Histone H3K56 acetylation, Rad52, and non-DNA repair factors control double-strand break repair choice with the sister chromatid

Maf1 phosphorylation is regulated through the action of prefoldin-like Bud27 on PP4 phosphatase in Saccharomyces cerevisiae

Bud27 is a prefoldin-like protein that participates in transcriptional regulation mediated by the three RNA polymerases in Saccharomyces cerevisiae. Lack of Bud27 significantly affects RNA pol III transcription, although the involved mechanisms have not been characterized. Here, we show that Bud27 regulates the phosphorylation state of the RNA pol III transcriptional repressor, Maf1, influences its nuclear localization, and likely its activity. We demonstrate that Bud27 is associated with the Maf1 main phosphatase PP4 in vivo, and that this interaction is required for proper Maf1 dephosphorylation. Lack of Bud27 decreases the interaction among PP4 and Maf1, Maf1 dephosphorylation, and its nuclear entry. Our data uncover a new nuclear function of Bud27, identify PP4 as a novel Bud27 interactor and demonstrate the effect of this prefoldin-like protein on the posttranslational regulation of Maf1. Finally, our data reveal a broader effect of Bud27 on PP4 activity by influencing, at least, the phosphorylation of Rad53.

Strand asymmetry in the repair of replication dependent double-strand breaks

Single-strand breaks (SSBs) are one of the most common endogenous lesions and have the potential to give rise to cytotoxic double-strand breaks (DSBs) during DNA replication. To investigate the mechanism of replication fork collapse at SSBs and subsequent repair, we employed Cas9 nickase (nCas9) to generate site and strand-specific nicks in the budding yeast genome. We show that nCas9-induced nicks are converted to mostly double-ended DSBs during S-phase. We find that repair of replication-dependent DSBs requires homologous recombination (HR) and is independent of canonical non-homologous end joining. Consistent with a strong bias to repair these lesions using a sister chromatid template, we observe minimal induction of inter-chromosomal HR by nCas9. Using nCas9 and a gRNA to nick either the leading or lagging strand template, we carried out a genome-wide screen to identify factors necessary for the repair of replication-dependent DSBs. All the core HR genes were recovered in the screen with both gRNAs, but we recovered components of the replication-coupled nucleosome assembly (RCNA) pathway with only the gRNA targeting the leading strand template. By use of additional gRNAs, we find that the RCNA pathway is especially important to repair a leading strand fork collapse.

Persistent Acetylation of Histone H3 Lysine 56 Compromises the Activity of DNA Replication Origins

In Saccharomyces cerevisiae, newly synthesized histones H3 are acetylated on lysine 56 (H3 K56ac) by the Rtt109 acetyltransferase prior to their deposition on nascent DNA behind replication forks. Two deacetylases of the sirtuin family, Hst3 and Hst4, remove H3 K56ac from chromatin after S phase. hst3Δ hst4Δ cells present constitutive H3 K56ac, which sensitizes cells to replicative stress via unclear mechanisms. A chemogenomic screen revealed that DBF4 heterozygosity sensitizes cells to NAM-induced inhibition of sirtuins. DBF4 and CDC7 encode subunits of the Dbf4-dependent kinase (DDK), which activates origins of DNA replication during S phase. We show that (i) cells harboring the dbf4-1 or cdc7-4 hypomorphic alleles are sensitized to NAM, and that (ii) the sirtuins Sir2, Hst1, Hst3, and Hst4 promote DNA replication in cdc7-4 cells. We further demonstrate that Rif1, an inhibitor of DDK-dependent activation of origins, causes DNA damage and replication defects in NAM-treated cells and hst3Δ hst4Δ mutants. cdc7-4 hst3Δ hst4Δ cells are shown to display delayed initiation of DNA replication, which is not due to intra-S checkpoint activation but requires Rtt109-dependent H3 K56ac. Our results suggest that constitutive H3 K56ac sensitizes cells to replicative stress in part by negatively influencing the activation of origins of DNA replication.

Chromatin assembly factor-1 preserves genome stability in ctf4Δ cells by promoting sister chromatid cohesion

Chromatin assembly and the establishment of sister chromatid cohesion are intimately connected to the progression of DNA replication forks. Here we examined the genetic interaction between the heterotrimeric chromatin assembly factor-1 (CAF-1), a central component of chromatin assembly during replication, and the core replisome component Ctf4. We find that CAF-1 deficient cells as well as cells affected in newly-synthesized H3-H4 histones deposition during DNA replication exhibit a severe negative growth with ctf4Δ mutant. We dissected the role of CAF-1 in the maintenance of genome stability in ctf4Δ yeast cells. In the absence of CTF4, CAF-1 is essential for viability in cells experiencing replication problems, in cells lacking functional S-phase checkpoint or functional spindle checkpoint, and in cells lacking DNA repair pathways involving homologous recombination. We present evidence that CAF-1 affects cohesin association to chromatin in a DNA-damage-dependent manner and is essential to maintain cohesion in the absence of CTF4. We also show that Eco1-catalyzed Smc3 acetylation is reduced in absence of CAF-1. Furthermore, we describe genetic interactions between CAF-1 and essential genes involved in cohesin loading, cohesin stabilization, and cohesin component indicating that CAF-1 is crucial for viability when sister chromatid cohesion is affected. Finally, our data indicate that the CAF-1-dependent pathway required for cohesion is functionally distinct from the Rtt101-Mms1-Mms22 pathway which functions in replicated chromatin assembly. Collectively, our results suggest that the deposition by CAF-1 of newly-synthesized H3-H4 histones during DNA replication creates a chromatin environment that favors sister chromatid cohesion and maintains genome integrity.

Genetic inhibitors of APOBEC3B-induced mutagenesis

The cytidine deaminases APOBEC3A (A3A) and APOBEC3B (A3B) are prominent mutators of human cancer genomes. However, tumor-specific genetic modulators of APOBEC-induced mutagenesis are poorly defined. Here, we used a screen to identify 61 gene deletions that increase A3B-induced mutations in yeast. We also determined whether each deletion was epistatic with Ung1 loss, which indicated whether the encoded factors participate in the homologous recombination (HR)-dependent bypass of A3B/Ung1-dependent abasic sites or suppress A3B-catalyzed deamination by protecting against aberrant formation of single-stranded DNA (ssDNA). We found that the mutation spectra of A3B-induced mutations revealed genotype-specific patterns of strand-specific ssDNA formation and nucleotide incorporation across APOBEC-induced lesions. Combining these three metrics, we were able to establish a multifactorial signature of APOBEC-induced mutations specific to (1) failure to remove H3K56 acetylation, (2) defective CTF18-RFC complex function, and (3) defective HR-mediated bypass of APOBEC-induced lesions. We extended these results by analyzing mutation data for human tumors and found BRCA1/2-deficient breast cancers display three- to fourfold more APOBEC-induced mutations. Mirroring our results in yeast, Rev1-mediated C-to-G substitutions are mainly responsible for increased APOBEC-signature mutations in BRCA1/2-deficient tumors, and these mutations associate with lagging strand synthesis during replication. These results identify important factors that influence DNA replication dynamics and likely the abundance of APOBEC-induced mutation during tumor progression. They also highlight a novel role for BRCA1/2 during HR-dependent lesion bypass of APOBEC-induced lesions during cancer cell replication.

Histone H3 serine-57 is a CHK1 substrate whose phosphorylation affects DNA repair

Histone post-translational modifications promote a chromatin environment that controls transcription, DNA replication and repair, but surprisingly few phosphorylations have been documented. We report the discovery of histone H3 serine-57 phosphorylation (H3S57ph) and show that it is implicated in different DNA repair pathways from fungi to vertebrates. We identified CHK1 as a major human H3S57 kinase, and disrupting or constitutively mimicking H3S57ph had opposing effects on rate of recovery from replication stress, 53BP1 chromatin binding, and dependency on RAD52. In fission yeast, mutation of all H3 alleles to S57A abrogated DNA repair by both non-homologous end-joining and homologous recombination, while cells with phospho-mimicking S57D alleles were partly compromised for both repair pathways, presented aberrant Rad52 foci and were strongly sensitised to replication stress. Mechanistically, H3S57ph loosens DNA-histone contacts, increasing nucleosome mobility, and interacts with H3K56. Our results suggest that dynamic phosphorylation of H3S57 is required for DNA repair and recovery from replication stress, opening avenues for investigating the role of this modification in other DNA-related processes.

Genetic modifiers of APOBEC-induced mutagenesis

The cytidine deaminases APOBEC3A and APOBEC3B (A3B) are prominent mutators of human cancer genomes. However, tumor-specific genetic modulators of APOBEC-induced mutagenesis are poorly defined. Here, we utilized a screen to identify 61 gene deletions that increase A3B-induced mutations in yeast. Also, we determined whether each deletion was epistatic with UNG1 loss, which indicated whether the encoded factors participate in the error-free bypass of A3B/Ung1-dependent abasic sites or suppress A3B-catalyzed deamination by protecting against aberrant formation of single stranded DNA (ssDNA). Additionally, we determined that the mutation spectra of A3B-induced mutations revealed genotype-specific patterns of strand-specific ssDNA formation and nucleotide incorporation across APOBEC-induced lesions. Combining these three metrics we were able to establish a multifactorial signature of APOBEC-induced mutations specific to (1) failure to remove H3K56 acetylation, which results in extremely high A3B-induced mutagenesis, (2) defective CTF18-RFC complex function, which results in high levels of A3B induced mutations specifically on the leading strand template that synergistically increase with loss of UNG1, and (3) defective HR-mediated bypass of APOBEC-induced lesions, which were epistatic with Ung1 loss and result from increased Rev1-mediated C-to-G substitutions. We extended these results by analyzing mutation data for human tumors and found BRCA1/2-deficient breast cancer tumors display 3- to 4-fold more APOBEC-induced mutations. Mirroring our results in yeast, for BRCA1/2 deficient tumors Rev1-mediated C-to-G substitutions are solely responsible for increased APOBEC-signature mutations and these mutations occur on the lagging strand during DNA replication. Together these results identify important factors that influence the dynamics of DNA replication and likely the abundance of APOBEC-induced mutation during tumor progression as well as a novel mechanistic role for BRCA1/2 during HR-dependent lesion bypass of APOBEC-induced lesions during cancer cell replication.

Histone acetylation dynamics in repair of DNA double-strand breaks

Packaging of eukaryotic genome into chromatin is a major obstacle to cells encountering DNA damage caused by external or internal agents. For maintaining genomic integrity, the double-strand breaks (DSB) must be efficiently repaired, as these are the most deleterious type of DNA damage. The DNA breaks have to be detected in chromatin context, the DNA damage response (DDR) pathways have to be activated to repair breaks either by non‐ homologous end joining and homologous recombination repair. It is becoming clearer now that chromatin is not a mere hindrance to DDR, it plays active role in sensing, detection and repair of DNA damage. The repair of DSB is governed by the reorganization of the pre-existing chromatin, leading to recruitment of specific machineries, chromatin remodelling complexes, histone modifiers to bring about dynamic alterations in histone composition, nucleosome positioning, histone modifications. In response to DNA break, modulation of chromatin occurs via various mechanisms including post-translational modification of histones. DNA breaks induce many types of histone modifications, such as phosphorylation, acetylation, methylation and ubiquitylation on specific histone residues which are signal and context dependent. DNA break induced histone modifications have been reported to function in sensing the breaks, activating processing of breaks by specific pathways, and repairing damaged DNA to ensure integrity of the genome. Favourable environment for DSB repair is created by generating open and relaxed chromatin structure. Histone acetylation mediate de-condensation of chromatin and recruitment of DSB repair proteins to their site of action at the DSB to facilitate repair. In this review, we will discuss the current understanding on the critical role of histone acetylation in inducing changes both in chromatin organization and promoting recruitment of DSB repair proteins to sites of DNA damage. It consists of an overview of function and regulation of the deacetylase enzymes which remove these marks and the function of histone acetylation and regulators of acetylation in genome surveillance.

Histone deacetylase SirE regulates development, DNA damage response and aflatoxin production in Aspergillus flavus

Aspergillus flavus is a ubiquitous saprotrophic soil-borne pathogenic fungus that causes crops contamination with the carcinogen aflatoxins. Although Sirtuin E (SirE) is known to be a NAD-dependent histone deacetylase involved in global transcriptional regulation. Its biological functions in A. flavus are not fully understood. To explore the effects of SirE, we found that SirE was located in the nucleus and increased the level of H3K56 acetylation. The ΔsirE mutant had the most severe growth defect in the sirtuin family. The RNA-Seq revealed that sirE was crucial for secondary metabolism production as well as genetic information process and oxidation-reduction in A. flavus. Further analysis revealed that the ΔsirE mutant increased aflatoxin production. Both the sirE deletion and H3K56 mutants were highly sensitive to DNA damage and oxidative stresses, indicating that SirE was required for DNA damage and redox reaction by the H3K56 locus. Furthermore, the ΔsirE mutant displayed high sensitivity to osmotic stress and cell wall stress, but they may not be associated with the H3K56. Finally, the catalytic activity site N192 of SirE was required for regulating growth, deacetylase function and aflatoxin production. Together, SirE is essential for histone deacetylation and biological function in A. flavus.

A role for the Saccharomyces cerevisiae Rtt109 histone acetyltransferase in R-loop homeostasis and associated genome instability

The stability of the genome is occasionally challenged by the formation of DNA–RNA hybrids and R-loops, which can be influenced by the chromatin context. This is mainly due to the fact that DNA–RNA hybrids hamper the progression of replication forks, leading to fork stalling and, ultimately, DNA breaks. Through a specific screening of chromatin modifiers performed in the yeast Saccharomyces cerevisiae, we have found that the Rtt109 histone acetyltransferase is involved in several steps of R-loop-metabolism and their associated genetic instability. On the one hand, Rtt109 prevents DNA–RNA hybridization by the acetylation of histone H3 lysines 14 and 23 and, on the other hand, it is involved in the repair of replication-born DNA breaks, such as those that can be caused by R-loops, by acetylating lysines 14 and 56. In addition, Rtt109 loss renders cells highly sensitive to replication stress in combination with R-loop-accumulating THO-complex mutants. Our data evidence that the chromatin context simultaneously influences the occurrence of DNA–RNA hybrid-associated DNA damage and its repair, adding complexity to the source of R-loop-associated genetic instability.

Acetylation of H3K56 orchestrates UV-responsive chromatin events that generate DNA accessibility during Nucleotide Excision Repair

Histone modifications have long been related to DNA damage response. Nucleotide excision repair pathway that removes helix-distorting lesions necessitates DNA accessibility through chromatin modifications. Previous studies have linked H3 tail residue acetylation to UV-induced NER. Here we present evidences that acetylation of H3K56 is crucial for early phases of NER. Using H3K56 mutants K56Q and K56R, which mimic acetylated and unacetylated lysines respectively, we show that recruitment of the repair factor Rad16, a Swi/Snf family member is dependent on H3K56 acetylation. With constitutive H3K56 acetylation, Rad16 recruitment became UV-independent. Furthermore, H3K56 acetylation promoted UV-induced hyperacetylation of H3K9 and H3K14. Importantly, constitutive H3K56 acetylation prominently increased chromatin accessibility. During NER, lack of H3K56 acetylation that effectively aborted H3 tail residue acetylation and Rad16 recruitment, thus failed to impart essential chromatin modulations. The NER-responsive oscillation of chromatin structure observed in wild type, was distinctly eliminated in absence of H3K56 acetylation. In vitro assay with wild type and H3K56 mutant cell extracts further indicated that absence of H3K56 acetylation negatively affected DNA relaxation during NER. Overall, H3K56 acetylation regulates Rad16 redistribution and UV-induced H3 tail residue hyperacetylation, and the resultant modification code promotes chromatin accessibility and recruitment of subsequent repair factors during NER.

vp1524, a Vibrio parahaemolyticus NAD+ dependent deacetylase, regulates host response during infection by induction of host histone deacetylation

Gram negative intracellular pathogen V. parahaemolyticus manifests its infection through a series of effector proteins released into the host via the type III secretion system. Most of these effector proteins alter signalling pathways of the host to facilitate survival and proliferation of bacteria inside host cells. Here, we report V. parahaemolyticus (serotype O3:K6) infection induced histone deacetylation in host intestinal epithelial cells, particularly deacetylation of H3K9, H3K56, H3K18 and H4K16 residues. We found a putative NAD+ dependent deacetylase, vp1524 (vpCobB) of Vibrio parahaemolyticus, was overexpressed during infection. Biochemical assays revealed that Vp1524 is a functional NAD+ dependent Sir2 family deacetylase in vitro, which was capable of deacetylating acetylated histones. Furthermore, we observed that vp1524 is expressed and localized to the nuclear periphery of the host cells during infection. Consequently, Vp1524 translocated to nuclear compartments of transfected cells, deacetylated histones, specifically causing deacetylation of those residues (K56, K16, K18) associated with V. parahaemolyticus infection. This infection induced deacetylation resulted in transcriptional repression of several host genes involved in epigenetic regulation, immune response, autophagy etc. Thus, our study shows that a V. parahaemolyticus lysine deacetylase Vp1524 is secreted inside the host cells during infection, modulating host gene expression through histone deacetylation.

Increased Agrobacterium-mediated transformation of Saccharomyces cerevisiae after deletion of the yeast ADA2 gene

Agrobacterium tumefaciens is the causative agent of crown gall disease and is widely used as a vector to create transgenic plants. Under laboratory conditions, the yeast Saccharomyces cerevisiae and other yeasts and fungi can also be transformed, and Agrobacterium-mediated transformation (AMT) is now considered the method of choice for genetic transformation of many fungi. Unlike plants, in S. cerevisiae, T-DNA is integrated preferentially by homologous recombination and integration by non-homologous recombination is very inefficient. Here we report that upon deletion of ADA2, encoding a component of the ADA and SAGA transcriptional adaptor/histone acetyltransferase complexes, the efficiency of AMT significantly increased regardless of whether integration of T-DNA was mediated by homologous or non-homologous recombination. This correlates with an increase in double-strand DNA breaks, the putative entry sites for T-DNA, in the genome of the ada2Δ deletion mutant, as visualized by the number of Rad52-GFP foci. Our observations may be useful to enhance the transformation of species that are difficult to transform.

Histone lysine modifying enzymes and their critical roles in DNA double-strand break repair

Cells protect the integrity of the genome against DNA double-strand breaks through several well-characterized mechanisms including nonhomologous end-joining repair, homologous recombination repair, microhomology-mediated end-joining and single-strand annealing. However, aberrant DNA damage responses (DDRs) lead to genome instability and tumorigenesis. Clarification of the mechanisms underlying the DDR following lethal damage will facilitate the identification of therapeutic targets for cancer. Histones are small proteins that play a major role in condensing DNA into chromatin and regulating gene function. Histone modifications commonly occur in several residues including lysine, arginine, serine, threonine and tyrosine, which can be acetylated, methylated, ubiquitinated and phosphorylated. Of these, lysine modifications have been extensively explored during DDRs. Here, we focus on discussing the roles of lysine modifying enzymes involved in acetylation, methylation, and ubiquitination during the DDR. We provide a comprehensive understanding of the basis of potential epigenetic therapies driven by histone lysine modifications.

Heterogeneity of DNA damage incidence and repair in different chromatin contexts

It has been long known that some regions of the genome are more susceptible to damage and mutagenicity than others. Recent advances have determined a critical role of chromatin both in the incidence of damage and in its repair. Thus, chromatin arises as a guardian of the stability of the genome, which is altered in cancer cells. In this review, we focus into the mechanisms by which chromatin influences the occurrence and repair of the most cytotoxic DNA lesions, double-strand breaks, in particular at actively transcribed chromatin or related to DNA replication.

Analysis of repair of replication-born double-strand breaks by sister chromatid recombination in yeast

The repair of DNA double-strand breaks is crucial for cell viability and the maintenance of genome integrity. When present, the intact sister chromatid is used as the preferred repair template to restore the genetic information by homologous recombination. Although the study of the factors involved in sister chromatid recombination is hampered by the fact that both sister chromatids are indistinguishable, genetic and molecular systems based on DNA repeats have been developed to overcome this problem. In particular, the use of site-specific nucleases capable of inducing DNA nicks that replication converts into double-strand breaks has enabled the specific study of the repair of such replication-born double strand breaks by sister chromatid recombination. In this chapter, we describe detailed protocols for determining the efficiency and kinetics of this recombination reaction as well as for the genetic quantification of recombination products.

Potential functions of histone H3.3 lysine 56 acetylation in mammals

H3K56 acetylation (H3K56Ac) was first identified in yeast and has recently been reported to play important roles in maintaining genomic stability, chromatin assembly, DNA replication, cell cycle progression and DNA repair. Although H3.1K56Ac has been relatively well studied, the function of H3.3K56Ac remains mostly unknown in mammals. In this study, we used H3.3K56Q and H3.3K56R mutants to study the possible function of H3.3K56 acetylation. The K-to-Q substitution mimics a constitutively acetylated lysine, while the K-to-R replacement mimics a constitutively unmodified lysine. We report that cell lines harbouring mutation of H3.3K56R exhibit increased cell death and dramatic morphology changes. Using a Tet-Off inducible system, we found an increased population of polyploid/aneuploid cells and decreased cell viability in H3.3K56R mutant cells. Consistent with these results, the H3.3K56R mutant had compromised H3.3 incorporation into several pericentric and centric heterochromatin regions we tested. Moreover, mass spectrometry analysis coupled with label-free quantification revealed that biological processes regulated by the H3.3-associating proteins, whose interaction with H3.3 was markedly increased by H3.3K56Q mutation but decreased by H3.3K56R mutation, include sister chromatid cohesion, mitotic nuclear division, and mitotic nuclear envelope disassembly. These results suggest that H3.3K56 acetylation is crucial for chromosome segregation and cell division in mammals.

Chromatin dynamics and DNA replication roadblocks

A broad spectrum of spontaneous and genotoxin-induced DNA lesions impede replication fork progression. The DNA damage response that acts to promote completion of DNA replication is associated with dynamic changes in chromatin structure that include two distinct processes which operate genome-wide during S-phase. The first, often referred to as histone recycling or parental histone segregation, is characterized by the transfer of parental histones located ahead of replication forks onto nascent DNA. The second, known as de novo chromatin assembly, consists of the deposition of new histone molecules onto nascent DNA. Because these two processes occur at all replication forks, their potential to influence a multitude of DNA repair and DNA damage tolerance mechanisms is considerable. The purpose of this review is to provide a description of parental histone segregation and de novo chromatin assembly, and to illustrate how these processes influence cellular responses to DNA replication roadblocks.

Wss1 Promotes Replication Stress Tolerance by Degrading Histones

Timely completion of DNA replication is central to accurate cell division and to the maintenance of genomic stability. However, certain DNA-protein interactions can physically impede DNA replication fork progression. Cells remove or bypass these physical impediments by different mechanisms to preserve DNA macromolecule integrity and genome stability. In Saccharomyces cerevisiae, Wss1, the DNA-protein crosslink repair protease, allows cells to tolerate hydroxyurea-induced replication stress, but the underlying mechanism by which Wss1 promotes this function has remained unknown. Here, we report that Wss1 provides cells tolerance to replication stress by directly degrading core histone subunits that non-specifically and non-covalently bind to single-stranded DNA. Unlike Wss1-dependent proteolysis of covalent DNA-protein crosslinks, proteolysis of histones does not require Cdc48 nor SUMO-binding activities. Wss1 thus acts as a multi-functional protease capable of targeting a broad range of covalent and non-covalent DNA-binding proteins to preserve genome stability during adverse conditions.

The Biological Significance of Targeting Acetylation-Mediated Gene Regulation for Designing New Mechanistic Tools and Potential Therapeutics

The molecular interplay between nucleosomal packaging and the chromatin landscape regulates the transcriptional programming and biological outcomes of downstream genes. An array of epigenetic modifications plays a pivotal role in shaping the chromatin architecture, which controls DNA access to the transcriptional machinery. Acetylation of the amino acid lysine is a widespread epigenetic modification that serves as a marker for gene activation, which intertwines the maintenance of cellular homeostasis and the regulation of signaling during stress. The biochemical horizon of acetylation ranges from orchestrating the stability and cellular localization of proteins that engage in the cell cycle to DNA repair and metabolism. Furthermore, lysine acetyltransferases (KATs) modulate the functions of transcription factors that govern cellular response to microbial infections, genotoxic stress, and inflammation. Due to their central role in many biological processes, mutations in KATs cause developmental and intellectual challenges and metabolic disorders. Despite the availability of tools for detecting acetylation, the mechanistic knowledge of acetylation-mediated cellular processes remains limited. This review aims to integrate molecular and structural bases of KAT functions, which would help design highly selective tools for understanding the biology of KATs toward developing new disease treatments.

DNA replication, transcription, and H3K56 acetylation regulate copy number and stability at tandem repeats

Tandem repeats are inherently unstable and exhibit extensive copy number polymorphisms. Despite mounting evidence for their adaptive potential, the mechanisms associated with regulation of the stability and copy number of tandem repeats remain largely unclear. To study copy number variation at tandem repeats, we used two well-studied repetitive arrays in the budding yeast genome, the ribosomal DNA (rDNA) locus, and the copper-inducible CUP1 gene array. We developed powerful, highly sensitive, and quantitative assays to measure repeat instability and copy number and used them in multiple high-throughput genetic screens to define pathways involved in regulating copy number variation. These screens revealed that rDNA stability and copy number are regulated by DNA replication, transcription, and histone acetylation. Through parallel studies of both arrays, we demonstrate that instability can be induced by DNA replication stress and transcription. Importantly, while changes in stability in response to stress are observed within a few cell divisions, a change in steady state repeat copy number requires selection over time. Further, H3K56 acetylation is required for regulating transcription and transcription-induced instability at the CUP1 array, and restricts transcription-induced amplification. Our work suggests that the modulation of replication and transcription is a direct, reversible strategy to alter stability at tandem repeats in response to environmental stimuli, which provides cells rapid adaptability through copy number variation. Additionally, histone acetylation may function to promote the normal adaptive program in response to transcriptional stress. Given the omnipresence of DNA replication, transcription, and chromatin marks like histone acetylation, the fundamental mechanisms we have uncovered significantly advance our understanding of the plasticity of tandem repeats more generally.

Histones on fire: the effect of Dun1 and Mrc1 on origin firing and replication of hyper-acetylated genomes

As cells replicate their DNA, there is a need to synthesize new histones with which to wrap it. Newly synthesized H3 histones that are incorporated into the assembling chromatin behind the replication fork are acetylated at lysine 56. The acetylation is removed by two deacetylases, Hst3 and Hst4. This process is tightly regulated and any perturbation leads to genomic instability and replicative stress. We recently showed that Dun1, a kinase implicated mainly in the regulation of dNTPs, is vital in cells with hyper-acetylation, to counteract Rad53's inhibition on late-firing origins of replication. Our work showed that ∆hst3 ∆hst4 cells depend on late origin firing for survival, and are unable to prevent Rad53's inhibition when Dun1 is inactive. Thus, our work describes a role for Dun1 that is independent on its known function as a regulator of dNTP levels. Here we show that Mrc1 (Claspin in mammals), a protein that moves with the replicating fork and participates in both replication and checkpoint functions, plays also an essential role in the absence of H3K56Ac deacetylation. The sum of the results shown here and in our recent publication suggests that dormant origins are also utilized in these cells, making Mrc1, which regulates firing from these origins, also essential when histone H3 is hyper-acetylated. Thus, cells suffering from hyper-acetylation of H3K56 experience replication stress caused by a combination of prone-to-collapse forks and limited replication tracts. This combination makes both Dun1 and Mrc1, each acting on different targets, essential for viability.

The Amazing Acrobat: Yeast's Histone H3K56 Juggles Several Important Roles While Maintaining Perfect Balance

Acetylation on lysine 56 of histone H3 of the yeast Saccharomyces cerevisiae has been implicated in many cellular processes that affect genome stability. Despite being the object of much research, the complete scope of the roles played by K56 acetylation is not fully understood even today. The acetylation is put in place at the S-phase of the cell cycle, in order to flag newly synthesized histones that are incorporated during DNA replication. The signal is removed by two redundant deacetylases, Hst3 and Hst4, at the entry to G2/M phase. Its crucial location, at the entry and exit points of the DNA into and out of the nucleosome, makes this a central modification, and dictates that if acetylation and deacetylation are not well concerted and executed in a timely fashion, severe genomic instability arises. In this review, we explore the wealth of information available on the many roles played by H3K56 acetylation and the deacetylases Hst3 and Hst4 in DNA replication and repair.

A novel role for Dun1 in the regulation of origin firing upon hyper-acetylation of H3K56

During DNA replication newly synthesized histones are incorporated into the chromatin of the replicating sister chromatids. In the yeast Saccharomyces cerevisiae new histone H3 molecules are acetylated at lysine 56. This modification is carefully regulated during the cell cycle, and any disruption of this process is a source of genomic instability. Here we show that the protein kinase Dun1 is necessary in order to maintain viability in the absence of the histone deacetylases Hst3 and Hst4, which remove the acetyl moiety from histone H3. This lethality is not due to the well-characterized role of Dun1 in upregulating dNTPs, but rather because Dun1 is needed in order to counteract the checkpoint kinase Rad53 (human CHK2) that represses the activity of late firing origins. Deletion of CTF18, encoding the large subunit of an alternative RFC-like complex (RLC), but not of components of the Elg1 or Rad24 RLCs, is enough to overcome the dependency of cells with hyper-acetylated histones on Dun1. We show that the detrimental function of Ctf18 depends on its interaction with the leading strand polymerase, Polε. Our results thus show that the main problem of cells with hyper-acetylated histones is the regulation of their temporal and replication programs, and uncover novel functions for the Dun1 protein kinase and the Ctf18 clamp loader.

Within the cell’s nucleus the DNA is wrapped around proteins called histones. Upon DNA replication, newly synthesized H3 histones are acetylated at lysine 56. This acetylation is significant for the cell because when it is not removed in a timely manner it leads to genomic instability. We have investigated the source of this instability and discovered that the kinase Dun1, usually implicated in the regulation of dNTPs, the building blocks of DNA, has a novel, dNTP-independent, essential role when histones are hyper-acetylated. The essential role of Dun1 is in the regulation of the temporal program of DNA replication. Thus, our results uncover what the main defect is in cells unable to regulate the acetylation of histones, while revealing new functions for well-characterized proteins with roles in genome stability maintenance.

Prefoldin-like Bud27 influences the transcription of ribosomal components and ribosome biogenesis in Saccharomyces cerevisiae

Understanding the functional connection that occurs for the three nuclear RNA polymerases to synthesize ribosome components during the ribosome biogenesis process has been the focal point of extensive research. To preserve correct homeostasis on the production of ribosomal components, cells might require the existence of proteins that target a common subunit of these RNA polymerases to impact their respective activities. This work describes how the yeast prefoldin-like Bud27 protein, which physically interacts with the Rpb5 common subunit of the three RNA polymerases, is able to modulate the transcription mediated by the RNA polymerase I, likely by influencing transcription elongation, the transcription of the RNA polymerase III, and the processing of ribosomal RNA. Bud27 also regulates both RNA polymerase II-dependent transcription of ribosomal proteins and ribosome biogenesis regulon genes, likely by occupying their DNA ORFs, and the processing of the corresponding mRNAs. With RNA polymerase II, this association occurs in a transcription rate-dependent manner. Our data also indicate that Bud27 inactivation alters the phosphorylation kinetics of ribosomal protein S6, a readout of TORC1 activity. We conclude that Bud27 impacts the homeostasis of the ribosome biogenesis process by regulating the activity of the three RNA polymerases and, in this way, the synthesis of ribosomal components. This quite likely occurs through a functional connection of Bud27 with the TOR signaling pathway.

Homologous recombination and Mus81 promote replication completion in response to replication fork blockage

Impediments to DNA replication threaten genome stability. The homologous recombination (HR) pathway has been involved in the restart of blocked replication forks. Here, we used a method to increase yeast cell permeability in order to study at the molecular level the fate of replication forks blocked by DNA topoisomerase I poisoning by camptothecin (CPT). Our results indicate that Rad52 and Rad51 HR factors are required to complete DNA replication in response to CPT. Recombination events occurring during S phase do not generally lead to the restart of DNA synthesis but rather protect blocked forks until they merge with convergent forks. This fusion generates structures requiring their resolution by the Mus81 endonuclease in G2 /M. At the global genome level, the multiplicity of replication origins in eukaryotic genomes and the fork protection mechanism provided by HR appear therefore to be essential to complete DNA replication in response to fork blockage.

Access to PCNA by Srs2 and Elg1 Controls the Choice between Alternative Repair Pathways in Saccharomyces cerevisiae

PCNA, the ring that encircles DNA maintaining the processivity of DNA polymerases, is modified by ubiquitin and SUMO. Whereas ubiquitin is required for bypassing lesions through the DNA damage tolerance (DDT) pathways, we show here that SUMOylation represses another pathway, salvage recombination. The Srs2 helicase is recruited to SUMOylated PCNA and prevents the salvage pathway from acting. The pathway can be induced by overexpressing the PCNA unloader Elg1, or the homologous recombination protein Rad52. Our results underscore the role of PCNA modifications in controlling the various bypass and DNA repair mechanisms.

During DNA replication, stalling can occur when the replicative DNA polymerases encounter lesions or hard-to replicate regions. Under these circumstances, the processivity factor PCNA gets ubiquitylated at lysine 164, inducing the DNA damage tolerance (DDT) mechanisms that can bypass lesions encountered during DNA replication. PCNA can also be SUMOylated at the same residue or at lysine 127. Surprisingly, pol30-K164R mutants display a higher degree of sensitivity to DNA-damaging agents than pol30-KK127,164RR strains, unable to modify any of the lysines. Here, we show that in addition to translesion synthesis and strand-transfer DDT mechanisms, an alternative repair mechanism (“salvage recombination”) that copies information from the sister chromatid is repressed by the recruitment of Srs2 to SUMOylated PCNA. Overexpression of Elg1, the PCNA unloader, or of the recombination protein Rad52 allows its activation. We dissect the genetic requirements for this pathway, as well as the interactions between Srs2 and Elg1.

Does genome surveillance explain the global discrepancy between binding and effect of chromatin factors?

Knocking out a chromatin factor often does not alter the transcription of its binding targets. What explains the observed disconnect between binding and effect? We hypothesize that this discrepancy could be associated with the role of chromatin factors in maintaining genetic and epigenetic integrity at promoters, and not necessarily with transcription. Through re-analysis of published datasets, we present several lines of evidence that support our hypothesis and deflate the popular assumptions. We also tested the hypothesis through mutation accumulation assays on yeast knockouts of chromatin factors. Altogether, the proposed hypothesis presents a simple explanation for the global discord between chromatin factor binding and effect. Future work in this direction might fortify the hypothesis and elucidate the underlying mechanisms.

The Nup84 complex coordinates the DNA damage response to warrant genome integrity

DNA lesions interfere with cellular processes such as transcription and replication and need to be adequately resolved to warrant genome integrity. Beyond their primary role in molecule transport, nuclear pore complexes (NPCs) function in other processes such as transcription, nuclear organization and DNA double strand break (DSB) repair. Here we found that the removal of UV-induced DNA lesions by nucleotide excision repair (NER) is compromised in the absence of the Nup84 nuclear pore component. Importantly, nup84Δ cells show an exacerbated sensitivity to UV in early S phase and delayed replication fork progression, suggesting that unrepaired spontaneous DNA lesions persist during S phase. In addition, nup84Δ cells are defective in the repair of replication-born DSBs by sister chromatid recombination (SCR) and rely on post-replicative repair functions for normal proliferation, indicating dysfunctions in the cellular pathways that enable replication on damaged DNA templates. Altogether, our data reveal a central role of the NPC in the DNA damage response to facilitate replication progression through damaged DNA templates by promoting efficient NER and SCR and preventing chromosomal rearrangements.

Rpd3L and Hda1 histone deacetylases facilitate repair of broken forks by promoting sister chromatid cohesion

Genome stability involves accurate replication and DNA repair. Broken replication forks, such as those encountering a nick, lead to double strand breaks (DSBs), which are preferentially repaired by sister-chromatid recombination (SCR). To decipher the role of chromatin in eukaryotic DSB repair, here we analyze a collection of yeast chromatin-modifying mutants using a previously developed system for the molecular analysis of repair of replication-born DSBs by SCR based on a mini-HO site. We confirm the candidates through FLP-based systems based on a mutated version of the FLP flipase that causes nicks on either the leading or lagging DNA strands. We demonstrate that Rpd3L and Hda1 histone deacetylase (HDAC) complexes contribute to the repair of replication-born DSBs by facilitating cohesin loading, with no effect on other types of homology-dependent repair, thus preventing genome instability. We conclude that histone deacetylation favors general sister chromatid cohesion as a necessary step in SCR.

Actin and Nuclear Envelope Components Influence Ectopic Recombination in the Absence of Swr1

The accuracy of most DNA processes depends on chromatin integrity and dynamics. Our analyses in the yeast Saccharomyces cerevisiae show that an absence of Swr1 (the catalytic and scaffold subunit of the chromatin-remodeling complex SWR) leads to the formation of long-duration Rad52, but not RPA, foci and to an increase in intramolecular recombination. These phenotypes are further increased by MMS, zeocin, and ionizing radiation, but not by double-strand breaks, HU, or transcription/replication collisions, suggesting that they are associated with specific DNA lesions. Importantly, these phenotypes can be specifically suppressed by mutations in: (1) chromatin-anchorage internal nuclear membrane components (mps3∆75-150 and src1∆); (2) actin and actin regulators (act1-157, act1-159, crn1∆, and cdc42-6); or (3) the SWR subunit Swc5 and the SWR substrate Htz1 However, they are not suppressed by global disruption of actin filaments or by the absence of Csm4 (a component of the external nuclear membrane that forms a bridging complex with Mps3, thus connecting the actin cytoskeleton with chromatin). Moreover, swr1∆-induced Rad52 foci and intramolecular recombination are not associated with tethering recombinogenic DNA lesions to the nuclear periphery. In conclusion, the absence of Swr1 impairs efficient recombinational repair of specific DNA lesions by mechanisms that are influenced by SWR subunits, including actin, and nuclear envelope components. We suggest that these recombinational phenotypes might be associated with a pathological effect on homologous recombination of actin-containing complexes.

Chromatin Modifiers Alter Recombination Between Divergent DNA Sequences

Recombination between divergent DNA sequences is actively prevented by heteroduplex rejection mechanisms. In baker's yeast, such antirecombination mechanisms can be initiated by the recognition of DNA mismatches in heteroduplex DNA by MSH proteins, followed by recruitment of the Sgs1-Top3-Rmi1 helicase-topoisomerase complex to unwind the recombination intermediate. We previously showed that the repair/rejection decision during single-strand annealing recombination is temporally regulated by MSH (MutShomolog) protein levels and by factors that excise nonhomologous single-stranded tails. These observations, coupled with recent studies indicating that mismatch repair (MMR) factors interact with components of the histone chaperone machinery, encouraged us to explore roles for epigenetic factors and chromatin conformation in regulating the decision to reject vs. repair recombination between divergent DNA substrates. This work involved the use of an inverted repeat recombination assay thought to measure sister chromatid repair during DNA replication. Our observations are consistent with the histone chaperones CAF-1 and Rtt106, and the histone deacetylase Sir2, acting to suppress heteroduplex rejection and the Rpd3, Hst3, and Hst4 deacetylases acting to promote heteroduplex rejection. These observations, and double-mutant analysis, have led to a model in which nucleosomes located at DNA lesions stabilize recombination intermediates and compete with MMR factors that mediate heteroduplex rejection.

Mutational signatures of redox stress in yeast single-strand DNA and of aging in human mitochondrial DNA share a common feature

Redox stress is a major hallmark of cancer. Analysis of thousands of sequenced cancer exomes and whole genomes revealed distinct mutational signatures that can be attributed to specific sources of DNA lesions. Clustered mutations discovered in several cancer genomes were linked to single-strand DNA (ssDNA) intermediates in various processes of DNA metabolism. Previously, only one clustered mutational signature had been clearly associated with a subclass of ssDNA-specific apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like (APOBEC) cytidine deaminases. Others remain to be elucidated. We report here deciphering of the mutational spectra and mutational signature of redox stress in ssDNA of budding yeast and the signature of aging in human mitochondrial DNA. We found that the predominance of C to T substitutions is a common feature of both signatures. Measurements of the frequencies of hydrogen peroxide-induced mutations in proofreading-defective yeast mutants supported the conclusion that hydrogen peroxide-induced mutagenesis is not the result of increased DNA polymerase misincorporation errors but rather is caused by direct damage to DNA. Proteins involved in modulation of chromatin status play a significant role in prevention of redox stress-induced mutagenesis, possibly by facilitating protection through modification of chromatin structure. These findings provide an opportunity for the search and identification of the mutational signature of redox stress in cancers and in other pathological conditions and could potentially be used for informing therapeutic decisions. In addition, the discovery of such signatures that may be present in related organisms should also advance our understanding of evolution.

Resveratrol improved the developmental potential of oocytes after vitrification by modifying the epigenetics

Resveratrol (Res) has been reported to be able to improve oocyte vitrification because of its antioxidative properties. The objective of this study was to further assess the positive effect of Res addition on the developmental potential of vitrified mouse oocytes from the perspective of epigenetic alterations. First, 2 μM Res was chosen as the optimal concentration on the basis of its effects on survival and its antioxidative properties. We found that Res addition significantly promoted fertilization (63.8% vs. 42.9%) and blastocyst formation (68.3% vs. 50.2%) after oocyte vitrification. The quality of the derived blastocysts was also higher after Res treatment. Regarding epigenetic aspects, the expression of the important deacetylase SIRT1 was found to decrease significantly upon vitrification, but it was rescued by Res. The abnormal levels of H3K9 acetylation and DNA methylation in vitrified oocytes were restored by Res addition. Moreover, the expression of several imprinted genes was affected by oocyte vitrification. Among them, abnormal Gtl2 and Peg3 expression levels were restored by Res addition. Therefore, the methylation of their imprinted control regions (ICRs) was examined. Surprisingly, the abnormal patterns of Gtl2 and Peg3 methylation in blastocysts developed from vitrified oocytes were both restored by Res addition. Finally, the full-term embryonic development showed that the birth rate was improved significantly by Res addition (56.2% vs. 38.1%). Collectively, Res was beneficial for the pre- and postimplantation embryonic development. Except for the antioxidative activity, Res also played a role in the correction of some abnormal epigenetic modifications caused by oocyte vitrification.

How Does Fusarium oxysporum Sense and Respond to Nicotinaldehyde, an Inhibitor of the NAD+ Salvage Biosynthesis Pathway?

Plant pathogenic fungi are a major threat to food security and impose a severe economic burden, thus there is a continuous need to develop new strategies to manage them. NAD+ is a co-factor in numerous enzymatic activities and determines the metabolic fate of the cell. Therefore, maintenance of NAD+ concentration is important for cellular viability. Consequently, the NAD+ biosynthetic pathway and redox homeostasis was suggested as a target for antifungal development. We aimed to study how Fusarium oxysporum senses and responds to nicotinaldehyde (NA), an inhibitor of Pnc1, a key enzyme in the salvage pathway of NAD+ biosynthesis. We were able to show that NA was inhibitory in high concentrations to several fungal plant pathogens, with much milder effects on tomato growth. Under low nutrient conditions NA reduced the total amounts of NAD+ in the fungal cell, a trend that was also observed in rich media, although without statistical significance. In low and high nutrient availability NA dramatically reduced the NAD+/NADH ratio. After exposure to NA, NADH levels were increased and NAD+ levels and the biomass were greatly reduced. Cells responded to NA by up-regulation of oxidoreductases, with hardly any up-regulation of the classic response to oxidative stress. Direct measurement of oxidative stress response showed that unlike formaldehyde and hydrogen peroxide, NA caused reductive rather than oxidative stress. Surprisingly, alcohol dehydrogenases were significantly up-regulated more than any other dehydrogenases, including aldehyde dehydrogenases. We propose that conidia of F. oxysporum efficiently detoxified the aldehyde group of NA by reducing NAD+ to NADH; the high concentrations of the latter provoked the expression of alcohol dehydrogenases that in yeast can act to reduce NADH and increase NAD+ amounts, respectively. Overall, the results suggest that targeting NAD+ biosynthesis pathway and redox homeostasis can be a potential approach to manage fungal plant pathogens. Many of the natural antifungal compounds produced by bio-control agents or even the natural biome are aldehydes, and thus the results presented here predict the possible response of Fusarium to wide sources of toxicity in the environment.

Guidelines for DNA recombination and repair studies: Cellular assays of DNA repair pathways

Understanding the plasticity of genomes has been greatly aided by assays for recombination, repair and mutagenesis. These assays have been developed in microbial systems that provide the advantages of genetic and molecular reporters that can readily be manipulated. Cellular assays comprise genetic, molecular, and cytological reporters. The assays are powerful tools but each comes with its particular advantages and limitations. Here the most commonly used assays are reviewed, discussed, and presented as the guidelines for future studies.

Role of the Histone Acetyltransferase Rtt109 in Development and Pathogenicity of the Rice Blast Fungus

Acetylation of histone H3 lysine 56 (H3K56) by the fungal-specific histone acetyltransferase Rtt109 plays important roles in maintaining genome integrity and surviving DNA damage. Here, we investigated the implications of Rtt109-mediated response to DNA damage on development and pathogenesis of the rice blast fungus Magnaporthe oryzae (anamorph: Pyricularia oryzae). The ortholog of Rtt109 in M. oryzae (MoRtt109) was found via sequence homology and its functionality was confirmed by phenotypic complementation of the Saccharomyces cerevisiae Rtt109 deletion strain. Targeted deletion of MoRtt109 resulted in a significant reduction in acetylation of H3K56 and rendered the fungus defective in hyphal growth and asexual reproduction. Furthermore, the deletion mutant displayed hypersensitivity to genotoxic agents, confirming the conserved importance of Rtt109 in genome integrity maintenance and genotoxic stress tolerance. Elevated expression of DNA repair genes and the results of the comet assay were consistent with constitutive endogenous DNA damage. Although the conidia produced from the mutant were not impaired in germination and appressorium morphogenesis, the mutant was significantly less pathogenic on rice leaves. Transcriptomic analysis provided insight into the factors underlying phenotypic defects that are associated with deficiency of H3K56 acetylation. Overall, our results indicate that MoRtt109 is a conserved histone acetyltransferase that affects proliferation and asexual fecundity of M. oryzae through maintenance of genome integrity and response to DNA damage.

The Main Role of Srs2 in DNA Repair Depends on Its Helicase Activity, Rather than on Its Interactions with PCNA or Rad51

Homologous recombination (HR) is a mechanism that repairs a variety of DNA lesions. Under certain circumstances, however, HR can generate intermediates that can interfere with other cellular processes such as DNA transcription or replication. Cells have therefore developed pathways that abolish undesirable HR intermediates. The Saccharomyces cerevisiae yeast Srs2 helicase has a major role in one of these pathways. Srs2 also works during DNA replication and interacts with the clamp PCNA. The relative importance of Srs2’s helicase activity, Rad51 removal function, and PCNA interaction in genome stability remains unclear. We created a new SRS2 allele [srs2(1-850)] that lacks the whole C terminus, containing the interaction site for Rad51 and PCNA and interactions with many other proteins. Thus, the new allele encodes an Srs2 protein bearing only the activity of the DNA helicase. We find that the interactions of Srs2 with Rad51 and PCNA are dispensable for the main role of Srs2 in the repair of DNA damage in vegetative cells and for proper completion of meiosis. On the other hand, it has been shown that in cells impaired for the DNA damage tolerance (DDT) pathways, Srs2 generates toxic intermediates that lead to DNA damage sensitivity; we show that this negative Srs2 activity requires the C terminus of Srs2. Dissection of the genetic interactions of the srs2(1-850) allele suggest a role for Srs2’s helicase activity in sister chromatid cohesion. Our results also indicate that Srs2’s function becomes more central in diploid cells.

Homologous recombination (HR) is a key mechanism that repairs damaged DNA. However, this process has to be tightly regulated; failure to regulate it can lead to genome instability. The Srs2 helicase is considered a regulator of HR; it was shown to be able to evict the recombinase Rad51 from DNA. Cells lacking Srs2 exhibit sensitivity to DNA-damaging agents, and in some cases, they display defects in DNA replication. The relative roles of the helicase and Rad51 removal activities of Srs2 in genome stability remain unclear. To address this question, we created a new Srs2 mutant which has only the DNA helicase domain. Our study shows that only the DNA helicase domain is needed to deal with DNA damage and assist in DNA replication during vegetative growth and in meiosis. Thus, our findings shift the view on the role of Srs2 in the maintenance of genome integrity.

The Yeast Prefoldin Bud27

Bud27 and its human orthologue URI (unconventional prefoldin RPB5-interactor) are members of the prefoldin (PFD) family of ATP-independent molecular chaperones binding the Rpb5 subunit to all three nuclear eukaryotic RNA polymerases (RNA pols). Bud27/URI are considered to function as a scaffold protein able to assemble additional members of the prefoldin (PDF) family in both human and yeast. Bud27 and URI are not subunits of the canonical PFD/GimC complex and not only the composition but also other functions independent of the PFD/GimC complex have been described for Bud27 and URI. Bud27 interacts only with Pfd6 but no other components of the R2TP/PFDL. Furthermore previously reported interaction between Bud27 and Pfd2 was not later confirmed. These results point to major differences in the prefoldin-like complex composition between yeast and other organisms, suggesting also important differences in functions. Furthermore, this assumption could be extended to the R2TP/PFDL complex, which has been shown to differ between different organisms and has not been identified in yeast. This casts doubt on whether Bud27 cooperation with prefoldin and other components of the R2TP/PFDL modules are required for its action. This could be extended to URI and point to a role of Bud27/URI in cell functions more relevant than this previously proposed as co-prefoldin.

Genome-wide mapping of sister chromatid exchange events in single yeast cells using Strand-seq

Homologous recombination involving sister chromatids is the most accurate, and thus most frequently used, form of recombination-mediated DNA repair. Despite its importance, sister chromatid recombination is not easily studied because it does not result in a change in DNA sequence, making recombination between sister chromatids difficult to detect. We have previously developed a novel DNA template strand sequencing technique, called Strand-seq, that can be used to map sister chromatid exchange (SCE) events genome-wide in single cells. An increase in the rate of SCE is an indicator of elevated recombination activity and of genome instability, which is a hallmark of cancer. In this study, we have adapted Strand-seq to detect SCE in the yeast Saccharomyces cerevisiae. We provide the first quantifiable evidence that most spontaneous SCE events in wild-type cells are not due to the repair of DNA double-strand breaks.

Pathways and Mechanisms that Prevent Genome Instability in Saccharomyces cerevisiae

Genome rearrangements result in mutations that underlie many human diseases, and ongoing genome instability likely contributes to the development of many cancers. The tools for studying genome instability in mammalian cells are limited, whereas model organisms such as Saccharomyces cerevisiae are more amenable to these studies. Here, we discuss the many genetic assays developed to measure the rate of occurrence of Gross Chromosomal Rearrangements (called GCRs) in S. cerevisiae. These genetic assays have been used to identify many types of GCRs, including translocations, interstitial deletions, and broken chromosomes healed by de novo telomere addition, and have identified genes that act in the suppression and formation of GCRs. Insights from these studies have contributed to the understanding of pathways and mechanisms that suppress genome instability and how these pathways cooperate with each other. Integrated models for the formation and suppression of GCRs are discussed.

A new role for Rrm3 in repair of replication-born DNA breakage by sister chromatid recombination

Replication forks stall at different DNA obstacles such as those originated by transcription. Fork stalling can lead to DNA double-strand breaks (DSBs) that will be preferentially repaired by homologous recombination when the sister chromatid is available. The Rrm3 helicase is a replisome component that promotes replication upon fork stalling, accumulates at highly transcribed regions and prevents not only transcription-induced replication fork stalling but also transcription-associated hyper-recombination. This led us to explore the possible role of Rrm3 in the repair of DSBs when originating at the passage of the replication fork. Using a mini-HO system that induces mainly single-stranded DNA breaks, we show that rrm3Δ cells are defective in DSB repair. The defect is clearly seen in sister chromatid recombination, the major repair pathway of replication-born DSBs. Our results indicate that Rrm3 recruitment to replication-born DSBs is crucial for viability, uncovering a new role for Rrm3 in the repair of broken replication forks.

DNA replication needs to be precise to ensure cell survival and to avoid genetic instability. Different DNA obstacles, such as those originated by transcription, frequently hamper replication fork progression leading to fork stalling or even fork breakage. This requires the homologous recombination machinery to repair the damage. Here, we uncovered a role for yeast Rrm3, a replisome component known to promote replication upon fork stalling, in the repair of replication-born double strand breaks. In particular, rrm3Δ cells show a defect in the recombination with the sister chromatid, the preferred template for the maintenance of genome integrity. Our results support the possibility that the known accumulation of Rrm3 at sites of active transcription reflects an active role of Rrm3 in the repair of broken forks.

Holliday junction trap shows how cells use recombination and a junction-guardian role of RecQ helicase

DNA repair by homologous recombination (HR) underpins cell survival and fuels genome instability, cancer, and evolution. However, the main kinds and sources of DNA damage repaired by HR in somatic cells and the roles of important HR proteins remain elusive. We present engineered proteins that trap, map, and quantify Holliday junctions (HJs), a central DNA intermediate in HR, based on catalytically deficient mutant RuvC protein of Escherichia coli. We use RuvCDefGFP (RDG) to map genomic footprints of HR at defined DNA breaks in E. coli and demonstrate genome-scale directionality of double-strand break (DSB) repair along the chromosome. Unexpectedly, most spontaneous HR-HJ foci are instigated, not by DSBs, but rather by single-stranded DNA damage generated by replication. We show that RecQ, the E. coli ortholog of five human cancer proteins, nonredundantly promotes HR-HJ formation in single cells and, in a novel junction-guardian role, also prevents apparent non-HR-HJs promoted by RecA overproduction. We propose that one or more human RecQ orthologs may act similarly in human cancers overexpressing the RecA ortholog RAD51 and find that cancer genome expression data implicate the orthologs BLM and RECQL4 in conjunction with EME1 and GEN1 as probable HJ reducers in such cancers. Our results support RecA-overproducing E. coli as a model of the many human tumors with up-regulated RAD51 and provide the first glimpses of important, previously elusive reaction intermediates in DNA replication and repair in single living cells.

Nicotinamide Suppresses the DNA Damage Sensitivity of Saccharomyces cerevisiae Independently of Sirtuin Deacetylases

Nicotinamide is both a reaction product and an inhibitor of the conserved sirtuin family of deacetylases, which have been implicated in a broad range of cellular functions in eukaryotes from yeast to humans. Phenotypes observed following treatment with nicotinamide are most often assumed to stem from inhibition of one or more of these enzymes. Here, we used this small molecule to inhibit multiple sirtuins at once during treatment with DNA damaging agents in the Saccharomyces cerevisiae model system. Since sirtuins have been previously implicated in the DNA damage response, we were surprised to observe that nicotinamide actually increased the survival of yeast cells exposed to the DNA damage agent MMS. Remarkably, we found that enhanced resistance to MMS in the presence of nicotinamide was independent of all five yeast sirtuins. Enhanced resistance was also independent of the nicotinamide salvage pathway, which uses nicotinamide as a substrate to generate NAD+, and of a DNA damage-induced increase in the salvage enzyme Pnc1 Our data suggest a novel and unexpected function for nicotinamide that has broad implications for its use in the study of sirtuin biology across model systems.

Multi-BRCT scaffolds use distinct strategies to support genome maintenance

Genome maintenance requires coordinated actions of diverse DNA metabolism processes. Scaffolding proteins, such as those containing multiple BRCT domains, can influence these processes by collaborating with numerous partners. The best-studied examples of multi-BRCT scaffolds are the budding yeast Dpb11 and its homologues in other organisms, which regulate DNA replication, repair, and damage checkpoints. Recent studies have shed light on another group of multi-BRCT scaffolds, including Rtt107 in budding yeast and related proteins in other organisms. These proteins also influence several DNA metabolism pathways, though they use strategies unlike those employed by the Dpb11 family of proteins. Yet, at the same time, these 2 classes of multi-BRCT proteins can collaborate under specific situations. This review summarizes recent advances in our understanding of how these multi-BRCT proteins function in distinct manners and how they collaborate, with a focus on Dpb11 and Rtt107.

Multiple Rad52-Mediated Homology-Directed Repair Mechanisms Are Required to Prevent Telomere Attrition-Induced Senescence in Saccharomyces cerevisiae

Most human somatic cells express insufficient levels of telomerase, which can result in telomere shortening and eventually senescence, both of which are hallmarks of ageing. Homology-directed repair (HDR) is important for maintaining proper telomere function in yeast and mammals. In Saccharomyces cerevisiae, Rad52 is required for almost all HDR mechanisms, and telomerase-null cells senesce faster in the absence of Rad52. However, its role in preventing accelerated senescence has been unclear. In this study, we make use of rad52 separation-of-function mutants to find that multiple Rad52-mediated HDR mechanisms are required to delay senescence, including break-induced replication and sister chromatid recombination. In addition, we show that misregulation of histone 3 lysine 56 acetylation, which is known to be defective in sister chromatid recombination, also causes accelerated senescence. We propose a model where Rad52 is needed to repair telomere attrition-induced replication stress.

Telomeres are essential structures located at the ends of chromosomes. The canonical DNA replication machinery is unable to fully replicate DNA at chromosome ends, causing telomeres to shorten with every round of cell division. This shortening can be counteracted by an enzyme called telomerase, but in most human somatic cells, there is insufficient expression of telomerase to prevent telomere shortening. Cells with critically short telomeres can enter an arrested state known as senescence. Telomere attrition has been identified as a hallmark of human ageing. Homologous recombination proteins are important for proper telomere function in yeast and mammals. Yeast lacking both telomerase and Rad52, required for almost all recombination, exhibits accelerated senescence, yet no apparent increase in the rate of telomere shortening. In this study, we explore the role of Rad52 during senescence by taking advantage of rad52 separation-of-function mutants. We find that Rad52 acts in multiple ways to overcome DNA replication problems at telomeres. Impediments to telomere replication can be dealt with by post-replication repair mechanisms, which use a newly synthesized sister chromatid as a template to replicate past the impediment, while telomere truncations, likely caused by the collapse of replication forks, can be extended by break-induced replication.

The Knowns Unknowns: Exploring the Homologous Recombination Repair Pathway in Toxoplasma gondii

Toxoplasma gondii is an apicomplexan parasite of medical and veterinary importance which causes toxoplasmosis in humans. Great effort is currently being devoted toward the identification of novel drugs capable of targeting such illness. In this context, we believe that the thorough understanding of the life cycle of this model parasite will facilitate the identification of new druggable targets in T. gondii. It is important to exploit the available knowledge of pathways which could modulate the sensitivity of the parasite to DNA damaging agents. The homologous recombination repair (HRR) pathway may be of particular interest in this regard as its inactivation sensitizes other cellular models such as human cancer to targeted therapy. Herein we discuss the information available on T. gondii's HRR pathway from the perspective of its conservation with respect to yeast and humans. Special attention was devoted to BRCT domain-containing and end-resection associated proteins in T. gondii as in other experimental models such proteins have crucial roles in early/late steps or HRR and in the pathway choice for double strand break resolution. We conclude that T. gondii HRR pathway is a source of several lines of investigation that allow to to comprehend the extent of diversification of HRR in T. gondii. Such an effort will serve to determine if HRR could represent a potential targer for the treatment of toxoplasmosis.

Boric acid-dependent decrease in regulatory histone H3 acetylation is not mutagenic in yeast

Candida albicans is a dimorphic yeast commonly found on human mucosal membranes that switches from yeast to hyphal morphology in response to environmental factors. The change to hyphal growth requires histone H3 modifications by the yeast-specific histone acetyltransferase Rtt109. In addition to its role in morphogenesis, Rtt109-dependent acetylation of histone H3 lysine residues 9 and 56 has regulatory functions during DNA replication and repair. Boric acid (BA) is a broad-spectrum agent that specifically inhibits C. albicans hyphal growth, locking the fungus in its harmless commensal yeast state. The present study characterizes the effect of BA on C. albicans histone acetylation in respect to specificity, time-course and significance. We demonstrate that sublethal concentrations of BA reduce H3K9/H3K56 acetylation, both on a basal level and in response to genotoxic stress. Acetylation at other selected histone sites were not affected by BA. qRT-PCR expression analysis of the DNA repair gene Rad51 indicated no elevated level of genotoxic stress during BA exposure. A forward-mutation analysis demonstrated the BA does not increase spontaneous or induced mutations. The findings suggest that DNA repair remains effective even when histone H3 acetylation decreases and dispels the notion that BA treatment impairs genome integrity in yeast.

Functions of Ubiquitin and SUMO in DNA Replication and Replication Stress

Complete and faithful duplication of its entire genetic material is one of the essential prerequisites for a proliferating cell to maintain genome stability. Yet, during replication DNA is particularly vulnerable to insults. On the one hand, lesions in replicating DNA frequently cause a stalling of the replication machinery, as most DNA polymerases cannot cope with defective templates. This situation is aggravated by the fact that strand separation in preparation for DNA synthesis prevents common repair mechanisms relying on strand complementarity, such as base and nucleotide excision repair, from working properly. On the other hand, the replication process itself subjects the DNA to a series of hazardous transformations, ranging from the exposure of single-stranded DNA to topological contortions and the generation of nicks and fragments, which all bear the risk of inducing genomic instability. Dealing with these problems requires rapid and flexible responses, for which posttranslational protein modifications that act independently of protein synthesis are particularly well suited. Hence, it is not surprising that members of the ubiquitin family, particularly ubiquitin itself and SUMO, feature prominently in controlling many of the defensive and restorative measures involved in the protection of DNA during replication. In this review we will discuss the contributions of ubiquitin and SUMO to genome maintenance specifically as they relate to DNA replication. We will consider cases where the modifiers act during regular, i.e., unperturbed stages of replication, such as initiation, fork progression, and termination, but also give an account of their functions in dealing with lesions, replication stalling and fork collapse.