Fission yeast Swi2 designates cell-type specific donor and stimulates Rad51-driven strand exchange for mating-type switching.

A haploid of the fission yeast Schizosaccharomyces pombe expresses either the P or M mating-type, determined by the active, euchromatic, mat1 cassette. Mating-type is switched by Rad51-driven gene conversion of mat1 using a heterochromatic donor cassette, mat2-P or mat3-M. The Swi2-Swi5 complex, a mating-type switching factor, is central to this process by designating a preferred donor in a cell-type-specific manner. Swi2-Swi5 selectively enables one of two cis-acting recombination enhancers, SRE2 adjacent to mat2-P or SRE3 adjacent to mat3-M. Here, we identified two functionally important motifs in Swi2, a Swi6 (HP1 homolog)-binding site and two DNA-binding AT-hooks. Genetic analysis demonstrated that the AT-hooks were required for Swi2 localization at SRE3 to select the mat3-M donor in P cells, while the Swi6-binding site was required for Swi2 localization at SRE2 to select mat2-P in M cells. In addition, the Swi2-Swi5 complex promoted Rad51-driven strand exchange in vitro. Taken together, our results show how the Swi2-Swi5 complex would localize to recombination enhancers through a cell-type specific binding mechanism and stimulate Rad51-driven gene conversion at the localization site.

Euchromatin factors HULC and Set1C affect heterochromatin organization and mating-type switching in fission yeast Schizosaccharomyces pombe.

Mating-type (P or M) of fission yeast Schizosaccharomyces pombe is determined by the transcriptionally active mat1 cassette and is switched by gene conversion using a donor, either mat2 or mat3, located in an adjacent heterochromatin region (mating-type switching; MTS). In the switching process, heterochromatic donors of genetic information are selected based on the P or M cell type and on the action of two recombination enhancers, SRE2 promoting the use of mat2-P and SRE3 promoting the use of mat3-M, leading to replacement of the content of the expressed mat1 cassette. Recently, we found that the histone H3K4 methyltransferase complex Set1C participates in donor selection, raising the question of how a complex best known for its effects in euchromatin controls recombination in heterochromatin. Here, we report that the histone H2BK119 ubiquitin ligase complex HULC functions with Set1C in MTS, as mutants in the shf1, brl1, brl2 and rad6 genes showed defects similar to Set1C mutants and belonged to the same epistasis group as set1Δ. Moreover, using H3K4R and H2BK119R histone mutants and a Set1-Y897A catalytic mutant, we found that ubiquitylation of histone H2BK119 by HULC and methylation of histone H3K4 by Set1C are functionally coupled in MTS. Cell-type biases in MTS in these mutants suggested that HULC and Set1C inhibit the use of the SRE3 recombination enhancer in M cells, thus favoring SRE2 and mat2-P. Consistent with this, imbalanced switching in the mutants was traced to compromised association of the directionality factor Swi6 with the recombination enhancers in M cells. Based on their known effects at other chromosomal locations, we speculate that HULC and Set1C control nucleosome mobility and strand invasion near the SRE elements. In addition, we uncovered distinct effects of HULC and Set1C on histone H3K9 methylation and gene silencing, consistent with additional functions in the heterochromatic domain.

The transcription factor Atf1 lowers the transition barrier for nucleosome-mediated establishment of heterochromatin.

Transcription factors can exert opposite effects depending on the chromosomal context. The fission yeast transcription factor Atf1 both activates numerous genes in response to stresses and mediates heterochromatic gene silencing in the mating-type region. Investigating this context dependency, we report here that the establishment of silent heterochromatin in the mating-type region occurs at a reduced rate in the absence of Atf1 binding. Quantitative modeling accounts for the observed establishment profiles by a combinatorial recruitment of histone-modifying enzymes: locally by Atf1 at two binding sites and over the whole region by dynamically appearing heterochromatic nucleosomes, a source of which is the RNAi-dependent cenH element. In the absence of Atf1 binding, the synergy is lost, resulting in a slow rate of heterochromatin formation. The system shows how DNA-binding proteins can influence local nucleosome states and thereby potentiate long-range positive feedback on histone-modification reactions to enable heterochromatin formation over large regions in a context-dependent manner.

Establishment of heterochromatin in domain-size-dependent bursts.

Methylation of histone H3K9 is a hallmark of epigenetic silencing in eukaryotes. Nucleosome modifications often rely on positive feedback where enzymes are recruited by modified nucleosomes. A combination of local and global feedbacks has been proposed to account for some dynamic properties of heterochromatin, but the range at which the global feedbacks operate and the exact mode of heterochromatin propagation are not known. We investigated these questions in fission yeast. Guided by mathematical modeling, we incrementally increased the size of the mating-type region and profiled heterochromatin establishment over time. We observed exponential decays in the proportion of cells with active reporters, with rates that decreased with domain size. Establishment periods varied from a few generations in wild type to >200 generations in the longest region examined, and highly correlated silencing of two reporters located outside the nucleation center was observed. On a chromatin level, this indicates that individual regions are silenced in sudden bursts. Mathematical modeling accounts for these bursts if heterochromatic nucleosomes facilitate a deacetylation or methylation reaction at long range, in a distance-independent manner. A likely effector of three-dimensional interactions is the evolutionarily conserved Swi6HP1 H3K9me reader, indicating the bursting behavior might be a general mode of heterochromatin propagation.

Integrity of a heterochromatic domain ensured by its boundary elements.

In fission yeast, the inverted repeats IR-L and IR-R function as boundary elements at the edges of a 20-kb silent heterochromatic domain where nucleosomes are methylated at histone H3K9. Each repeat contains a series of B-box motifs physically associated with the architectural TFIIIC complex and with other factors including the replication regulator Sap1 and the Rix1 complex (RIXC). We demonstrate here the activity of these repeats in heterochromatin formation and maintenance. Deletion of the entire IR-R repeat or, to a lesser degree, deletion of just the B boxes impaired the de novo establishment of the heterochromatic domain. Nucleation proceeded normally at the RNA interference (RNAi)-dependent element cenH but subsequent propagation to the rest of the region occurred at reduced rates in the mutants. Once established, heterochromatin was unstable in the mutants. These defects resulted in bistable populations of cells occupying alternate "on" and "off" epigenetic states. Deleting IR-L in combination with IR-R synergistically tipped the balance toward the derepressed state, revealing a concerted action of the two boundaries at a distance. The nuclear rim protein Amo1 has been proposed to tether the mating-type region and its boundaries to the nuclear envelope, where Amo1 mutants displayed milder phenotypes than boundary mutants. Thus, the boundaries might facilitate heterochromatin propagation and maintenance in ways other than just through Amo1, perhaps by constraining a looped domain through pairing.

Mating-type switching by homology-directed recombinational repair: a matter of choice.

In eukaryotes, all DNA transactions happen in the context of chromatin that often takes part in regulatory mechanisms. In particular, chromatin structure can regulate exchanges of DNA occurring through homologous recombination. Few systems have provided as detailed a view on this phenomenon as mating-type switching in yeast. Mating-type switching entails the choice of a template for the gene conversions of the expressed mating-type locus. In the fission yeast Schizosaccharomyces pombe, correct template choice requires two competing small recombination enhancers, SRE2 and SRE3, that function in the context of heterochromatin. These two enhancers act with the Swi2/Swi5 recombination accessory complex to initiate strand exchange in a cell-type-specific manner, from SRE2 in M cells and SRE3 in P cells. New research indicates that the Set1C complex, responsible for H3K4 methylation, and the Brl2 ubiquitin ligase, that catalyzes H2BK119 ubiquitylation, participate in the cell-type-specific selection of SRE2 or SRE3. Here, we review these findings, compare donor preference in S. pombe to the distantly related budding yeast Saccharomyces cerevisiae, and contrast the positive effects of heterochromatin on the donor selection process with other situations, where heterochromatin represses recombination.

Structure of the replication regulator Sap1 reveals functionally important interfaces.

The mechanism by which specific protein-DNA complexes induce programmed replication fork stalling in the eukaryotic genome remains poorly understood. In order to shed light on this process we carried out structural investigations on the essential fission yeast protein Sap1. Sap1 was identified as a protein involved in mating-type switching in Schizosaccharomyces pombe, and has been shown to be involved in programmed replication fork stalling. Interestingly, Sap1 assumes two different DNA binding modes. At the mating-type locus dimers of Sap1 bind the SAS1 sequence in a head-to-head arrangement, while they bind to replication fork blocking sites at rDNA and Tf2 transposons in a head-to-tail mode. In this study, we have solved the crystal structure of the Sap1 DNA binding domain and we observe that Sap1 molecules interact in the crystal using a head-to-tail arrangement that is compatible with DNA binding. We find that Sap1 mutations which alleviate replication-fork blockage at Tf2 transposons in CENP-B mutants map to the head-to-tail interface. Furthermore, several other mutations introduced in this interface are found to be lethal. Our data suggests that essential functions of Sap1 depend on its head-to-tail oligomerization.

New insights into donor directionality of mating-type switching in Schizosaccharomyces pombe.

Mating-type switching in Schizosaccharomyces pombe entails programmed gene conversion events regulated by DNA replication, heterochromatin, and the HP1-like chromodomain protein Swi6. The whole mechanism remains to be fully understood. Using a gene deletion library, we screened ~ 3400 mutants for defects in the donor selection step where a heterochromatic locus, mat2-P or mat3-M, is chosen to convert the expressed mat1 locus. By measuring the biases in mat1 content that result from faulty directionality, we identified in total 20 factors required for donor selection. Unexpectedly, these included the histone H3 lysine 4 (H3K4) methyltransferase complex subunits Set1, Swd1, Swd2, Swd3, Spf1 and Ash2, the BRE1-like ubiquitin ligase Brl2 and the Elongator complex subunit Elp6. The mutant defects were investigated in strains with reversed donor loci (mat2-M mat3-P) or when the SRE2 and SRE3 recombination enhancers, adjacent to the donors, were deleted or transposed. Mutants in Set1C, Brl2 or Elp6 altered balanced donor usage away from mat2 and the SRE2 enhancer, towards mat3 and the SRE3 enhancer. The defects in these mutants were qualitatively similar to heterochromatin mutants lacking Swi6, the NAD+-dependent histone deacetylase Sir2, or the Clr4, Raf1 or Rik1 subunits of the histone H3 lysine 9 (H3K9) methyltransferase complex, albeit not as extreme. Other mutants showed clonal biases in switching. This was the case for mutants in the NAD+-independent deacetylase complex subunits Clr1, Clr2 and Clr3, the casein kinase CK2 subunit Ckb1, the ubiquitin ligase component Pof3, and the CENP-B homologue Cbp1, as well as for double mutants lacking Swi6 and Brl2, Pof3, or Cbp1. Thus, we propose that Set1C cooperates with Swi6 and heterochromatin to direct donor choice to mat2-P in M cells, perhaps by inhibiting the SRE3 recombination enhancer, and that in the absence of Swi6 other factors are still capable of imposing biases to donor choice.

Dependency of Heterochromatin Domains on Replication Factors.

Chromatin structure regulates both genome expression and dynamics in eukaryotes, where large heterochromatic regions are epigenetically silenced through the methylation of histone H3K9, histone deacetylation, and the assembly of repressive complexes. Previous genetic screens with the fission yeast Schizosaccharomyces pombe have led to the identification of key enzymatic activities and structural constituents of heterochromatin. We report here on additional factors discovered by screening a library of deletion mutants for silencing defects at the edge of a heterochromatic domain bound by its natural boundary-the IR-R+ element-or by ectopic boundaries. We found that several components of the DNA replication progression complex (RPC), including Mrc1/Claspin, Mcl1/Ctf4, Swi1/Timeless, Swi3/Tipin, and the FACT subunit Pob3, are essential for robust heterochromatic silencing, as are the ubiquitin ligase components Pof3 and Def1, which have been implicated in the removal of stalled DNA and RNA polymerases from chromatin. Moreover, the search identified the cohesin release factor Wpl1 and the forkhead protein Fkh2, both likely to function through genome organization, the Ssz1 chaperone, the Fkbp39 proline cis-trans isomerase, which acts on histone H3P30 and P38 in Saccharomyces cerevisiae, and the chromatin remodeler Fft3. In addition to their effects in the mating-type region, to varying extents, these factors take part in heterochromatic silencing in pericentromeric regions and telomeres, revealing for many a general effect in heterochromatin. This list of factors provides precious new clues with which to study the spatiotemporal organization and dynamics of heterochromatic regions in connection with DNA replication.

Mating-Type Determination in Schizosaccharomyces pombe.

Here we describe how mating-type tests are conducted in Schizosaccharomyces pombe Two methods can be employed: matings with h- and h+ tester strains and polymerase chain reaction (PCR) for mat1 content.

Ethyl Methanesulfonate Mutagenesis in Schizosaccharomyces pombe.

Here we provide an ethyl methanesulfonate (EMS) mutagenesis protocol for Schizosaccharomyces pombe cells.

Genetic Analysis of Schizosaccharomyces pombe.

In this introduction we discuss some basic genetic tools and techniques that are used with the fission yeast Schizosaccharomyces pombe Genes commonly used for selection or as reporters are discussed, with an emphasis on genes that permit counterselection, intragenic complementation, or colony-color assays. S. pombe is most stable as a haploid organism. We describe its mating-type system, how to perform genetic crosses and methods for selecting and propagating diploids. We discuss the relative merits of tetrad dissection and random spore preparation in strain construction and genetic analyses. Finally, we present several types of mutant screens, with an evaluation of their respective strengths and limitations in the light of emerging technologies such as next-generation sequencing.

Selecting Schizosaccharomyces pombe Diploids.

Here we describe procedures for the selection of diploid Schizosaccharomyces pombeade6-M210/ade6-M216 heteroallelic complementation is widely used to select for Ade+ diploids. Such diploids will readily sporulate when starved of nitrogen. For some investigations, stable diploids are preferable (e.g., for genetic complementation tests), and in these cases mating an h- strain with an h90 mat2-Pi-102 strain can be used to prevent sporulation. When ade6-M210/ade6-M216 mutations impact on, or show synthetic interactions with, the gene of interest, two different auxotrophic markers can be used to select complementation.

Setting up Schizosaccharomyces pombe Crosses/Matings.

Here we provide methods for setting up standard crosses with Schizosaccharomyces pombe strains. All strain genotypes and pedigrees should be recorded in a laboratory strain book. Matings between two haploid strains of interest are induced on solid medium poor in nitrogen. Usually, sporulation agar (SPA) plates are preferred, but for difficult matings it is advisable to try several mating media in parallel because one medium might allow for more efficient mating. Protoplast fusion can be used to produce zygotes from sterile mutants that fail to mate.

Spore Analysis and Tetrad Dissection of Schizosaccharomyces pombe.

Here we describe the processing of Schizosaccharomyces pombe spores in batches (random spore analysis) or through tetrad dissections. Spores are usually prepared from matings between haploid strains (producing zygotic asci) or from sporulating diploids (producing azygotic asci). In random spore analysis, a snail enzyme preparation is used to digest the walls of asci to release free spores that are diluted and plated to form colonies. In tetrad dissection, a needle attached to a micromanipulator is used to pick asci and separate spores. Tetrad dissection has traditionally been the method of choice for genetic mapping and is very useful in the study of genetic interactions (e.g., suppressor analysis). It is also the preferred method for routine crosses because it ensures that every colony stems from a single spore. This can never be certain in random spore analysis.

Establishment of expression-state boundaries by Rif1 and Taz1 in fission yeast.

The Shelterin component Rif1 has emerged as a global regulator of the replication-timing program in all eukaryotes examined to date, possibly by modulating the 3D-organization of the genome. In fission yeast a second Shelterin component, Taz1, might share similar functions. Here, we identified unexpected properties for Rif1 and Taz1 by conducting high-throughput genetic screens designed to identify cis- and trans-acting factors capable of creating heterochromatin-euchromatin boundaries in fission yeast. The preponderance of cis-acting elements identified in the screens originated from genomic loci bound by Taz1 and associated with origins of replication whose firing is repressed by Taz1 and Rif1. Boundary formation and gene silencing by these elements required Taz1 and Rif1 and coincided with altered replication timing in the region. Thus, small chromosomal elements sensitive to Taz1 and Rif1 (STAR) could simultaneously regulate gene expression and DNA replication over a large domain, at the edge of which they established a heterochromatin-euchromatin boundary. Taz1, Rif1, and Rif1-associated protein phosphatases Sds21 and Dis2 were each sufficient to establish a boundary when tethered to DNA. Moreover, efficient boundary formation required the amino-terminal domain of the Mcm4 replicative helicase onto which the antagonistic activities of the replication-promoting Dbf4-dependent kinase and Rif1-recruited phosphatases are believed to converge to control replication origin firing. Altogether these observations provide an insight into a coordinated control of DNA replication and organization of the genome into expression domains.

Nucleation and spreading of a heterochromatic domain in fission yeast.

Outstanding questions in the chromatin field bear on how large heterochromatin domains are formed in space and time. Positive feedback, where histone-modifying enzymes are attracted to chromosomal regions displaying the modification they catalyse, is believed to drive the formation of these domains; however, few quantitative studies are available to assess this hypothesis. Here we quantified the de novo establishment of a naturally occurring ∼20-kb heterochromatin domain in fission yeast through single-cell analyses, measuring the kinetics of heterochromatin nucleation in a region targeted by RNAi and its subsequent expansion. We found that nucleation of heterochromatin is stochastic and can take from one to ten cell generations. Further silencing of the full region takes another one to ten generations. Quantitative modelling of the observed kinetics emphasizes the importance of local feedback, where a nucleosome-bound enzyme modifies adjacent nucleosomes, combined with a feedback where recruited enzymes can act at a distance.

Targeting of SUMO substrates to a Cdc48-Ufd1-Npl4 segregase and STUbL pathway in fission yeast.

In eukaryotes, the conjugation of proteins to the small ubiquitin-like modifier (SUMO) regulates numerous cellular functions. A proportion of SUMO conjugates are targeted for degradation by SUMO-targeted ubiquitin ligases (STUbLs) and it has been proposed that the ubiquitin-selective chaperone Cdc48/p97-Ufd1-Npl4 facilitates this process. However, the extent to which the two pathways overlap, and how substrates are selected, remains unknown. Here we address these questions in fission yeast through proteome-wide analyses of SUMO modification sites. We identify over a thousand sumoylated lysines in a total of 468 proteins and quantify changes occurring in the SUMO modification status when the STUbL or Ufd1 pathways are compromised by mutations. The data suggest the coordinated processing of several classes of SUMO conjugates, many dynamically associated with centromeres or telomeres. They provide new insights into subnuclear organization and chromosome biology, and, altogether, constitute an extensive resource for the molecular characterization of SUMO function and dynamics.

Two portable recombination enhancers direct donor choice in fission yeast heterochromatin.

Mating-type switching in fission yeast results from gene conversions of the active mat1 locus by heterochromatic donors. mat1 is preferentially converted by mat2-P in M cells and by mat3-M in P cells. Here, we report that donor choice is governed by two portable recombination enhancers capable of promoting use of their adjacent cassette even when they are transposed to an ectopic location within the mat2-mat3 heterochromatic domain. Cells whose silent cassettes are swapped to mat2-M mat3-P switch mating-type poorly due to a defect in directionality but cells whose recombination enhancers were transposed together with the cassette contents switched like wild type. Trans-acting mutations that impair directionality affected the wild-type and swapped cassettes in identical ways when the recombination enhancers were transposed together with their cognate cassette, showing essential regulatory steps occur through the recombination enhancers. Our observations lead to a model where heterochromatin biases competitions between the two recombination enhancers to achieve directionality.

Concerted action of the ubiquitin-fusion degradation protein 1 (Ufd1) and Sumo-targeted ubiquitin ligases (STUbLs) in the DNA-damage response.

In eukaryotes many players in the DNA-damage response (DDR) catalyze protein sumoylation or ubiquitylation. Emphasis has been placed on how these modifications orchestrate the sequential recruitment of repair factors to sites of DNA damage or stalled replication forks. Here, we shed light on a pathway in which sumoylated factors are eliminated through the coupled action of Sumo-targeted ubiquitin ligases (STUbLs) and the ubiquitin-fusion degradation protein 1 (Ufd1). Ufd1 is a subunit of the Cdc48-Ufd1-Npl4 complex implicated in the sorting of ubiquitylated substrates for degradation by the proteasome. We find that in fission yeast, Ufd1 interacts physically and functionally with the Sumo-targeted ubiquitin ligase (STUbL) Rfp1, homologous to human RNF4, and with the Sumo E3 ligase Pli1, homologous to human PIAS1. Deleting a C-terminal domain of Ufd1 that mediates the interaction of Ufd1 with Rfp1, Pli1, and Sumo (ufd1ΔCt(213-342) ) lead to an accumulation of high-molecular-weight Sumo conjugates and caused severe genomic instabilities. The spectrum of sensitivity of ufd1ΔCt(213-342) cells to genotoxins, the epistatic relationships of ufd1ΔCt(213-342) with mutations in DNA repair factors, and the localization of the repair factor Rad22 in ufd1ΔCt(213-342) cells point to ufd1ΔCt(213-342) cells accumulating aberrant structures during replication that require homologous recombination (HR) for their repair. We present evidence that HR is however often not successful in ufd1ΔCt(213-342) cells and we identify Rad22 as one of the high-molecular-weight conjugates accumulating in the ufd1ΔCt(213-342) mutant consistent with Rad22 being a STUbL/Ufd1 substrate. Suggesting a direct role of Ufd1 in the processing of Sumo-conjugates, Ufd1 formed nuclear foci colocalizing with Sumo during the DDR, and Sumo-conjugates accumulated in foci in the ufd1ΔCt(213-342) mutant. Broader functional relationships between Ufd1 and STUbLs conceivably affect numerous cellular processes beyond the DDR.