome duplication. G-quadruplexes are higher-order nucleic acid structures that form in guanine-rich regions of DNA and have been shown to act as obstacles, interfering with genomic maintenance pathways. This study introduces a real-time, fluorescence microscopy-based method to observe DNA polymerase interactions with G-quadruplex structures. Short, primed DNA oligonucleotides containing a G-quadruplex were immobilized on functionalized glass coverslips within a microfluidic flow cell. Fluorescently labeled DNA polymerases were introduced, allowing their behavior and stoichiometry to be monitored over time. This approach enabled the observation of polymerase behavior as it was stalled by a G-quadruplex. Specifically, using fluorescently labeled yeast polymerase δ, it was found that upon encountering a G-quadruplex, the polymerase undergoes a continuous cycle of binding and unbinding. This single-molecule assay can be adapted to study interactions between various DNA-maintenance proteins and obstacles on the DNA substrate.

Replication of cellular chromosomes requires a primase to generate short RNA primers to initiate genomic replication. While bacterial and archaeal primase generate short RNA primers, the eukaryotic primase, Polα-primase, contains both RNA primase and DNA polymerase (Pol) subunits that function together to form a >20 base hybrid RNA-DNA primer. Interestingly, the DNA Pol1 subunit of Polα lacks a 3'-5' proofreading exonuclease, contrary to the high-fidelity normally associated with DNA replication. However, Polδ and Polε synthesize the majority of the eukaryotic genome, and both contain 3'-5' exonuclease activity for high fidelity. Nonetheless, even the small amount of DNA produced by Pol1 in each of the many RNA/DNA primers during chromosome replication adds up to tens of millions of nucleotides in a human genome. Thus, it has been a longstanding question why Pol1 lacks a proofreading exonuclease. We show here that Polα is uniquely capable of traversing common oxidized or hydrolyzed template nucleotides and propose that Polα evolved to bypass these common template lesions when they are encountered during chromosome replication. Additionally, we show a unique ability of replication factor C (RFC) to stimulate Polδ lesion bypass, independent of its sliding clamp. This suggests that there may be a coordination between Polδ and RFC that does not involve RFC loading of PCNA.

The eukaryotic DNA damage and replication stress checkpoint is an essential component of the DNA damage response and crucial for genome maintenance. In budding yeast, the apical kinase Mec1 (ATR ortholog), along with binding partner Ddc2 (ATRIP ortholog), senses persistent RPA-bound ssDNA in the cell. Mec1 is activated by interaction with a Mec1-activating protein. One such activator, Dpb11 (TopBP1 ortholog), is recruited to a 5' ss-dsDNA junction via the 9-1-1 checkpoint clamp. Due to their differential DNA binding preferences, it remains to be determined how Mec1 encounters its activators on damaged DNA. Using real-time single-molecule imaging of checkpoint proteins binding to dsDNA containing a long ssDNA gap, we show that, even in the absence of 9-1-1, Dpb11 binds to ssDNA and localizes to ss-dsDNA junctions in an RPA-dependent manner. Importantly, we directly visualize that Dpb11 recruits Mec1-Ddc2 to ss-dsDNA junctions. Additionally, single-molecule force spectroscopy was used to demonstrate that Dpb11 can interact with multiple DNA sites simultaneously to form bridges both alone and in the presence of RPA, stabilizing ssDNA loops and reducing the end-to-end distance of gapped DNA. Taken together, these data support a model in which Dpb11 facilitates Mec1 colocalization with its activators both directly by recruiting Mec1 to gap junctions and indirectly by decreasing the effective gap length.

During DNA replication, the proliferating cell nuclear antigen (PCNA) clamps are loaded onto primed sites for each Okazaki fragment synthesis by the AAA + heteropentamer replication factor C (RFC). PCNA encircling duplex DNA is quite stable and is removed from DNA by the dedicated clamp unloader Elg1-RFC. Here, we show the cryo-EM structure of Elg1-RFC in various states with PCNA. The structures reveal essential features of Elg1-RFC that explain how it is dedicated to PCNA unloading. Specifically, Elg1 contains two external loops that block opening of the Elg1-RFC complex for DNA binding, and an “Elg1 plug” domain that fills the central DNA binding chamber, thereby reinforcing the exclusive PCNA unloading activity of Elg1-RFC. Elg1-RFC was capable of unloading PCNA using non-hydrolyzable AMP-PNP. Both RFC and Elg1-RFC could remove PCNA from covalently closed circular DNA, indicating that PCNA unloading occurs by a mechanism that is distinct from PCNA loading. Implications for the PCNA unloading mechanism are discussed.

The proliferating cell nuclear antigen (PCNA) clamp encircles DNA to hold DNA polymerases (Pols) to DNA for processivity. The Ctf18-RFC PCNA loader, a replication factor C (RFC) variant, is specific to the leading-strand Pol (Polε). We reveal here the underlying mechanism of Ctf18-RFC specificity to Polε using cryo-electron microscopy and biochemical studies. We found that both Ctf18-RFC and Polε contain specific structural features that direct PCNA loading onto DNA. Unlike other clamp loaders, Ctf18-RFC has a disordered ATPase associated with a diverse cellular activities (AAA+) motor that requires Polε to bind and stabilize it for efficient PCNA loading. In addition, Ctf18-RFC can pry prebound Polε off of DNA, then load PCNA onto DNA and transfer the PCNA-DNA back to Polε. These elements in both Ctf18-RFC and Polε provide specificity in loading PCNA onto DNA for Polε.

Cell biology and genetic studies have demonstrated that DNA double strand break (DSB) repair can be performed using an RNA transcript that spans the site of the DNA break as a template for repair. This type of DSB repair requires a reverse transcriptase to convert an RNA sequence into DNA to facilitate repair of the break, rather than copying from a DNA template as in canonical DSB repair. Translesion synthesis (TLS) DNA polymerases (Pol) are often more promiscuous than DNA Pols, raising the notion that reverse transcription could be performed by a TLS Pol. Indeed, several studies have demonstrated that human Pol η has reverse transcriptase activity, while others have suggested that the yeast TLS Pol ζ is involved. Here, we purify all seven known nuclear DNA Pols of Saccharomyces cerevisiae and compare their reverse transcriptase activities. The comparison shows that Pol ζ far surpasses Pol η and all other DNA Pols in reverse transcriptase activity. We find that Pol ζ reverse transcriptase activity is not affected by RPA or RFC/PCNA and acts distributively to make DNA complementary to an RNA template strand. Consistent with prior S. cerevisiae studies performed in vivo, we propose that Pol ζ is the major DNA Pol that functions in the RNA templated DSB repair pathway.

Clamp loaders are AAA+ ATPases that facilitate high-speed DNA replication. In eukaryotic and bacteriophage clamp loaders, ATP hydrolysis requires interactions between aspartate residues in one protomer, present in conserved ‘DEAD-box’ motifs, and arginine residues in adjacent protomers. We show that functional defects resulting from a DEAD-box mutation in the T4 bacteriophage clamp loader can be compensated by widely distributed single mutations in the ATPase domain. Using cryo-EM, we discovered an unsuspected inactive conformation of the clamp loader, in which DNA binding is blocked and the catalytic sites are disassembled. Mutations that restore function map to regions of conformational change upon activation, suggesting that these mutations may increase DNA affinity by altering the energetic balance between inactive and active states. Our results show that there are extensive opportunities for evolution to improve catalytic efficiency when an inactive intermediate is involved. Here, using deep mutagenesis and cryo-EM, the authors unveil an autoinhibited conformation of a clamp loader from T4 bacteriophage, which is characterized by disassembled catalytic sites and blocked DNA binding.

Cell biology and genetic studies have demonstrated that DNA double-strand break (DSB) repair can be performed using an RNA transcript that spans the site of the DNA break as a template for repair. This type of DSB repair requires a reverse transcriptase to convert an RNA sequence into DNA to facilitate repair of the break, rather than copying from a DNA template as in canonical DSB repair. Translesion synthesis (TLS) DNA polymerases (Pol) are often more promiscuous than DNA Pols, raising the notion that reverse transcription could be performed by a TLS Pol. Indeed, several studies have demonstrated that human Pol η has reverse transcriptase activity, while others have suggested that the yeast TLS Pol ζ is involved. Here, we purify all seven known nuclear DNA Pols of Saccharomyces cerevisiae and compare their reverse transcriptase activities. The comparison shows that Pol ζ far surpasses Pol η and all other DNA Pols in reverse transcriptase activity. We find that Pol ζ reverse transcriptase activity is not affected by RPA or RFC/PCNA and acts distributively to make DNA complementary to an RNA template strand. Consistent with prior S. cerevisiae studies performed in vivo, we propose that Pol ζ is the major DNA Pol that functions in the RNA-templated DSB repair pathway.

The DNA sliding clamp PCNA is a multipurpose platform for DNA polymerases and many other proteins involved in DNA metabolism. The topologically closed PCNA ring needs to be cracked open and loaded onto DNA by a clamp loader, e.g., the well-studied pentameric ATPase complex RFC (RFC1-5). The CTF18-RFC complex is an alternative clamp loader found recently to bind the leading strand DNA polymerase ε and load PCNA onto leading strand DNA, but its structure and the loading mechanism have been unknown. By cryo-EM analysis of in vitro assembled human CTF18-RFC-DNA-PCNA complex, we have captured seven loading intermediates, revealing a detailed PCNA loading mechanism onto a 3'-ss/dsDNA junction by CTF18-RFC. Interestingly, the alternative loader has evolved a highly mobile CTF18 AAA+ module likely to lower the loading activity, perhaps to avoid competition with the RFC and to limit its role to leading strand clamp loading. To compensate for the lost stability due to the mobile AAA+ module, CTF18 has evolved a unique β-hairpin motif that reaches across RFC2 to interact with RFC5, thereby stabilizing the pentameric complex. Further, we found that CTF18 also contains a separation pin to locally melt DNA from the 3'-end of the primer; this ensures its ability to load PCNA to any 3'-ss/dsDNA junction, facilitated by the binding energy of the E-plug to the major groove. Our study reveals unique structural features of the human CTF18-RFC and contributes to a broader understanding of PCNA loading by the alternative clamp loaders.