Repression of a mating type cassette in the fission yeast by four DNA elements

The Fission Yeast Mating-Type Switching Motto: "One-for-Two" and "Two-for-One"

Schizosaccharomyces pombe is an ascomycete fungus that divides by medial fission; it is thus commonly referred to as fission yeast, as opposed to the distantly related budding yeast Saccharomyces cerevisiae. The reproductive lifestyle of S. pombe relies on an efficient genetic sex determination system generating a 1:1 sex ratio and using alternating haploid/diploid phases in response to environmental conditions. In this review, we address how one haploid cell manages to generate two sister cells with opposite mating types, a prerequisite to conjugation and meiosis. This mating-type switching process depends on two highly efficient consecutive asymmetric cell divisions that rely on DNA replication, repair, and recombination as well as the structure and components of heterochromatin. We pay special attention to the intimate interplay between the genetic and epigenetic partners involved in this process to underscore the importance of basic research and its profound implication for a better understanding of chromatin biology.

Noncoding RNA-nucleated heterochromatin spreading is intrinsically labile and requires accessory elements for epigenetic stability

The heterochromatin spreading reaction is a central contributor to the formation of gene-repressive structures, which are re-established with high positional precision, or fidelity, following replication. How the spreading reaction contributes to this fidelity is not clear. To resolve the origins of stable inheritance of repression, we probed the intrinsic character of spreading events in fission yeast using a system that quantitatively describes the spreading reaction in live single cells. We show that spreading triggered by noncoding RNA-nucleated elements is stochastic, multimodal, and fluctuates dynamically across time. This lack of stability correlates with high histone turnover. At the mating type locus, this unstable behavior is restrained by an accessory cis-acting element REIII, which represses histone turnover. Further, REIII safeguards epigenetic memory against environmental perturbations. Our results suggest that the most prevalent type of spreading, driven by noncoding RNA-nucleators, is epigenetically unstable and requires collaboration with accessory elements to achieve high fidelity.

HDAC genes play distinct and redundant roles in Cryptococcus neoformans virulence

The human fungal pathogen Cryptococcus neoformans undergoes many phenotypic changes to promote its survival in specific ecological niches and inside the host. To explore the role of chromatin remodeling on the expression of virulence-related traits, we identified and deleted seven genes encoding predicted class I/II histone deacetylases (HDACs) in the C. neoformans genome. These studies demonstrated that individual HDACs control non-identical but overlapping cellular processes associated with virulence, including thermotolerance, capsule formation, melanin synthesis, protease activity and cell wall integrity. We also determined the HDAC genes necessary for C. neoformans survival during in vitro macrophage infection and in animal models of cryptococcosis. Our results identified the HDA1 HDAC gene as a central mediator controlling several cellular processes, including mating and virulence. Finally, a global gene expression profile comparing the hda1Δ mutant versus wild-type revealed altered transcription of specific genes associated with the most prominent virulence attributes in this fungal pathogen. This study directly correlates the effects of Class I/II HDAC-mediated chromatin remodeling on the marked phenotypic plasticity and virulence potential of this microorganism. Furthermore, our results provide insights into regulatory mechanisms involved in virulence gene expression that are likely shared with other microbial pathogens.

Combinatorial, site-specific requirement for heterochromatic silencing factors in the elimination of nucleosome-free regions

High-resolution nucleosome occupancy maps of heterochromatic regions of wild-type and silencing-defective mutants of the fission yeast Schizosaccharomyces pombe revealed that heterochromatin induces the elimination of nucleosome-free regions (NFRs). NFRs associated with transcription initiation sites as well as those not associated with promoters are affected. We dissected the roles of the histone H3K9 methyltransferase Clr4 and the HP1 proteins Swi6 and Chp2, as well as the two catalytic activities of the SHREC histone deacetylase (HDAC)/ATPase effector complex. Strikingly, different DNA sites have distinct combinatorial requirements for these factors: Five classes of NFRs were identified that are eliminated by silencing factors through a mechanistic hierarchy governed by Clr4. The SHREC HDAC activity plays a major role in the elimination of class I-IV NFRs by antagonizing the action of RSC, a remodeling complex implicated in NFR formation. We propose that heterochromatin formation involves the deployment in several sequence-specific mechanisms to eliminate gaps between nucleosomes, thereby blocking access to the DNA.

Lessons learned from studies of fission yeast mating-type switching and silencing

Stably maintaining specific states of gene expression during cell division is crucial for cellular differentiation. In fission yeast, such patterns result from directed gene rearrangements and chromosomally inherited epigenetic gene control mechanisms that control mating cell type. Recent advances have shown that a specific DNA strand at the mat1 locus is "differentiated" by a novel strand-specific imprint so that nonequivalent sister chromatids are produced. Therefore, cellular differentiation is a natural consequence of the fact that DNA strands are complementary and nonequivalent. Another epigenetic control that "silences" library copies of mat-information is due to heterochromatin organization. This is a clear case where Mendel's gene is composed of DNA plus the associated epigenetic moiety. Following up on initial genetic studies with more recent molecular investigations, this system has become one of the prominent models to understand mechanisms of gene regulation, genome integrity, and cellular differentiation. By applying lessons learned from these studies, such epigenetic gene control mechanisms, which must be installed in somatic cells, might explain mechanisms of cellular differentiation and development in higher eukaryotes.

Eukaryotic DNA Replication in a Chromatin Context

There has been remarkable progress in the last 20 years in defining the molecular mechanisms that regulate initiation of DNA synthesis in eukaryotic cells. Replication origins in the DNA nucleate the ordered assembly of protein factors to form a prereplication complex (preRC) that is poised for DNA synthesis. Transition of the preRC to an active initiation complex is regulated by cyclin-dependent kinases and other signaling molecules, which promote further protein assembly and activate the mini chromosome maintenance helicase. We will review these mechanisms and describe the state of knowledge about the proteins involved. However, we will also consider an additional layer of complexity. The DNA in the cell is packaged with histone proteins into chromatin. Chromatin structure provides an additional layer of heritable information with associated epigenetic modifications. Thus, we will begin by describing chromatin structure, and how the cell generally controls access to the DNA. Access to the DNA requires active chromatin remodeling, specific histone modifications, and regulated histone deposition. Studies in transcription have revealed a variety of mechanisms that regulate DNA access, and some of these are likely to be shared with DNA replication. We will briefly describe heterochromatin as a model for an epigenetically inherited chromatin state. Next, we will describe the mechanisms of replication initiation and how these are affected by constraints of chromatin. Finally, chromatin must be reassembled with appropriate modifications following passage of the replication fork, and our third major topic will be the reassembly of chromatin and its associated epigenetic marks. Thus, in this chapter, we seek to bring together the studies of replication initiation and the studies of chromatin into a single holistic narrative.

Selective gene expression in multigene families from yeast to mammals

Exclusive gene expression, where only one member of a gene or gene cassette family is selected for expression, plays an important role in the establishment of cell identity in several biological systems. Here, we compare four such systems: mating-type switching in fission and budding yeast, where cells choose between expressing one of the two different mating-type cassettes, and immunoglobulin and odorant receptor gene expression in mammals, where the number of gene choices is substantially higher. The underlying mechanisms that establish this selective expression pattern in each system differ in almost every detail. In all four systems, once a successful gene activation event has taken place, a feedback mechanism affects the fate of the cell. In the mammalian systems, feedback is mediated by the expressed cell surface receptor to ensure monoallelic gene expression, whereas in the yeasts, the expressed gene cassette at the mating-type locus affects donor choice during the subsequent switching event.

Regulation of Swi6/HP1-dependent Heterochromatin Assembly by Cooperation of Components of the Mitogen-activated Protein Kinase Pathway and a Histone Deacetylase Clr6

A study of gene silencing within the mating-type region of fission yeast defines two distinct pathways responsible for the establishment of heterochromatin assembly. One is RNA interference-dependent and acts on centromere-homologous repeats (cenH). The other is a stochastic Swi6 (the fission yeast HP1 homolog)-dependent mechanism that is not fully understood. Here we find that activating transcription factor (Atf1) and Pcr1, the fission yeast bZIP transcription factors homologous to human ATF-2, are crucial for proper histone deacetylation of both H3 and H4. This deacetylation is a prerequisite for subsequent H3 lysine 9 methylation and Swi6-dependent heterochromatin assembly across the rest of the silent mating-type (mat) region lacking the RNA interference-dependent cenH repeat. Moreover, Atf1 and Pcr1 can form complexes with both a histone deacetylase, Clr6, and Swi6, and clr6 mutations affected the H3/H4 acetylation patterns, similar to the atf1 and pcr1 deletion mutant phenotypes at the endogenous mat loci and at the ctt1+ promoter region surrounding ATF/CRE-binding site. These data suggest that Atf1 and Pcr1 participate in an early step essential for heterochromatin assembly at the mat locus and silencing of transcriptional targets of Atf1. Furthermore, a phosphorylation event catalyzed by the conserved mitogen-activated protein kinase pathway is important for regulation of heterochromatin silencing by Atf1 and Pcr1. These findings suggest a role for the mitogen-activated protein kinase pathway and histone deacetylase in Swi6-based heterochromatin assembly.

Identification of Uhp1, a ubiquitinated histone-like protein, as a target/mediator of Rhp6 in mating-type silencing in fission yeast

Mating-type silencing in Schizosaccharomyces pombe is brought about by cooperative interactions between cis-acting DNA sequences flanking mat2P and mat3M and the trans-acting factors, namely Swi6, Clr1-Clr4, Clr6, and Rik1. In addition, DNA repair gene rhp6, which plays a role in post-replication DNA repair and ubiquitination of proteins including histones, is also involved in silencing, albeit in a unique way; its effect on silencing and chromatin structure of the donor loci is dependent on their switching competence. Earlier, we hypothesized the existence of a mediator of Rhp6 that plays a role in reestablishment of the chromatin structure coincidentally with DNA replication associated with mating-type switching. Here we report the identification of a 22-kDa protein as an in vivo target and mediator of Rhp6 in mating-type silencing. The level of this protein is greatly elevated in sng1-1/rhp6(-) mutant and rhp6Delta as compared with wild type strain. Both the deletion and overexpression of the gene encoding this protein elicit switching-dependent loss of silencing. Furthermore, the 22-kDa protein undergoes Rhp6-dependent multiubiquitination and associates with mat2 locus during S phase in wild type cells. Interestingly, it contains a histone-fold motif similar to that of histone H2A, and like histone H2A, it interacts strongly with histone H2B in vitro. These results indicate that the 22-kDa protein, renamed as the ubiquitinated histone-like protein Uhp1, is an in vivo target/mediator of Rhp6 in silencing. Thus, regulation of association of Uhp1 with chromatin and ubiquitination followed by degradation may play a role in reestablishment of inactive chromatin structure at the silent mating-type loci.

DNA sequence of themat2,3 region ofSchizosaccharomyces kambucha shares high homology with the corresponding sequence fromSz. pombe

To define conserved sequences for mat1 imprinting and silencing of the mat2,3 region of Schizosaccharomyces pombe, we determined the DNA sequence of the cognate region (mat2,3 region) of another fission yeast, Sz. kambucha, a yeast species isolated from Kambucha tea mix. The entire mat2,3 region shows more than 98% identity between the two species. Sequence similarity is even higher (99.3%) for mating-type cassettes; deduced amino acid sequences of three of the four Mat peptides (Pi, Pc and Mi) are identical between the two species, while the fourth (Mc) has a single amino acid polymorphism. Comparison of the sequence motif of the imprint site essential for mat1 switching shows that mat-P of Sz. kambucha has a sequence identical to the conserved motif present in Sz. pombe. However, this sequence motif of nine bases differs by one base for mat-M of Sz. kambucha. The sequence of the K region shows about 98% identity between the two species, with the cenH region showing 98.3% homology. Thus, the arrangement of the mat2,3 region in both yeasts is conserved and shows 1-2% nucleotide sequence variation throughout the region. The DNA sequence of the mat2,3 region from Sz. kambucha has been submitted to GenBank under Accession No. AY271822.

The 2.1-kb inverted repeat DNA sequences flank the mat2,3 silent region in two species of Schizosaccharomyces and are involved in epigenetic silencing in Schizosaccharomyces pombe

The mat2,3 region of the fission yeast Schizosaccharomyces pombe exhibits a phenomenon of transcriptional silencing. This region is flanked by two identical DNA sequence elements, 2.1 kb in length, present in inverted orientation: IRL on the left and IRR on the right of the silent region. The repeats do not encode any ORF. The inverted repeat DNA region is also present in a newly identified related species, which we named S. kambucha. Interestingly, the left and right repeats share perfect identity within a species, but show approximately 2% bases interspecies variation. Deletion of IRL results in variegated expression of markers inserted in the silent region, while deletion of the IRR causes their derepression. When deletions of these repeats were genetically combined with mutations in different trans-acting genes previously shown to cause a partial defect in silencing, only mutations in clr1 and clr3 showed additive defects in silencing with the deletion of IRL. The rate of mat1 switching is also affected by deletion of repeats. The IRL or IRR deletion did not cause significant derepression of the mat2 or mat3 loci. These results implicate repeats for maintaining full repression of the mat2,3 region, for efficient mat1 switching, and further support the notion that multiple pathways cooperate to silence the mat2,3 domain.

The fission yeast ubiquitin-conjugating enzymes UbcP3, Ubc15, and Rhp6 affect transcriptional silencing of the mating-type region

Genes transcribed by RNA polymerase II are silenced when introduced near the mat2 or mat3 mating-type loci of the fission yeast Schizosaccharomyces pombe. Silencing is mediated by a number of gene products and cis-acting elements. We report here the finding of novel trans-acting factors identified in a screen for high-copy-number disruptors of silencing. Expression of cDNAs encoding the putative E2 ubiquitin-conjugating enzymes UbcP3, Ubc15 (ubiquitin-conjugating enzyme), or Rhp6 (Rad homolog pombe) from the strong nmt1 promoter derepressed the silent mating-type loci mat2 and mat3 and reporter genes inserted nearby. Deletion of rhp6 slightly derepressed an ade6 reporter gene placed in the mating-type region, whereas disruption of ubcP3 or ubc15 had no obvious effect on silencing. Rhp18 is the S. pombe homolog of Saccharomyces cerevisiae Rad18p, a DNA-binding protein that physically interacts with Rad6p. Rhp18 was not required for the derepression observed when UbcP3, Ubc15, or Rhp6 was overproduced. Overexpressing Rhp6 active-site mutants showed that the ubiquitin-conjugating activity of Rhp6 is essential for disruption of silencing. However, high dosage of UbcP3, Ubc15, or Rhp6 was not suppressed by a mutation in the 26S proteasome, suggesting that loss of silencing is not due to an increased degradation of silencing factors but rather to the posttranslational modification of proteins by ubiquitination. We discuss the implications of these results for the possible modes of action of UbcP3, Ubc15, and Rhp6.

Transcriptional silencing in Saccharomyces cerevisiae and Schizosaccharomyces pombe

Transcriptional silencing is a heritable form of gene inactivation that involves the assembly of large regions of DNA into a specialized chromatin structure that inhibits transcription. This phenomenon is responsible for inhibiting transcription at silent mating-type loci, telomeres and rDNA repeats in both budding yeast Saccharomyces cerevisiae and fission yeast Schizosaccharomyces pombe, as well as at centromeres in fission yeast. Although transcriptional silencing in both S.cerevisiae and S.pombe involves modification of chromatin, no apparent amino acid sequence similarities have been reported between the proteins involved in establishment and maintenance of silent chromatin in these two distantly related yeasts. Silencing in S.cerevisiae is mediated by Sir2p-containing complexes, whereas silencing in S.pombe is mediated primarily by Swi6-containing complexes. The Swi6 complexes of S.pombe contain proteins closely related to their counterparts in higher eukaryotes, but have no apparent orthologs in S.cerevisiae. Silencing proteins from both yeasts are also actively involved in other chromosome-related nuclear functions, including DNA repair and the regulation of chromatin structure.

Chromodomain protein Swi6-mediated role of DNA polymerase alpha in establishment of silencing in fission Yeast

Although DNA replication has been thought to play an important role in the silencing of mating type loci in Saccharomyces cerevisiae, recent studies indicate that silencing can be decoupled from replication. In Schizosaccharomyces pombe, mating type silencing is brought about by the trans-acting proteins, namely Swi6, Clr1-Clr4, and Rhp6, in cooperation with the cis-acting silencers. The latter contain an autonomous replication sequence, suggesting that DNA replication may be critical for silencing in S. pombe. To investigate the connection between DNA replication and silencing in S. pombe, we analyzed several temperature-sensitive mutants of DNA polymerase alpha. We find that one such mutant, swi7H4, exhibits silencing defects at mat, centromere, and telomere loci. This effect is independent of the checkpoint and replication defects of the mutant. Interestingly, the extent of the silencing defect in the swi7H4 mutant at the silent mat2 locus is further enhanced in absence of the cis-acting, centromere-proximal silencer. The chromodomain protein Swi6, which is required for silencing and is localized to mat and other heterochromatin loci, interacts with DNA polymerase alpha in vivo and in vitro in wild type cells. However, it does not interact with the mutant pol alpha and is delocalized away from the silent mat loci in the mutant. Our results demonstrate a role of DNA polymerase alpha in the establishment of silencing. We propose a recruitment model for the coupling of DNA replication with the establishment of silencing by the chromodomain protein Swi6, which may be applicable to higher eukaryotes.

A fission yeast repression element cooperates with centromere-like sequences and defines a mat silent domain boundary

REII is a Schizosaccharomyces pombe repression element located at the centromere-proximal end of the mat silent domain. Here we show that inversion of REII enhances silencing on its centromere-proximal side while suppressing silencing on its centromere-distal side. Transplacement of REII to a position 2.5 kb from its native locus extends the region of stringent repression to the new REII site. These results suggest that REII defines a mat silent domain boundary by acting preferentially toward its centromere-distal side. To investigate cooperation between REII and a K-region sequence that shares homology with the centromeric dg dh repeats (cen2 homology), we targeted combinations of these elements to an ectopic site and monitored expression of an adjacent reporter gene. Centromeric dh-like sequences conferred low-level silencing on the adjacent reporter gene, and REII, which did not display silencing activity on its own, enhanced cen2 homology-mediated silencing. Cooperation was also apparent at the mat locus, where deletion of REII impaired repression stability. We propose that REII and the cen2 homology play different yet complementary roles in silencing establishment and inheritance at the mat locus.

Transcriptional silencing in fission yeast

In eukaryotes, epigenetic events govern diverse processes, ranging from gene expression to other aspects of global chromosome architecture essential for preserving the integrity of the genome. Transcriptional silencing at the mating-type locus, centromeres, and telomeres of the fission yeast is regulated by epigenetic mechanisms. Epigenetic states are inherited in cis during mitosis and, remarkably, even through meiosis. Several trans-acting genes that affect silencing are found to encode either chromatin proteins such as chromodomain proteins Swi6 and Clr4 or the factors that affect chromatin assembly, including histone deacetylase homologs Clr3 and Clr6. A recent study showed that Swi6 is involved in imprinting at the mating-type locus and contributes to the cellular memory responsible for maintenance of the silenced state. The "gene" in this instance thus comprises DNA plus the associated Swi6-containing protein complex.

Position effect variegation at the mating-type locus of fission yeast: a cis-acting element inhibits covariegated expression of genes in the silent and expressed domains

Schizosaccharomyces pombe switches its mating type by transposing a copy of unexpressed genes from the respective mat2 or mat3 cassettes to mat1. The donor cassettes are located in a silent domain that is separated from the expressed mat1 cassette by the L region. We monitored the expression of ade6 from sites in the L region and examined the relationship between the expression state at these sites and at sites within the silent domain. Results indicate that: (1) the silent domain extends into the L region, but repression is gradually alleviated with increasing distance from mat2, and overexpression of swi6 enhances PEV in the L region; (2) a transcriptionally active chromatin state, associated with reporter gene expression in the L region, spreads toward the silent domain; (3) a cis-acting element, located at the junction between the L region and mat2-P, ensures repression in the silent domain, regardless of the expression state in the L region; and (4) repression in mat1-P cells is less stringently controlled than in mat1-M cells. We discuss the functional organization of the mat region and genetic elements that ensure separation between repressed and derepressed domains.

Localization and properties of a silencing element near the mat3-M mating-type cassette of Schizosaccharomyces pombe

Transcription is repressed in a segment of Schizosaccharomyces pombe chromosome II that encompasses the mat2-P and mat3-M mating-type cassettes. Chromosomal deletion analysis revealed the presence of a repressor element within 500 bp of mat3-M. This element acted in synergy with the trans-acting factors Swi6, Clr1, Clr2, Clr3, and Clr4 and had several properties characteristic of silencers: it did not display promoter specificity, being able to silence not only the M mating-type genes but also the S. pombe ura4 and ade6 genes placed on the centromere-distal side of the mat3-M cassette; it could repress a gene when placed further than 2.6 kb from the promoter and it acted in both orientations, although with different efficiencies, the natural orientation repressing more stringently than the reverse. Following deletion of this element, two semistable states of expression of the mat3-M region were observed and these two states could interconvert. The deletion did not affect gene expression in the vicinity of the mat2-P cassette, 11 kb away from mat3-M. Conversely, deleting 1.5 kb on the centromere-proximal side of the mat2-P cassette, which was previously shown to partially derepress transcription around mat2-P, had no effect on gene expression near mat3-M. A double deletion removing the mat2-P and mat3-M repressor elements had the same effect as the single deletions on their respective cassettes when assayed in cells of the M mating type. These observations allow us to refine a model proposing that redundant pathways silence the mating type region of S. pombe.

A novel function of the DNA repair gene rhp6 in mating-type silencing by chromatin remodeling in fission yeast

Recent studies have indicated that the DNA replication machinery is coupled to silencing of mating-type loci in the budding yeast Saccharomyces cerevisiae, and a similar silencing mechanism may operate in the distantly related yeast Schizosaccharomyces pombe. Regarding gene regulation, an important function of DNA replication may be in coupling of faithful chromatin assembly to reestablishment of the parental states of gene expression in daughter cells. We have been interested in isolating mutants that are defective in this hypothesized coupling. An S. pombe mutant fortuitously isolated from a screen for temperature-sensitive growth and silencing phenotype exhibited a novel defect in silencing that was dependent on the switching competence of the mating-type loci, a property that differentiates this mutant from other silencing mutants of S. pombe as well as of S. cerevisiae. This unique mutant phenotype defined a locus which we named sng1 (for silencing not governed). Chromatin analysis revealed a switching-dependent unfolding of the donor loci mat2P and mat3M in the sng1(-) mutant, as indicated by increased accessibility to the in vivo-expressed Escherichia coli dam methylase. Unexpectedly, cloning and sequencing identified the gene as the previously isolated DNA repair gene rhp6. RAD6, an rhp6 homolog in S. cerevisiae, is required for postreplication DNA repair and ubiquitination of histones H2A and H2B. This study implicates the Rad6/rhp6 protein in gene regulation and, more importantly, suggests that a transient window of opportunity exists to ensure the remodeling of chromatin structure during chromosome replication and recombination. We propose that the effects of the sng1(-)/rhp6(-) mutation on silencing are indirect consequences of changes in chromatin structure.

Elements of chromosome structure and function in fission yeast

The investigation of fission yeast chromosome structure and function has moved rapidly over the past 10 years. The isolation of replication origins, telomeres and centromeres has allowed the development of minichromosomes, a yeast artificial chromosome (YAC)-like cloning system and investigations into chromosome segregation and behaviour during mitosis and meiosis. Many mutants have been isolated which are defective in chromosome segregation. The development of the fluorescent in-situ hybridization (FISH) technique for use in S. pombe has allowed the localization of centromeres and telomeres throughout mitosis and meiosis. In combination with indirect immunofluorescence to detect spindle and chromosomal proteins, the FISH technique should further advance our understanding of fission yeast chromosome structure and function. The recent discovery of a heterochromatin-like structure mediating transcriptional repression at centromeres reinforces the notion that fission yeast centromeres are similar to those of larger eukaryotes. Further characterization of such phenomena will accelerate the genetic dissection of this important chromosomal element.

The silent P mating type locus in fission yeast contains two autonomously replicating sequences

We show that in fission yeast two DNA fragments at the silent P mating type locus provide plasmids with the capability of autonomous replication. Bacterial vectors containing these sequences replicate in a polymeric form in fission yeast very much like plasmids with the commonly used replication sequence ars1, do. There are, however, several differences between the two new ars sequences. The percentage of cells containing the plasmid during selection, the plasmid copy number and the plasmid segregation during mitosis are all dependent on the choice of the ars sequence. A DNA fragment with ars activity from the left side of the silent P cassette represses the expression of the marker gene, ura4+, at least three hundred fold compared to plasmids containing only the other new ars sequence or only ars1. The importance of replication in this promoter independent transcriptional regulation is further substantiated by the fact that the repression is partially released in the presence of ars1 on the same plasmid.

Trans-acting factors and properly positioned DNA elements repress mating-type genes in fission yeast

Repression of the mating-type P genes at the silent mat2-P locus in fission yeast is dependent on four cis-acting DNA elements, two on each side of the coding sequences. The mechanism by which these elements exert their influence on the mating-type promoter is studied here by insertion of a bacterial antibiotic resistance gene at several positions in the silent region. The behavior of the resistance gene itself, and the changes its insertion causes in mating-type expression, reveal that the repressive elements have a limited range of action and that the four elements have unequal effects on gene expression. Repression of the antibiotic resistance gene inside the silent region leads to an antibiotic-sensitive phenotype and facilitates the selection of resistant mutants. These mutants can de-repress the resistance gene at other positions than the one used for their selection. Strong antibiotic resistance correlates with derepression of the plasmid-borne mating-type cassette. These data argue that mat2-P repression is dependent on trans-acting factors and the positioning of the repressive DNA elements, but less dependent on the nature of the affected promoter.