DNA replication in all cells begins with the melting of base pairs at the duplex origin to allow access to single-stranded DNA templates which are replicated by DNA polymerases. In bacteria, origin DNA is presumed to be melted by accessory proteins that allow loading of two ring-shaped replicative helicases around single-strand DNA (ssDNA) for bidirectional unwinding and DNA replication. In eukaryotes, by contrast, two replicative CMG (Cdc45-Mcm2-7-GINS) helicases are initially loaded head to head around origin double-strand DNA (dsDNA), and there does not appear to be a separate origin unwinding factor. This led us to investigate whether head-to-head CMGs use their adenosine triphosphate (ATP)-driven motors to initiate duplex DNA unwinding at the origin. Here, we show that CMG tracks on one strand of the duplex while surrounding it, and this feature allows two head-to-head CMGs to unwind dsDNA by using their respective motors to pull on opposite strands of the duplex. We further show that while CMG is capable of limited duplex unwinding on its own, the extent of unwinding is greatly and rapidly stimulated by addition of the multifunctional CMG-binding protein Mcm10 that is critical for productive initiation of DNA replication in vivo. On the basis of these findings, we propose that Mcm10 is a processivity or positioning factor that helps translate the work performed by the dual CMG motors at the origin into productive unwinding that facilitates bidirectional DNA replication.

Duplication of DNA genomes requires unwinding of the double-strand (ds) DNA so that each single strand (ss) can be copied by a DNA polymerase. The genomes of eukaryotic cells are unwound by two ring-shaped hexameric helicases that initially encircle dsDNA but transition to ssDNA for function as replicative helicases. How the duplex is initially unwound, and the role of the two helicases in this process, is poorly understood. We recently described an initiation mechanism for eukaryotes in which the two helicases are directed inward toward one another and shear the duplex open by pulling on opposite strands of the duplex while encircling dsDNA [L. D. Langston, M. E. O'Donnell, eLife 8 , e46515 (2019)]. Two head-to-head T-Antigen helicases are long known to be loaded at the SV40 origin. We show here that T-Antigen tracks head (N-tier) first on ssDNA, opposite the direction proposed for decades. We also find that SV40 T-Antigen tracks directionally while encircling dsDNA and mainly tracks on one strand of the duplex in the same orientation as during ssDNA translocation. Further, two inward directed T-Antigen helicases on dsDNA are able to melt a 150-bp duplex. These findings explain the “rabbit ear” DNA loops observed at the SV40 origin by electron microscopy and reconfigure how the DNA loops emerge from the double hexamer relative to earlier models. Thus, the mechanism of DNA shearing by two opposing helicases is conserved in a eukaryotic viral helicase and may be widely used to initiate origin unwinding of dsDNA genomes.

Twin CMG complexes are assembled head-to-head around duplex DNA at eukaryotic origins of replication. Mcm10 activates CMGs to form helicases that encircle single-strand (ss) DNA and initiate bidirectional forks. How the CMGs melt duplex DNA while encircling it is unknown. Here we show that S. cerevisiae CMG tracks with force while encircling double-stranded (ds) DNA and that in the presence of Mcm10 the CMG melts long blocks of dsDNA while it encircles dsDNA. We demonstrate that CMG tracks mainly on the 3’−5’ strand during duplex translocation, predicting that head-to-head CMGs at an origin exert force on opposite strands. Accordingly, we show that CMGs that encircle double strand DNA in a head-to-head orientation melt the duplex in an Mcm10-dependent reaction.

Our earlier study demonstrated that head-to-head CMGs that encircle duplex DNA and track inward at origins, melt double-strand (ds) DNA while encircling the duplex by pulling on opposite strands and shearing DNA apart (Langston and O’Donnell (2019) eLife 9, e46515). We show here that increasing the methylphosphonate neutral DNA from 10 nucleotides in the previous report, to 20 nucleotides, reveals that CMG encircling duplex DNA only interacts with the tracking strand compared to the non-tracking strand. This significantly enhances support that CMG tracks on duplex DNA by binding only one strand. Furthermore, EMSA assays using AMPPNP to load CMG onto DNA shows a stoichiometry of only 2 CMGs on an origin mimic DNA, containing a 150 bp duplex with two 3 prime single-strand (ss) tails, one on each end, enabling assay of dsDNA unwinding by a shearing force produced by only two head-to-head CMGs. The use of non-hydrolysable AMPPNP enabled a preincubation for CMG binding the two 3 prime tailed origin mimic DNA, and gave robust unwinding of dsDNA by head-to-head CMG-Mcm10’s. With this precedent, it is possible to envision that the cell may utilize opposing dsDNA motors to unwind DNA for other types of DNA transactions besides origin unwinding.

The 11-subunit eukaryotic replicative helicase CMG (Cdc45, Mcm2-7, GINS) tightly binds Mcm10, an essential replication protein in all eukaryotes. Here we show that Mcm10 has a potent strand-annealing activity both alone and in complex with CMG. CMG-Mcm10 unwinds and then reanneals single strands soon after they have been unwound in vitro. Given the DNA damage and replisome instability associated with loss of Mcm10 function, we examined the effect of Mcm10 on fork regression. Fork regression requires the unwinding and pairing of newly synthesized strands, performed by a specialized class of ATP-dependent DNA translocases. We show here that Mcm10 inhibits fork regression by the well-known fork reversal enzyme SMARCAL1. We propose that Mcm10 inhibits the unwinding of nascent strands to prevent fork regression at normal unperturbed replication forks, either by binding the fork junction to form a block to SMARCAL1 or by reannealing unwound nascent strands to their parental template. Analysis of the CMG-Mcm10 complex by cross-linking mass spectrometry reveals Mcm10 interacts with six CMG subunits, with the DNA-binding region of Mcm10 on the N-face of CMG. This position on CMG places Mcm10 at the fork junction, consistent with a role in regulating fork regression.

Replicative helicases are ring-shaped hexamers that encircle DNA for duplex unwinding. The currently accepted view of hexameric helicase function is by steric exclusion, where the helicase encircles one DNA strand and excludes the other, acting as a wedge with an external DNA unwinding point during translocation. Accordingly, strand-specific blocks only affect these helicases when placed on the tracking strand, not the excluded strand. We examined the effect of blocks on the eukaryotic CMG and, contrary to expectations, blocks on either strand inhibit CMG unwinding. A recent cryoEM structure of yeast CMG shows that duplex DNA enters the helicase and unwinding occurs in the central channel. The results of this report inform important aspects of the structure, and we propose that CMG functions by a modified steric exclusion process in which both strands enter the helicase and the duplex unwinding point is internal, followed by exclusion of the non-tracking strand.

Replicative helicases in all cell types are hexameric rings that unwind DNA by steric exclusion in which the helicase encircles the tracking strand only and excludes the other strand from the ring. This mode of translocation allows helicases to bypass blocks on the strand that is excluded from the central channel. Unlike other replicative helicases, eukaryotic CMG helicase partially encircles duplex DNA at a forked junction and is stopped by a block on the non-tracking (lagging) strand. This report demonstrates that Mcm10, an essential replication protein unique to eukaryotes, binds CMG and greatly stimulates its helicase activity in vitro. Most significantly, Mcm10 enables CMG and the replisome to bypass blocks on the non-tracking DNA strand. We demonstrate that bypass occurs without displacement of the blocks and therefore Mcm10 must isomerize the CMG-DNA complex to achieve the bypass function.

Replicative helicases in all domains of life are hexameric rings that encircle DNA while unwinding it, and studies have sought to distinguish between two different modes of action. In the steric exclusion model the helicase encircles and tracks along one strand and excludes the other strand from the inner channel. The ploughshare model posits that the helicase encircles both strands and a separation pin divides the two strands and directs them into distinct channels. Using purified CMG on model DNA substrates, we find evidence that the CMG helicase switches between both modes. In the ploughshare mode it is capable of stripping streptavidin from biotin on either DNA strand, indicating that it encircles both strands and proceeds with substantial force. In the steric exclusion mode it can simply bypass streptavidin on the lagging strand. The helicase appears to utilize the ploughshare mode by default, facilitating removal of obstacles on both strands at the replication fork, but can switch to the steric exclusion mode depending on the severity of the block. These results point to an unexpected degree of flexibility in the mode of helicase translocation that may also extend to other replicative helicases. Support or Funding Information This work was supported by NIH Grant GM38839 and the Howard Hughes Medical Institute

Several important physiological transactions, including control of replicative life span (RLS), prevention of collision between replication and transcription, and cellular differentiation, require programmed replication fork arrest (PFA). However, a general mechanism of PFA has remained elusive. We previously showed that the Tof1-Csm3 fork protection complex is essential for PFA by antagonizing the Rrm3 helicase that displaces nonhistone protein barriers that impede fork progression. Here we show that mutations of Dbf4-dependent kinase (DDK) of Saccharomyces cerevisiae, but not other DNA replication factors, greatly reduced PFA at replication fork barriers in the spacer regions of the ribosomal DNA array. A key target of DDK is the mini chromosome maintenance (Mcm) 2-7 complex, which is known to require phosphorylation by DDK to form an active CMG [Cdc45 (cell division cycle gene 45), Mcm2-7, GINS (Go, Ichi, Ni, and San)] helicase. In vivo experiments showed that mutational inactivation of DDK caused release of Tof1 from the chromatin fractions. In vitro binding experiments confirmed that CMG and/or Mcm2-7 had to be phosphorylated for binding to phospho-Tof1-Csm3 but not to its dephosphorylated form. Suppressor mutations that bypass the requirement for Mcm2-7 phosphorylation by DDK restored PFA in the absence of the kinase. Retention of Tof1 in the chromatin fraction and PFA in vivo was promoted by the suppressor mcm5-bob1, which bypassed DDK requirement, indicating that under this condition a kinase other than DDK catalyzed the phosphorylation of Tof1. We propose that phosphorylation regulates the recruitment and retention of Tof1-Csm3 by the replisome and that this complex antagonizes the Rrm3 helicase, thereby promoting PFA, by preserving the integrity of the Fob1-Ter complex.

We have reconstituted a eukaryotic leading/lagging strand replisome comprising 31 distinct polypeptides. This study identifies a process unprecedented in bacterial replisomes. While bacteria and phage simply recruit polymerases to the fork, we find that suppression mechanisms are used to position the distinct eukaryotic polymerases on their respective strands. Hence, Pol ε is active with CMG on the leading strand, but it is unable to function on the lagging strand, even when Pol δ is not present. Conversely, Pol δ-PCNA is the only enzyme capable of extending Okazaki fragments in the presence of Pols ε and α. We have shown earlier that Pol δ-PCNA is suppressed on the leading strand with CMG (Georgescu et al., 2014). We propose that CMG, the 11-subunit helicase, is responsible for one or both of these suppression mechanisms that spatially control polymerase occupancy at the fork.

Processivity clamps that hold DNA polymerases to DNA for processivity were the first proteins known to encircle the DNA duplex. At the time, polymerase processivity was thought to be the only function of ring shaped processivity clamps. But studies from many laboratories have identified numerous proteins that bind and function with sliding clamps. Among these processes are mismatch repair and nucleosome assembly. Interestingly, there exist polymerases that are highly processive and do not require clamps. Hence, DNA polymerase processivity does not intrinsically require that sliding clamps evolved for this purpose. We propose that polymerases evolved to require clamps as a way of ensuring that clamps are deposited on newly replicated DNA. These clamps are then used on the newly replicated daughter strands, for processes important to genomic integrity, such as mismatch repair and the assembly of nucleosomes to maintain epigenetic states of replicating cells during development.

Significance All cells must replicate their chromosomes prior to cell division. This process is carried out by a collection of proteins, known as the replisome, that act together to unwind the double helix and synthesize two new DNA strands complementary to the two parental strands. The details of replisome function have been worked out for bacteria but are much less well understood for eukaryotic cells. We have developed a system for studying eukaryotic replisome function in vitro using purified proteins. Using this system, we have identified a direct interaction between the component that unwinds the DNA, the CMG (Cdc45-MCM-GINS) helicase, and the component that replicates the leading strand, DNA polymerase ε, to form a large helicase–polymerase holoenzyme comprising 15 separate proteins.

Eukaryotic DNA replication is carried out by two DNA polymerases, Pol ɛ and Pol δ. An in vitro–replication system reconstituted with purified yeast components identifies the factors that selectively recruit each polymerase for leading- or lagging-strand synthesis. Eukaryotes use distinct polymerases for leading- and lagging-strand replication, but how they target their respective strands is uncertain. We reconstituted Saccharomyces cerevisiae replication forks and found that CMG helicase selects polymerase (Pol) ɛ to the exclusion of Pol δ on the leading strand. Even if Pol δ assembles on the leading strand, Pol ɛ rapidly replaces it. Pol δ–PCNA is distributive with CMG, in contrast to its high stability on primed ssDNA. Hence CMG will not stabilize Pol δ, instead leaving the leading strand accessible for Pol ɛ and stabilizing Pol ɛ. Comparison of Pol ɛ and Pol δ on a lagging-strand model DNA reveals the opposite. Pol δ dominates over excess Pol ɛ on PCNA-primed ssDNA. Thus, PCNA strongly favors Pol δ over Pol ɛ on the lagging strand, but CMG over-rides and flips this balance in favor of Pol ɛ on the leading strand.

Seiji Tanaka and Hiroyuki Araki Regulating DNA Replication in Eukarya Khalid Siddiqui, Kin Fan On and John F.X. Diffley Poxvirus DNA Replication Bernard Moss Archaeology of Eukaryotic DNA Replication Kira S. Makarova and Eugene V. Koonin Helicase The Minichromosome Maintenance Replicative Stephen D. Bell and Michael R. Botchan Translesion DNA Polymerases Myron F. Goodman and Roger Woodgate DNA Replication Origins Alan C. Leonard and Marcel Méchali

The accurate copying of genetic information in the double helix of DNA is essential for inheritance of traits that define the phenotype of cells and the organism. The core machineries that copy DNA are conserved in all three domains of life: bacteria, archaea, and eukaryotes. This article outlines the general nature of the DNA replication machinery, but also points out important and key differences. The most complex organisms, eukaryotes, have to coordinate the initiation of DNA replication from many origins in each genome and impose regulation that maintains genomic integrity, not only for the sake of each cell, but for the organism as a whole. In addition, DNA replication in eukaryotes needs to be coordinated with inheritance of chromatin, developmental patterning of tissues, and cell division to ensure that the genome replicates once per cell division cycle.

To preserve genetic information through generations, DNA must be duplicated with precision, and thus DNA polymerases (Pols) that replicate chromosomes are highly accurate. However, DNA is constantly bombarded by damage and undergoing repair, and, if the replication fork encounters a damaged base before repair, highly accurate replicases stall and cannot continue fork progression. All cells contain specialized translesion (TLS) Pols that have low accuracy but can extend DNA over template lesions (1, 2). Although some TLS Pols have evolved to insert the correct nucleotide opposite particular lesions, they can also be mutagenic by inserting an incorrect base. The conservation of TLS Pols in all cells suggests that single-base mutations may offer a better outcome than fork collapse, cell death, or recombination that results in gross chromosome abnormalities. However, there has been longstanding confusion regarding the participation of the high-fidelity eukaryotic replicase Pol δ in the mutagenic TLS process, a dilemma that is largely resolved by data presented in PNAS showing sharing of subunits between Pol δ and TLS Pol ζ (3).

The conversion of chemical energy into mechanical force by AAA+ (ATPases associated with diverse cellular activities) ATPases is integral to cellular processes, including DNA replication, protein unfolding, cargo transport and membrane fusion. The AAA+ ATPase motor cytoplasmic dynein regulates ciliary trafficking, mitotic spindle formation and organelle transport, and dissecting its precise functions has been challenging because of its rapid timescale of action and the lack of cell-permeable, chemical modulators. Here we describe the discovery of ciliobrevins, the first specific small-molecule antagonists of cytoplasmic dynein. Ciliobrevins perturb protein trafficking within the primary cilium, leading to their malformation and Hedgehog signalling blockade. Ciliobrevins also prevent spindle pole focusing, kinetochore-microtubule attachment, melanosome aggregation and peroxisome motility in cultured cells. We further demonstrate the ability of ciliobrevins to block dynein-dependent microtubule gliding and ATPase activity in vitro. Ciliobrevins therefore will be useful reagents for studying cellular processes that require this microtubule motor and may guide the development of additional AAA+ ATPase superfamily inhibitors.

Seiji Tanaka and Hiroyuki Araki Regulating DNA Replication in Eukarya Khalid Siddiqui, Kin Fan On and John F.X. Diffley Poxvirus DNA Replication Bernard Moss Archaeology of Eukaryotic DNA Replication Kira S. Makarova and Eugene V. Koonin Helicase The Minichromosome Maintenance Replicative Stephen D. Bell and Michael R. Botchan Translesion DNA Polymerases Myron F. Goodman and Roger Woodgate DNA Replication Origins Alan C. Leonard and Marcel Méchali

Chromosomal replicases utilize circular sliding clamps for high‐processivity during replication. Sliding clamps not only bind the chromosomal replicase, but they also function with all 5 DNA polymerases in E. coli as well as other proteins for repair and lesion bypass. This presentation focuses on the use of the sliding clamp by a variety of DNA polymerases in crossing barriers that are encountered during replication. We find that the replicase, Pol III, acting in the context of a replisome including helicase, primase and the sliding clamp at a replication fork, can rapidly exchange with other DNA polymerases, such as translesion enzymes, that can allow the replisome to bypass lesions that it encounters during replication. We demonstrate that RecA, the recombinase, dictates the polymerase occupancy at the replication fork. To perform this function, RecA must be in the nucleofilament form, RecA*, which should only be present during DNA damage; it acts in trans. RecA* inhibits the Pol III chromosomal replicase, while greatly promoting translesion polymerases (TLS Pols), Pols II and IV. Interestingly, the polymerase that occupies the replisome dictates the speed of the helicase. We also find that RecA* converts TLS Pol V to a strand displacement enzyme, an activity needed for recombinational strand exchange. Finally, we present single‐molecule studies that examine the function of the third polymerase within the replisome.