A second proliferating cell nuclear antigen loader complex, Ctf18-replication factor C, stimulates DNA polymerase eta activity

Replication factor C (RFC) loads the clamp protein PCNA onto DNA structures. Ctf18-RFC, which consists of the chromosome cohesion factors Ctf18, Dcc1, and Ctf8 and four small RFC subunits, functions as a second proliferating cell nuclear antigen (PCNA) loader. To identify potential targets of Ctf18-RFC, human cell extracts were assayed for DNA polymerase activity specifically stimulated by Ctf18-RFC in conjunction with PCNA. After several chromatography steps, an activity stimulated by Ctf18-RFC but not by RFC was identified. Liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis revealed the presence of two DNA polymerases, eta and lambda, in the most purified fraction, but experiments with purified recombinant proteins demonstrated that only polymerase (pol) eta was responsible for activity. Ctf18-RFC alone stimulated pol eta, and the addition of PCNA cooperatively increased stimulation. Furthermore, Ctf18-RFC interacted physically with pol eta, as indicated by co-precipitation in human cells. We propose that this novel loader-DNA polymerase interaction allows DNA replication forks to overcome interference by various template structures, including damaged DNA and DNA-protein complexes that maintain chromosome cohesion.     

Identification of RFC(Ctf18p, Ctf8p, Dcc1p): an alternative RFC complex required for sister chromatid cohesion in S. cerevisiae

We have identified and characterized an alternative RFC complex RFC(Ctf18p, Ctf8p, Dcc1p) that is required for sister chromatid cohesion and faithful chromosome transmission. Ctf18p, Ctf8p, and Dcc1p interact physically in a complex with Rfc2p, Rfc3p, Rfc4p, and Rfc5p but not with Rfc1p or Rad24p. Deletion of CTF18, CTF8, or DCC1 singly or in combination (ctf18Deltactf8Deltadcc1Delta) leads to sensitivity to microtubule depolymerizing drugs and a severe sister chromatid cohesion defect. Furthermore, temperature-sensitive mutations in RFC4 result in precocious sister chromatid separation. Our results highlight a novel function of the RFC proteins and support a model in which sister chromatid cohesion is established at the replication fork via a polymerase switching mechanism and a replication-coupled remodeling of chromatin.     

Structural studies of RFCCtf18 reveal a novel chromatin recruitment role for Dcc1

Replication factor C complexes load and unload processivity clamps from DNA and are involved in multiple DNA replication and repair pathways. The RFC^Ctf18 variant complex is required for activation of the intra‐S‐phase checkpoint at stalled replication forks and aids the establishment of sister chromatid cohesion. Unlike other RFC complexes, RFC^Ctf18 contains two non‐Rfc subunits, Dcc1 and Ctf8. Here, we present the crystal structure of the Dcc1‐Ctf8 heterodimer bound to the C‐terminus of Ctf18. We find that the C‐terminus of Dcc1 contains three‐winged helix domains, which bind to both ssDNA and dsDNA. We further show that these domains are required for full recruitment of the complex to chromatin, and correct activation of the replication checkpoint. These findings provide the first structural data on a eukaryotic seven‐subunit clamp loader and define a new biochemical activity for Dcc1.     

Physical links between the nuclear envelope protein Mps3, three alternate replication factor C complexes, and a variant histone in Saccharomyces cerevisiae

Viability of cell progeny upon cell division require that genomes are replicated, repaired, and maintained with high fidelity. Central to both DNA replication and repair are Replication Factor C (RFC) complexes which catalyze the unloading/loading of sliding clamps such as PCNA or 9-1-1 complexes on DNA. Budding yeast contain four alternate RFC complexes which play partially redundant roles. Rfc1, Ctf18, Rad24, and Elg1 are all large subunits that bind, in a mutually exclusive fashion to RFC 2-5 small subunits. Ctf18, Rad24, and Elg1 are of particular interest because, in addition to their roles in maintaining genome integrity, all three play critical roles in sister chromatid tethering reactions that appear coupled to their roles in DNA replication/repair. Intriguingly, the nuclear envelope protein Mps3 similarly exhibits roles in repair and cohesion, leading us to hypothesize that Mps3 and RFCs function through a singular mechanism. Here we report that the nuclear envelope protein Mps3 physically associates with all three of these large RFC complex subunits (Ctf18, Elg1, and Rad24). In addition we report a physical interaction between Mps3 and the histone variant Htz1, a factor previously shown to promote DNA repair. In combination, these findings reveal a direct link between the nuclear envelope and chromatin and provide support for a model that telomeres and chromatin interact with the nuclear envelope during both DNA repair and sister chromatid pairing reactions.     

A conserved Pol? binding module in Ctf18-RFC is required for S-phase checkpoint activation downstream of Mec1

Defects during chromosome replication in eukaryotes activate a signaling pathway called the S-phase checkpoint, which produces a multifaceted response that preserves genome integrity at stalled DNA replication forks. Work with budding yeast showed that the ‘alternative clamp loader’ known as Ctf18-RFC acts by an unknown mechanism to activate the checkpoint kinase Rad53, which then mediates much of the checkpoint response. Here we show that budding yeast Ctf18-RFC associates with DNA polymerase epsilon, via an evolutionarily conserved ‘Pol ϵ binding module’ in Ctf18-RFC that is produced by interaction of the carboxyl terminus of Ctf18 with the Ctf8 and Dcc1 subunits. Mutations at the end of Ctf18 disrupt the integrity of the Pol ϵ binding module and block the S-phase checkpoint pathway, downstream of the Mec1 kinase that is the budding yeast orthologue of mammalian ATR. Similar defects in checkpoint activation are produced by mutations that displace Pol ϵ from the replisome. These findings indicate that the association of Ctf18-RFC with Pol ϵ at defective replication forks is a key step in activation of the S-phase checkpoint.     

Mechanical link between cohesion establishment and DNA replication: Ctf7p/Eco1p,a cohesion establishment factor,associates with three different replication factor C complexes

CTF7/ECO1 is an essential yeast gene required for the establishment of sister chromatid cohesion. The findings that CTF7/ECO1, POL30 (PCNA), and CHL12/CTF18 (a replication factor C [RFC] homolog) genetically interact provided the first evidence that the processes of cohesion establishment and DNA replication are intimately coupled-a link now confirmed by other studies. To date, however, it is unknown how Ctf7p/Eco1p function is coupled to DNA replication or whether Ctf7p/Eco1p physically associates with any components of the DNA replication machinery. Here, we report that Ctf7p/Eco1p associates with proteins that perform partially redundant functions in DNA replication. Chl12p/Ctf18p combines with Rfc2p to Rfc5p to form one of three independent RFC complexes. By chromatographic methods, Ctf7p/Eco1p was found to associate with Chl12/Ctf18p and with Rfc2p, Rfc3p, Rfc4p, and Rfc5p. The association between Ctf7p/Eco1p and this RFC complex is biologically relevant in that (i) Ctf7p/Eco1p cosediments with Chl12p/Ctf18p in vivo and (ii) rfc5-1 mutant cells exhibit precocious sister separation. Previous studies revealed that Rfc1p or Rad24p associates with Rfc2p to Rfc5p to form two other RFC complexes independent of Ctf18p-RFC complexes. These Rfc1p-RFC and Rad24p-RFC complexes function in DNA replication or repair and DNA damage checkpoint pathways. Importantly, Ctf7p/Eco1p also associates with Rfc1p and Rad24p, suggesting that these RFC complexes also play critical roles in cohesion establishment. The associations between Ctf7p/Eco1p and RFC subunits provide novel evidence regarding the physical linkage between cohesion establishment and DNA replication. Furthermore, the association of Ctf7p/Eco1p with each of three RFC complexes supplies new insights into the functional redundancy of RFC complexes in cohesion establishment.     

Two different replication factor C proteins, Ctf18 and RFC1, separately control PCNA-CRL4Cdt2-mediated Cdt1 proteolysis during S phase and following UV irradiation

Recent work identified the E3 ubiquitin ligase CRL4(Cdt2) as mediating the timely degradation of Cdt1 during DNA replication and following DNA damage. In both cases, proliferating cell nuclear antigen (PCNA) loaded on chromatin mediates the CRL4(Cdt2)-dependent proteolysis of Cdt1. Here, we demonstrate that while replication factor C subunit 1 (RFC1)-RFC is required for Cdt1 degradation after UV irradiation during the nucleotide excision repair process, another RFC complex, Ctf18-RFC, which is known to be involved in the establishment of cohesion, has a key role in Cdt1 degradation in S phase. Cdt1 segments having only the degron, a specific sequence element in target protein for ubiquitination, for CRL4(Cdt2) were stabilized during S phase in Ctf18-depleted cells. Additionally, endogenous Cdt1 was stabilized when both Skp2 and Ctf18 were depleted. Since a substantial amount of PCNA was detected on chromatin in Ctf18-depleted cells, Ctf18 is required in addition to loaded PCNA for Cdt1 degradation in S phase. Our data suggest that Ctf18 is involved in recruiting CRL4(Cdt2) to PCNA foci during S phase. Ctf18-mediated Cdt1 proteolysis occurs independent of cohesion establishment, and depletion of Ctf18 potentiates rereplication. Our findings indicate that individual RFC complexes differentially control CRL4(Cdt2)-dependent proteolysis of Cdt1 during DNA replication and repair.     

RFCCtf18 and the Swi1-Swi3 complex function in separate and redundant pathways required for the stabilization of replication forks to facilitate sister chromatid cohesion in Schizosaccharomyces pombe

Sister chromatid cohesion is established during S phase near the replication fork. However, how DNA replication is coordinated with chromosomal cohesion pathway is largely unknown. Here, we report studies of fission yeast Ctf18, a subunit of the RFC(Ctf18) replication factor C complex, and Chl1, a putative DNA helicase. We show that RFC(Ctf18) is essential in the absence of the Swi1-Swi3 replication fork protection complex required for the S phase stress response. Loss of Ctf18 leads to an increased sensitivity to S phase stressing agents, a decreased level of Cds1 kinase activity, and accumulation of DNA damage during S phase. Ctf18 associates with chromatin during S phase, and it is required for the proper resumption of replication after fork arrest. We also show that chl1Delta is synthetically lethal with ctf18Delta and that a dosage increase of chl1(+) rescues sensitivities of swi1Delta to S phase stressing agents, indicating that Chl1 is involved in the S phase stress response. Finally, we demonstrate that inactivation of Ctf18, Chl1, or Swi1-Swi3 leads to defective centromere cohesion, suggesting the role of these proteins in chromosome segregation. We propose that RFC(Ctf18) and the Swi1-Swi3 complex function in separate and redundant pathways essential for replication fork stabilization to facilitate sister chromatid cohesion in fission yeast.     

New functions of Ctf18-RFC in preserving genome stability outside its role in sister chromatid cohesion

Expansion of DNA trinucleotide repeats causes at least 15 hereditary neurological diseases, and these repeats also undergo contraction and fragility. Current models to explain this genetic instability invoke erroneous DNA repair or aberrant replication. Here we show that CAG/CTG tracts are stabilized in Saccharomyces cerevisiae by the alternative clamp loader/unloader Ctf18-Dcc1-Ctf8-RFC complex (Ctf18-RFC). Mutants in Ctf18-RFC increased all three forms of triplet repeat instability--expansions, contractions, and fragility--with effect over a wide range of allele lengths from 20-155 repeats. Ctf18-RFC predominated among the three alternative clamp loaders, with mutants in Elg1-RFC or Rad24-RFC having less effect on trinucleotide repeats. Surprisingly, chl1, scc1-73, or scc2-4 mutants defective in sister chromatid cohesion (SCC) did not increase instability, suggesting that Ctf18-RFC protects triplet repeats independently of SCC. Instead, three results suggest novel roles for Ctf18-RFC in facilitating genomic stability. First, genetic instability in mutants of Ctf18-RFC was exacerbated by simultaneous deletion of the fork stabilizer Mrc1, but suppressed by deletion of the repair protein Rad52. Second, single-cell analysis showed that mutants in Ctf18-RFC had a slowed S phase and a striking G2/M accumulation, often with an abnormal multi-budded morphology. Third, ctf18 cells exhibit increased Rad52 foci in S phase, often persisting into G2, indicative of high levels of DNA damage. The presence of a repeat tract greatly magnified the ctf18 phenotypes. Together these results indicate that Ctf18-RFC has additional important functions in preserving genome stability, besides its role in SCC, which we propose include lesion bypass by replication forks and post-replication repair.     

Replication factor C complexes play unique pro- and anti-establishment roles in sister chromatid cohesion

Recent studies have lead to a rapid expansion of sister chromatid cohesion pathways. Of particular interest is the growth in classifications of anti-establishment factors-now including those that are cohesin-associated (Rad61/WAPL and Pds5) or DNA replication fork-associated (Elg1-RFC). In this study, we show that the two classes of anti-establishment complexes are indistinguishable when challenged both genetically and functionally. These findings suggest that both classes function in a singular pathway that is centered on Ctf7/Eco1 (herein termed Ctf7) regulation. The anti-establishment activity of Elg1-RFC complex is particular intriguing given that an alternate Ctf18-RFC complex exhibits robust pro-establishment activity. Here, we provide several lines of evidence, including the use of Ctf7 bypass suppressors, indicating that these activities are not simply antagonistic. Moreover, the results suggest that Ctf18-RFC is capable of promoting sister chromatid pairing reactions independent of Ctf7. The combination of these studies suggest a new model of sister chromatid pairing regulation.     

Studies with the human cohesin establishment factor, ChlR1. Association of ChlR1 with Ctf18-RFC and Fen1

Human ChlR1 (hChlR1), a member of the DEAD/DEAH subfamily of helicases, was shown to interact with components of the cohesin complex and play a role in sister chromatid cohesion. In order to study the biochemical and biological properties of hChlR1, we purified the protein from 293 cells and demonstrated that hChlR1 possesses DNA-dependent ATPase and helicase activities. This helicase translocates on single-stranded DNA in the 5' to 3' direction in the presence of ATP and, to a lesser extent, dATP. Its unwinding activity requires a 5'-singlestranded region for helicase loading, since flush-ended duplex structures do not support unwinding. The helicase activity of hChlR1 is capable of displacing duplex regions up to 100 bp, which can be extended to 500 bp by RPA or the cohesion establishment factor, the Ctf18-RFC (replication factor C) complex. We show that hChlR1 interacts with the hCtf18-RFC complex, human proliferating cell nuclear antigen, and hFen1. The interactions between Fen1 and hChlR1 stimulate the flap endonuclease activity of Fen1. Selective depletion of either hChlR1 or Fen1 by targeted small interfering RNA treatment results in the precocious separation of sister chromatids. These findings are consistent with a role of hChlR1 in the establishment of sister chromatid cohesion and suggest that its action may contribute to lagging strand processing events important in cohesion.     

Saccharomyces cerevisiae CTF18 and CTF4 are required for sister chromatid cohesion

CTF4 and CTF18 are required for high-fidelity chromosome segregation. Both exhibit genetic and physical ties to replication fork constituents. We find that absence of either CTF4 or CTF18 causes sister chromatid cohesion failure and leads to a preanaphase accumulation of cells that depends on the spindle assembly checkpoint. The physical and genetic interactions between CTF4, CTF18, and core components of replication fork complexes observed in this study and others suggest that both gene products act in association with the replication fork to facilitate sister chromatid cohesion. We find that Ctf18p, an RFC1-like protein, directly interacts with Rfc2p, Rfc3p, Rfc4p, and Rfc5p. However, Ctf18p is not a component of biochemically purified proliferating cell nuclear antigen loading RF-C, suggesting the presence of a discrete complex containing Ctf18p, Rfc2p, Rfc3p, Rfc4p, and Rfc5p. Recent identification and characterization of the budding yeast polymerase kappa, encoded by TRF4, strongly supports a hypothesis that the DNA replication machinery is required for proper sister chromatid cohesion. Analogous to the polymerase switching role of the bacterial and human RF-C complexes, we propose that budding yeast RF-C(CTF18) may be involved in a polymerase switch event that facilities sister chromatid cohesion. The requirement for CTF4 and CTF18 in robust cohesion identifies novel roles for replication accessory proteins in this process.     

ATP utilization by yeast replication factor C. I. ATP-mediated interaction with DNA and with proliferating cell nuclear antigen.

Eukaryotic replication factor C is the heteropentameric complex that loads the replication clamp proliferating cell nuclear antigen (PCNA) onto primed DNA. In this study we used a derivative, designated RFC, with a N-terminal truncation of the Rfc1 subunit removing a DNA-binding domain not required for clamp loading. Interactions of yeast RFC with PCNA and DNA were studied by surface plasmon resonance. Binding of RFC to PCNA was stimulated by either adenosine (3-thiotriphosphate) (ATPgammaS) or ATP. RFC bound only to primer-template DNA coated with the single-stranded DNA-binding protein RPA if ATPgammaS was also present. Binding occurred without dissociation of RPA. ATP did not stimulate binding of RFC to DNA, suggesting that hydrolysis of ATP dissociated DNA-bound RFC. However, when RFC and PCNA together were flowed across the DNA chip in the presence of ATP, a signal was observed suggesting loading of PCNA by RFC. With ATPgammaS present instead of ATP, long-lived response signals were observed indicative of loading complexes arrested on the DNA. A primer with a 3' single-stranded extension also allowed loading of PCNA; yet turnover of the reaction intermediates was dramatically slowed down. Filter binding experiments and analysis of proteins bound to DNA-magnetic beads confirmed the conclusions drawn from the surface plasmon resonance studies.     

DNA damage-induced ubiquitylation of RFC2 subunit of replication factor C complex

Many proteins involved in DNA replication and repair undergo post-translational modifications such as phosphorylation and ubiquitylation. Proliferating cell nuclear antigen (PCNA; a homotrimeric protein that encircles double-stranded DNA to function as a sliding clamp for DNA polymerases) is monoubiquitylated by the RAD6-RAD18 complex and further polyubiquitylated by the RAD5-MMS2-UBC13 complex in response to various DNA-damaging agents. PCNA mono- and polyubiquitylation activate an error-prone translesion synthesis pathway and an error-free pathway of damage avoidance, respectively. Here we show that replication factor C (RFC; a heteropentameric protein complex that loads PCNA onto DNA) was also ubiquitylated in a RAD18-dependent manner in cells treated with alkylating agents or H(2)O(2). A mutant form of RFC2 with a D228A substitution (corresponding to a yeast Rfc4 mutation that reduces an interaction with replication protein A (RPA), a single-stranded DNA-binding protein) was heavily ubiquitylated in cells even in the absence of DNA damage. Furthermore RFC2 was ubiquitylated by the RAD6-RAD18 complex in vitro, and its modification was inhibited in the presence of RPA. The inhibitory effect of RPA on RFC2 ubiquitylation was relatively specific because RAD6-RAD18-mediated ubiquitylation of PCNA was RPA-insensitive. Our findings suggest that RPA plays a regulatory role in DNA damage responses via repression of RFC2 ubiquitylation in human cells.     

The Elg1 Clamp Loader Plays a Role in Sister Chromatid Cohesion

Mutations in the ELG1 gene of yeast lead to genomic instability, manifested in high levels of genetic recombination, chromosome loss, and gross chromosomal rearrangements. Elg1 shows similarity to the large subunit of the Replication Factor C clamp loader, and forms a RFC-like (RLC) complex in conjunction with the 4 small RFC subunits. Two additional RLCs exist in yeast: in one of them the large subunit is Ctf18, and in the other, Rad24. Ctf18 has been characterized as the RLC that functions in sister chromatid cohesion. Here we present evidence that the Elg1 RLC (but not Rad24) also plays an important role in this process. A genetic screen identified the cohesin subunit Mcd1/Scc1 and its loader Scc2 as suppressors of the synthetic lethality between elg1 and ctf4. We describe genetic interactions between ELG1 and genes encoding cohesin subunits and their accessory proteins. We also show that defects in Elg1 lead to higher precocious sister chromatid separation, and that Ctf18 and Elg1 affect cohesion via a joint pathway. Finally, we localize both Ctf18 and Elg1 to chromatin and show that Elg1 plays a role in the recruitment of Ctf18. Our results suggest that Elg1, Ctf4, and Ctf18 may coordinate the relative movement of the replication fork with respect to the cohesin ring.     

Molecular modeling-based analysis of interactions in the RFC-dependent clamp-loading process

Replication and related processes in eukaryotic cells require replication factor C (RFC) to load a molecular clamp for DNA polymerase in an ATP-driven process, involving multiple molecular interactions. The detailed understanding of this mechanism is hindered by the lack of data regarding structure, mutual arrangement, and dynamics of the players involved. In this study, we analyzed interactions that take place during loading onto DNA of either the PCNA clamp or the Rad9-Rad1-Hus1 checkpoint complex, using computationally derived molecular models. Combining the modeled structures for each RFC subunit with known structural, biochemical, and genetic data, we propose detailed models of how two of the RFC subunits, RFC1 and RFC3, interact with the C-terminal regions of PCNA. RFC1 is predicted to bind PCNA similarly to the p21-PCNA interaction, while the RFC3-PCNA binding is proposed to be similar to the E. coli delta-beta interaction. Additional sequence and structure analysis, supported by experimental data, suggests that RFC5 might be the third clamp loader subunit to bind the equivalent PCNA region. We discuss functional implications stemming from the proposed model of the RFC1-PCNA interaction and compare putative clamp-interacting regions in RFC1 and its paralogs, Rad17 and Ctf18. Based on the individual intermolecular interactions, we propose RFC and PCNA arrangement that places three RFC subunits in association with each of the three C-terminal regions in PCNA. The two other RFC subunits are positioned at the two PCNA interfaces, with the third PCNA interface left unobstructed. In addition, we map interactions at the level of individual subunits between the alternative clamp loader/clamp system, Rad17-RFC(2-5)/Rad9-Rad1-Hus1. The proposed models of interaction between two clamp/clamp loader pairs provide both structural framework for interpretation of existing experimental data and a number of specific findings that can be subjected to direct experimental testing.     

The INO80 Chromatin Remodeling Complex Functions in Sister Chromatid Cohesion

Here we identify a defect in sister chromatid cohesion in the Saccharomyces serevisiae arp8 mutant, which impairs the chromatin remodeling activity of the INO80 complex, and we report the direct association of Ino80 with centromeres and cohesin-associated regions. In early S phase, Ino80 is recruited to replication forks along with Ctf18 and PCNA, both of which are involved in the establishment of sister chromatid cohesion. The arp8 mutation perturbs the association of Ctf18 and PCNA but not of cohesin with replication forks. We propose that the INO80 complex is required for the proper establishment of sister chromatid cohesion.     

Cloning and characterization of hCTF18, hCTF8, and hDCC1. Human homologs of a Saccharomyces cerevisiae complex involved in sister chromatid cohesion establishment

A growing body of evidence suggests that establishment of sister chromatid cohesion is dependent on replication fork passage over a precohesion area. In Saccharomyces cerevisiae, this process involves an alternative replication factor C (RFC) complex that contains the four small RFC subunits as well as CTF18, CTF8, and DCC1. Here, we show that an evolutionarily conserved homologous complex exists in the nucleus of human cells. We demonstrate that hCTF18, hCTF8, and hDCC1 interact with each other as well as with the p38 subunit of RFC. This alternative RFC-containing complex interacts with proliferating cell nuclear antigen but not with the Rad9/Rad1/Hus1 complex, a proliferating cell nuclear antigen-like clamp involved in the DNA damage response. hCTF18 preferentially binds chromatin during S phase, suggesting a role during replication. Our data provide evidence for the existence of an alternative RFC complex with a probable role in mammalian sister chromatid cohesion establishment.     

Identification of replication factor C from Saccharomyces cerevisiae: a component of the leading-strand DNA replication complex.

A number of proteins have been isolated from human cells on the basis of their ability to support DNA replication in vitro of the simian virus 40 (SV40) origin of DNA replication. One such protein, replication factor C (RFC), functions with the proliferating cell nuclear antigen (PCNA), replication protein A (RPA), and DNA polymerase delta to synthesize the leading strand at a replication fork. To determine whether these proteins perform similar roles during replication of DNA from origins in cellular chromosomes, we have begun to characterize functionally homologous proteins from the yeast Saccharomyces cerevisiae. RFC from S. cerevisiae was purified by its ability to stimulate yeast DNA polymerase delta on a primed single-stranded DNA template in the presence of yeast PCNA and RPA. Like its human-cell counterpart, RFC from S. cerevisiae (scRFC) has an associated DNA-activated ATPase activity as well as a primer-template, structure-specific DNA binding activity. By analogy with the phage T4 and SV40 DNA replication in vitro systems, the yeast RFC, PCNA, RPA, and DNA polymerase delta activities function together as a leading-strand DNA replication complex. Now that RFC from S. cerevisiae has been purified, all seven cellular factors previously shown to be required for SV40 DNA replication in vitro have been identified in S. cerevisiae.     

An Eco1-independent sister chromatid cohesion establishment pathway in S. cerevisiae

Cohesion between sister chromatids, mediated by the chromosomal cohesin complex, is a prerequisite for their alignment on the spindle apparatus and segregation in mitosis. Budding yeast cohesin first associates with chromosomes in G1. Then, during DNA replication in S-phase, the replication fork-associated acetyltransferase Eco1 acetylates the cohesin subunit Smc3 to make cohesin’s DNA binding resistant to destabilization by the Wapl protein. Whether stabilization of cohesin molecules that happen to link sister chromatids is sufficient to build sister chromatid cohesion, or whether additional reactions are required to establish these links, is not known. In addition to Eco1, several other factors contribute to cohesion establishment, including Ctf4, Ctf18, Tof1, Csm3, Chl1 and Mrc1, but little is known about their roles. Here, we show that each of these factors facilitates cohesin acetylation. Moreover, the absence of Ctf4 and Chl1, but not of the other factors, causes a synthetic growth defect in cells lacking Eco1. Distinct from acetylation defects, sister chromatid cohesion in ctf4Δ and chl1Δ cells is not improved by removing Wapl. Unlike previously thought, we do not find evidence for a role of Ctf4 and Chl1 in Okazaki fragment processing, or of Okazaki fragment processing in sister chromatid cohesion. Thus, Ctf4 and Chl1 delineate an additional acetylation-independent pathway that might hold important clues as to the mechanism of sister chromatid cohesion establishment.