Fission Yeast Genome Research Articles

Spreading and epigenetic inheritance of heterochromatin require a critical density of histone H3 lysine 9 tri-methylation

Heterochromatin assembly requires methylation of histone H3 lysine 9 (H3K9me) and serves as a paradigm for understanding the importance of histone modifications in epigenetic genome control. Heterochromatin is nucleated at specific genomic sites and spreads across extended chromosomal domains to promote gene silencing. Moreover, heterochromatic structures can be epigenetically inherited in a self-templating manner, which is critical for stable gene repression. The spreading and inheritance of heterochromatin are believed to be dependent on preexisting H3K9 tri-methylation (H3K9me3), which is recognized by the histone methyltransferase Clr4/Suv39h via its chromodomain, to promote further deposition of H3K9me. However, the process involving the coupling of the "read" and "write" capabilities of histone methyltransferases is poorly understood. From an unbiased genetic screen, we characterize a dominant-negative mutation in histone H3 (H3G13D) that impairs the propagation of endogenous and ectopic heterochromatin domains in the fission yeast genome. H3G13D blocks methylation of H3K9 by the Clr4/Suv39h methyltransferase and acts in a dosage-dependent manner to interfere with the spreading and maintenance of heterochromatin. Our analyses show that the incorporation of unmethylatable histone H3G13D into chromatin decreases H3K9me3 density and thereby compromises the read-write capability of Clr4/Suv39h. Consistently, enhancing the affinity of Clr4/Suv39h for methylated H3K9 is sufficient to overcome the defects in heterochromatin assembly caused by H3G13D Our work directly implicates methylated histones in the transmission of epigenetic memory and shows that a critical density threshold of H3K9me3 is required to promote epigenetic inheritance of heterochromatin through the read-write mechanism.

LncRNA transcription induces meiotic recombination through chromatin remodelling in fission yeast

Noncoding RNAs (ncRNAs) are involved in various biological processes, including gene expression, development, and disease. Here, we identify a novel consensus sequence of a cis-element involved in long ncRNA (lncRNA) transcription and demonstrate that lncRNA transcription from this cis-element activates meiotic recombination via chromatin remodeling. In the fission yeast fbp1 gene, glucose starvation induces a series of promoter-associated lncRNAs, referred to as metabolic-stress-induced lncRNAs (mlonRNAs), which contribute to chromatin remodeling and fbp1 activation. Translocation of the cis-element required for mlonRNA into a well-characterized meiotic recombination hotspot, ade6-M26, further stimulates transcription and meiotic recombination via local chromatin remodeling. The consensus sequence of this cis-element (mlon-box) overlaps with meiotic recombination sites in the fission yeast genome. At one such site, the SPBC24C6.09c upstream region, meiotic double-strand break (DSB) formation is induced in an mlon-box-dependent manner. Therefore, mlonRNA transcription plays a universal role in chromatin remodeling and the regulation of transcription and recombination.

In silicoanalysis of DNA re-replication across a complete genome reveals cell-to-cell heterogeneity and genome plasticity

DNA replication is a complex and remarkably robust process: despite its inherent uncertainty, manifested through stochastic replication timing at a single-cell level, multiple control mechanisms ensure its accurate and timely completion across a population. Disruptions in these mechanisms lead to DNA re-replication, closely connected to genomic instability and oncogenesis. Here, we present a stochastic hybrid model of DNA re-replication that accurately portrays the interplay between discrete dynamics, continuous dynamics and uncertainty. Using experimental data on the fission yeast genome, model simulations show how different regions respond to re-replication and permit insight into the key mechanisms affecting re-replication dynamics. Simulated and experimental population-level profiles exhibit a good correlation along the genome, robust to model parameters, validating our approach. At a single-cell level, copy numbers of individual loci are affected by intrinsic properties of each locus, in cis effects from adjoining loci and in trans effects from distant loci. In silico analysis and single-cell imaging reveal that cell-to-cell heterogeneity is inherent in re-replication and can lead to genome plasticity and a plethora of genotypic variations.

Bioinformatical dissection of fission yeast DNA replication origins.

Replication origins in eukaryotes form a base for assembly of the pre-replication complex (pre-RC), thereby serving as an initiation site of DNA replication. Characteristics of replication origin vary among species. In fission yeast Schizosaccharomyces pombe, DNA of high AT content is a distinct feature of replication origins; however, it remains to be understood what the general molecular architecture of fission yeast origin is. Here, we performed ChIP-seq mapping of Orc4 and Mcm2, two representative components of the pre-RC, and described the characteristics of their binding sites. The analysis revealed that fission yeast efficient origins are associated with two similar but independent features: a ≥15 bp-long motif with stretches of As and an AT-rich region of a few hundred bp. The A-rich motif was correlated with chromosomal binding of Orc, a DNA-binding component in the pre-RC, whereas the AT-rich region was associated with efficient binding of the DNA replicative helicase Mcm. These two features, in combination with the third feature, a transcription-poor region of approximately 1 kb, enabled to distinguish efficient replication origins from the rest of chromosome arms with high accuracy. This study, hence, provides a model that describes how multiple functional elements specify DNA replication origins in fission yeast genome.

Detection of cellular G-quadruplex by using a loop structure as a structural determinant

G-quadrupex is now known to play crucial roles in various biological reactions. However, direct evidence for its presence in cells has been limited, due to the lack of versatile and non-biased methodology. We use Rif1 binding sites on the fission yeast genome, which has been shown to adopt G4 structures, as a model to prove that Rif1 BS indeed adopt G4 structure in cells. We take advantage of the presence of a single-stranded loop in the G4 structure. Rif1BS is unique in that they contain unusually long loop sequences, and we replace them with a 18 bp I-SceI restriction site. We show in vitro that I-SceI in the loop is not cleaved when G4 is formed on duplex Rif1BS DNA, but is cleaved when G4 is not formed due to a mutation in the G-tracts. This is observed both heat-induced and transcription-induced G4 structure, and gives proof of evidence for this procedure. We apply this strategy for detection of a G4 structure at the same Rif1BS in fission yeast cells. We present evidence that in vivo cleavage of I-SceI can be a measure for the presence of G4 at the target sequence in cells as well. The method described here gives a platform strategy for genome-wide analyses of cellular G4 and their dynamic formation and disruption.

Positioning Heterochromatin at the Nuclear Periphery Suppresses Histone Turnover to Promote Epigenetic Inheritance

In eukaryotes, heterochromatin is generally located at the nuclear periphery. This study investigates the biological significance of perinuclear positioning for heterochromatin maintenance and gene silencing. We identify the nuclear rim protein Amo1NUPL2 as a factor required for the propagation of heterochromatin at endogenous and ectopic sites in the fission yeast genome. Amo1 associates with the Rix1PELP1-containing RNA processing complex RIXC and with the histone chaperone complex FACT. RIXC, which binds to heterochromatin protein Swi6HP1 across silenced chromosomal domains and to surrounding boundary elements, connects heterochromatin with Amo1 at the nuclear periphery. In turn, the Amo1-enriched subdomain is critical for Swi6 association with FACT that precludes histone turnover to promote gene silencing and preserve epigenetic stability ofheterochromatin. In addition to uncovering conserved factors required for perinuclear positioning of heterochromatin, these analyses elucidate a mechanism by which a peripheral subdomain enforces stable gene repression and maintains heterochromatin in a heritable manner.

Mkt1 is required for RNAi-mediated silencing and establishment of heterochromatin in fission yeast.

Constitutive domains of repressive heterochromatin are maintained within the fission yeast genome through self-reinforcing mechanisms involving histone methylation and small RNAs. Non-coding RNAs generated from heterochromatic regions are processed into small RNAs by the RNA interference pathway, and are subject to silencing through both transcriptional and post-transcriptional mechanisms. While the pathways involved in maintenance of the repressive heterochromatin state are reasonably well understood, less is known about the requirements for its establishment. Here, we describe a novel role for the post-transcriptional regulatory factor Mkt1 in establishment of heterochromatin at pericentromeres in fission yeast. Loss of Mkt1 does not affect maintenance of existing heterochromatin, but does affect its recovery following depletion, as well as de novo establishment of heterochromatin on a mini-chromosome. Pathway dissection revealed that Mkt1 is required for RNAi-mediated post-transcriptional silencing, downstream of small RNA production. Mkt1 physically associates with pericentromeric transcripts, and is additionally required for maintenance of silencing and heterochromatin at centromeres when transcriptional silencing is impaired. Our findings provide new insight into the mechanism of RNAi-mediated post-transcriptional silencing in fission yeast, and unveil an important role for post-transcriptional silencing in establishment of heterochromatin that is dispensable when full transcriptional silencing is imposed.

Unprogrammed epigenetic variation mediated by stochastic formation of ectopic heterochromatin.

Changes in gene expression via chromatin-mediated mechanisms are important for reprogramming and differentiation, but uncontrolled changes can potentially lead to harmful or adaptive phenotypic alteration. Thus, diversification of the genome-wide chromatin state must be strictly limited, but the underlying mechanism of this regulation is largely unknown. In this review, we focused on distribution of heterochromatin, a tight chromatin structure that negatively regulates gene expression. Heterochromatin is characterized by methylation of histone H3 at lysine 9, and its formation and spreading are controlled by H3K9-specific methyltransferases and reversal factors such as histone demethylases. We summarize recent findings and discuss how variability in the heterochromatin distribution is controlled in the unicellular eukaryote fission yeast. In this context, we recently found that the anti-silencing factor Epe1 plays a key role in the formation of the individual-specific heterochromatin distribution. In conclusion, recent studies revealed that there are many potential heterochromatin formation sites in the fission yeast genome, and several proteins contribute to suppression of spreading and genome-wide dispersal of heterochromatin; knowledge from fission yeast studies may provide insights into the mechanisms regulating epigenetic diversification in multicellular eukaryotes.

Rif1 promotes association of G-quadruplex (G4) by its specific G4 binding and oligomerization activities

Rif1 is a conserved protein regulating replication timing and binds preferentially to the vicinity of late-firing/dormant origins in fission yeast. The Rif1 binding sites on the fission yeast genome have an intrinsic potential to generate G-quadruplex (G4) structures to which purified Rif1 preferentially binds. We previously proposed that Rif1 generates chromatin architecture that may determine replication timing by facilitating the chromatin loop formation. Here, we conducted detailed biochemical analyses on Rif1 and its G4 binding. Rif1 prefers sequences containing long stretches of guanines and binds preferentially to the multimeric G4 of parallel or hybrid/mix topology. Rif1 forms oligomers and binds simultaneously to multiple G4. We present a model on how Rif1 may facilitate the formation of chromatin architecture through its G4 binding and oligomerization properties.

Fitness Landscape of the Fission Yeast Genome.

The relationship between DNA sequence, biochemical function, and molecular evolution is relatively well-described for protein-coding regions of genomes, but far less clear in noncoding regions, particularly, in eukaryote genomes. In part, this is because we lack a complete description of the essential noncoding elements in a eukaryote genome. To contribute to this challenge, we used saturating transposon mutagenesis to interrogate the Schizosaccharomyces pombe genome. We generated 31 million transposon insertions, a theoretical coverage of 2.4 insertions per genomic site. We applied a five-state hidden Markov model (HMM) to distinguish insertion-depleted regions from insertion biases. Both raw insertion-density and HMM-defined fitness estimates showed significant quantitative relationships to gene knockout fitness, genetic diversity, divergence, and expected functional regions based on transcription and gene annotations. Through several analyses, we conclude that transposon insertions produced fitness effects in 66–90% of the genome, including substantial portions of the noncoding regions. Based on the HMM, we estimate that 10% of the insertion depleted sites in the genome showed no signal of conservation between species and were weakly transcribed, demonstrating limitations of comparative genomics and transcriptomics to detect functional units. In this species, 3′- and 5′-untranslated regions were the most prominent insertion-depleted regions that were not represented in measures of constraint from comparative genomics. We conclude that the combination of transposon mutagenesis, evolutionary, and biochemical data can provide new insights into the relationship between genome function and molecular evolution.

Immediate visualization of recombination events and chromosome segregation defects in fission yeast meiosis

Schizosaccharomyces pombe, also known as fission yeast, is an established model for studying chromosome biological processes. Over the years, research employing fission yeast has made important contributions to our knowledge about chromosome segregation during meiosis, as well as meiotic recombination and its regulation. Quantification of meiotic recombination frequency is not a straightforward undertaking, either requiring viable progeny for a genetic plating assay, or relying on laborious Southern blot analysis of recombination intermediates. Neither of these methods lends itself to high-throughput screens to identify novel meiotic factors. Here, we establish visual assays novel to Sz. pombe for characterizing chromosome segregation and meiotic recombination phenotypes. Genes expressing red, yellow, and/or cyan fluorophores from spore-autonomous promoters have been integrated into the fission yeast genomes, either close to the centromere of chromosome 1 to monitor chromosome segregation, or on the arm of chromosome 3 to form a genetic interval at which recombination frequency can be determined. The visual recombination assay allows straightforward and immediate assessment of the genetic outcome of a single meiosis by epi-fluorescence microscopy without requiring tetrad dissection. We also demonstrate that the recombination frequency analysis can be automatized by utilizing imaging flow cytometry to enable high-throughput screens. These assays have several advantages over traditional methods for analyzing meiotic phenotypes.

Phosphorylation of Arp2 is not essential for Arp2/3 complex activity in fission yeast.

LeClaire et al presented evidence that phosphorylation of three sites on the Arp2 subunit activates the Arp2/3 complex to nucleate actin filaments. We mutated the homologous residues of Arp2 (Y198, T233, and T234) in the fission yeast genome to amino acids that preclude or mimic phosphorylation. Arp2/3 complex is essential for the viability of fission yeast, yet strains unable to phosphorylate these sites grew normally. Y198F/T233A/T234A Arp2 was only nonfunctional if GFP-tagged, as observed by LeClaire et al in Drosophila cells. Replacing both T233 and T234 with aspartic acid was lethal, suggesting that phosphorylation might be inhibitory. Nevertheless, blocking phosphorylation at these sites had the same effect as mimicking it: slowing assembly of endocytic actin patches. Mass spectrometry revealed phosphorylation at a fourth conserved Arp2 residue, Y218, but both blocking and mimicking phosphorylation of Y218 only slowed actin patch assembly slightly. Therefore, phosphorylation of Y198, T233, T234, and Y218 is not required for the activity of fission yeast Arp2/3 complex.

Ccp1 modulates epigenetic stability at centromeres and affects heterochromatin distribution in Schizosaccharomyces pombe

Distinct chromatin organization features, such as centromeres and heterochromatin domains, are inherited epigenetically. However, the mechanisms that modulate the accuracy of epigenetic inheritance, especially at the individual nucleosome level, are not well-understood. Here, using ChIP and next-generation sequencing (ChIP-Seq), we characterized Ccp1, a homolog of the histone chaperone Vps75 in budding yeast that functions in centromere chromatin duplication and heterochromatin maintenance in fission yeast (Schizosaccharomyces pombe). We show that Ccp1 is enriched at the central core regions of the centromeres. Of note, among all histone chaperones characterized, deletion of the ccp1 gene uniquely reduced the rate of epigenetic switching, manifested as position effect variegation within the centromeric core region (CEN-PEV). In contrast, gene deletion of other histone chaperones either elevated the PEV switching rates or did not affect centromeric PEV. Ccp1 and the kinetochore components Mis6 and Sim4 were mutually dependent for centromere or kinetochore association at the proper levels. Moreover, Ccp1 influenced heterochromatin distribution at multiple loci in the genome, including the subtelomeric and the pericentromeric regions. We also found that Gar2, a protein predominantly enriched in the nucleolus, functions similarly to Ccp1 in modulating the epigenetic stability of centromeric regions, although its mechanism remained unclear. Together, our results identify Ccp1 as an important player in modulating epigenetic stability and maintaining proper organization of multiple chromatin domains throughout the fission yeast genome.

Utilizing an Optogenetic System for Manipulating Protein Localization in Fission Yeast

The cellular function of a protein is largely dependent upon proper localization, particularly in complex protein networks that regulate stages of cell division. Many of these regulatory proteins are highly conserved and essential for cell viability. As a result, manipulating protein function in vivo often involves controlling expression levels using inducible promoters or generating point mutations to disrupt function, both of which have limited temporal resolution for observing phenotypes. An emerging technology to manipulate the function of essential proteins involves the field of optogenetics. This approach utilizes light‐inducible dimerization modules to enable rapid, reversible spatial and temporal manipulation of in vivo protein localization. Transiently altering protein localization to temporarily disrupt protein function can provide insight into how that protein contributes to a cellular pathway.In this study, we utilize molecular cloning to adapt a CRY2‐CIBN optogenetic light‐inducible dimerization system into common fission yeast expression vectors. These vectors enable us to incorporate genetically encoded cryptochrome fusion tags onto genes of interest in their native locus of the fission yeast genome using homologous recombination. We can then induce dimerization of the CRY2‐CIBN domains in the resulting tagged‐proteins in a rapid and reversible manner using 488nm light, which alters their spatio‐temporal localization in vivo allowing for direct observation of the downstream effects on cellular processes in real time. We utilize this system to manipulate localization of an essential regulatory kinase involved in cytokinesis as a method to further characterize its contribution to successful cell division. This optogenetic system is a valuable tool for studying the fission yeast cell cycle that enables researchers to better understand the role proteins play in regulation of this essential, highly conserved process.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Functions for fission yeast splicing factors SpSlu7 and SpPrp18 in alternative splice-site choice and stress-specific regulated splicing.

Budding yeast spliceosomal factors ScSlu7 and ScPrp18 interact and mediate intron 3’ss choice during second step pre-mRNA splicing. The fission yeast genome with abundant multi-intronic transcripts, degenerate splice signals and SR proteins is an apt unicellular fungal model to deduce roles for core spliceosomal factors in alternative splice-site choice, intron retention and to study the cellular implications of regulated splicing. From our custom microarray data we deduce a stringent reproducible subset of S. pombe alternative events. We examined the role of factors SpSlu7 or SpPrp18 for these splice events and investigated the relationship to growth phase and stress. Wild-type log and stationary phase cells showed ats1+ exon 3 skipped and intron 3 retained transcripts. Interestingly the non-consensus 5’ss in ats1+ intron 3 caused SpSlu7 and SpPrp18 dependent intron retention. We validated the use of an alternative 5’ss in dtd1+ intron 1 and of an upstream alternative 3’ss in DUF3074 intron 1. The dtd1+ intron 1 non-canonical 5’ss yielded an alternative mRNA whose levels increased in stationary phase. Utilization of dtd1+ intron 1 sub-optimal 5’ ss required functional SpPrp18 and SpSlu7 while compromise in SpSlu7 function alone hampered the selection of the DUF3074 intron 1 non canonical 3’ss. We analysed the relative abundance of these splice isoforms during mild thermal, oxidative and heavy metal stress and found stress-specific splice patterns for ats1+ and DUF3074 intron 1 some of which were SpSlu7 and SpPrp18 dependent. By studying ats1+ splice isoforms during compromised transcription elongation rates in wild-type, spslu7-2 and spprp18-5 mutant cells we found dynamic and intron context-specific effects in splice-site choice. Our work thus shows the combinatorial effects of splice site strength, core splicing factor functions and transcription elongation kinetics to dictate alternative splice patterns which in turn serve as an additional recourse of gene regulation in fission yeast.

Architectural alterations of the fission yeast genome during the cell cycle.

Eukaryotic genomes are highly ordered through various mechanisms, including topologically associating domain (TAD) organization. We employed an in situ Hi-C approach to follow the 3D organization of the fission yeast genome during the cell cycle. We demonstrate that during mitosis, large domains of 300 kb-1 Mb are formed by condensin. This mitotic domain organization does not suddenly dissolve, but gradually diminishes until the next mitosis. By contrast, small domains of 30-40 kb that are formed by cohesin are relatively stable across the cell cycle. Condensin and cohesin mediate long- and short-range contacts, respectively, by bridging their binding sites, thereby forming the large and small domains. These domains are inversely regulated during the cell cycle but assemble independently. Our study describes the chromosomal oscillation between the formation and decay phases of the large and small domains, and we predict that the condensin-mediated domains serve as chromosomal compaction units.

The Fission Yeast Pre-mRNA-processing Factor 18 (prp18+) Has Intron-specific Splicing Functions with Links to G1-S Cell Cycle Progression

The fission yeast genome, which contains numerous short introns, is an apt model for studies on fungal splicing mechanisms and splicing by intron definition. Here we perform a domain analysis of the evolutionarily conserved Schizosaccharomyces pombe pre-mRNA-processing factor, SpPrp18. Our mutational and biophysical analyses of the C-terminal α-helical bundle reveal critical roles for the conserved region as well as helix five. We generate a novel conditional missense mutant, spprp18-5 To assess the role of SpPrp18, we performed global splicing analyses on cells depleted of prp18+ and the conditional spprp18-5 mutant, which show widespread but intron-specific defects. In the absence of functional SpPrp18, primer extension analyses on a tfIId+ intron 1-containing minitranscript show accumulated pre-mRNA, whereas the lariat intron-exon 2 splicing intermediate was undetectable. These phenotypes also occurred in cells lacking both SpPrp18 and SpDbr1 (lariat debranching enzyme), a genetic background suitable for detection of lariat RNAs. These data indicate a major precatalytic splicing arrest that is corroborated by the genetic interaction between spprp18-5 and spprp2-1, a mutant in the early acting U2AF59 protein. Interestingly, SpPrp18 depletion caused cell cycle arrest before S phase. The compromised splicing of transcripts coding for G1-S regulators, such as Res2, a transcription factor, and Skp1, a regulated proteolysis factor, are shown. The cumulative effects of SpPrp18-dependent intron splicing partly explain the G1 arrest upon the loss of SpPrp18. Our study using conditional depletion of spprp18+ and the spprp18-5 mutant uncovers an intron-specific splicing function and early spliceosomal interactions and suggests links with cell cycle progression.

Micro-C XL: assaying chromosome conformation from the nucleosome to the entire genome.

We present Micro-C XL, an improved method for analysis of chromosome folding at mononucleosome resolution. Using long crosslinkers and isolation of insoluble chromatin, Micro-C XL increases signal-to-noise ratio. Micro-C XL maps of budding and fission yeast genomes capture both short-range chromosome fiber features such as chromosomally interacting domains and higher order features such as centromere clustering. Micro-C XL provides a single assay to interrogate chromosome folding at length scales from the nucleosome to the full genome.

Taz1-Shelterin Promotes Facultative Heterochromatin Assembly at Chromosome-Internal Sites Containing Late Replication Origins

Facultative heterochromatin regulates gene expression, but its assembly is poorly understood. Previously, we identified facultative heterochromatin islands in the fission yeast genome and found that RNA elimination machinery promotes island assembly at meiotic genes. Here, we report that Taz1, a component of the telomere protection complex Shelterin, is required to assemble heterochromatin islands at regions corresponding to late replication origins that are sites of double-strand break formation during meiosis. The loss of Taz1 or other Shelterin subunits, including Ccq1 that interacts with Clr4/Suv39h, abolishes heterochromatin at late origins and causes derepression of associated genes. Moreover, the late-origin regulator Rif1 affects heterochromatin at Taz1-dependent islands and subtelomeric regions. We explore the connection between facultative heterochromatin and replication control and show that heterochromatin machinery affects replication timing. These analyses reveal the role of Shelterin in facultative heterochromatin assembly at late origins, which has important implications for genome stability and gene regulation.

G-rich telomeric and ribosomal DNA sequences from the fission yeast genome form stable G-quadruplex DNA structures in vitro and are unwound by the Pfh1 DNA helicase.

Certain guanine-rich sequences have an inherent propensity to form G-quadruplex (G4) structures. G4 structures are e.g. involved in telomere protection and gene regulation. However, they also constitute obstacles during replication if they remain unresolved. To overcome these threats to genome integrity, organisms harbor specialized G4 unwinding helicases. In Schizosaccharomyces pombe, one such candidate helicase is Pfh1, an evolutionarily conserved Pif1 homolog. Here, we addressed whether putative G4 sequences in S. pombe can adopt G4 structures and, if so, whether Pfh1 can resolve them. We tested two G4 sequences, derived from S. pombe ribosomal and telomeric DNA regions, and demonstrated that they form inter- and intramolecular G4 structures, respectively. Also, Pfh1 was enriched in vivo at the ribosomal G4 DNA and telomeric sites. The nuclear isoform of Pfh1 (nPfh1) unwound both types of structure, and although the G4-stabilizing compound Phen-DC3 significantly enhanced their stability, nPfh1 still resolved them efficiently. However, stable G4 structures significantly inhibited adenosine triphosphate hydrolysis by nPfh1. Because ribosomal and telomeric DNA contain putative G4 regions conserved from yeasts to humans, our studies support the important role of G4 structure formation in these regions and provide further evidence for a conserved role for Pif1 helicases in resolving G4 structures.