Chromosomal addresses of the cohesin component Mcd1p

Association of the RENT complex with nontranscribed and coding regions of rDNA and a regional requirement for the replication fork block protein Fob1 in rDNA silencing

Silencing within the yeast rDNA repeats inhibits hyperrecombination, represses transcription from foreign promoters, and extends replicative life span. rDNA silencing is mediated by a Sir2-containing complex called RENT (regulator of nucleolar silencing and telophase exit). We show that the Net1 (also called Cfi1) and Sir2 subunits of RENT localize primarily to two distinct regions within rDNA: in one of the nontranscribed spacers (NTS1) and around the Pol I promoter, extending into the 35S rRNA coding region. Binding to NTS1 overlaps the recombination hotspot and replication fork barrier elements, which have been shown previously to require the Fob1 protein for their activities. In cells lacking Fob1, silencing and the association of RENT subunits are abolished specifically at NTS1, while silencing and association at the Pol I promoter region are unaffected or increased. We find that Net1 and Sir2 are physically associated with Fob1 and subunits of RNA polymerase I. Together with the localization data, these results suggest the existence of two distinct modes for the recruitment of the RENT complex to rDNA and reveal a role for Fob1 in rDNA silencing and in the recruitment of the RENT complex. Furthermore, the Fob1-dependent associations of Net1 and Sir2 with the recombination hotspot region strongly suggest that Sir2 acts directly at this region to carry out its inhibitory effect on rDNA recombination and accelerated aging.

Condensin and cohesin: more than chromosome compactor and glue

Two related protein complexes, cohesin and condensin, are essential for separating identical copies of the genome into daughter cells during cell division. Cohesin glues replicated sister chromatids together until they split at anaphase, whereas condensin reorganizes chromosomes into their highly compact mitotic structure. Unexpectedly, mutations in the subunits of these complexes have been uncovered in genetic screens that target completely different processes. Exciting new evidence is emerging that cohesin and condensin influence crucial processes during interphase, and unforeseen aspects of mitosis. Each complex can perform several roles, and individual subunits can associate with different sets of proteins to achieve diverse functions, including the regulation of gene expression, DNA repair, cell-cycle checkpoints and centromere organization.

Saccharomyces cerevisiae DNA polymerase epsilon and polymerase sigma interact physically and functionally, suggesting a role for polymerase epsilon in sister chromatid cohesion

The large subunit of Saccharomyces cerevisiae DNA polymerase epsilon, Pol2, comprises two essential functions. The N terminus has essential DNA polymerase activity. The C terminus is also essential, but its function is unknown. We report here that the C-terminal domain of Pol2 interacts with polymerase sigma (Pol sigma), a recently identified, essential nuclear nucleotidyl transferase encoded by two redundant genes, TRF4 and TRF5. This interaction is functional, since Pol sigma stimulates the polymerase activity of the Pol epsilon holoenzyme significantly. Since Trf4 is required for sister chromatid cohesion as well as for completion of S phase and repair, the interaction suggested that Pol epsilon, like Pol sigma, might form a link between the replication apparatus and sister chromatid cohesion and/or repair machinery. We present evidence that pol2 mutants are defective in sister chromatid cohesion. In addition, Pol2 interacts with SMC1, a subunit of the cohesin complex, and with ECO1/CTF7, required for establishing sister chromatid cohesion; and pol2 mutations act synergistically with smc1 and scc1. We also show that trf5 Delta mutants, like trf4 Delta mutants, are defective in DNA repair and sister chromatid cohesion.

Centromeres and kinetochores: from epigenetics to mitotic checkpoint signaling

The centromere is a chromosomal locus that ensures delivery of one copy of each chromosome to each daughter at cell division. Efforts to understand the nature and specification of the centromere have demonstrated that this central element for ensuring inheritance is itself epigenetically determined. The kinetochore, the protein complex assembled at each centromere, serves as the attachment site for spindle microtubules and the site at which motors generate forces to power chromosome movement. Unattached kinetochores are also the signal generators for the mitotic checkpoint, which arrests mitosis until all kinetochores have correctly attached to spindle microtubules, thereby representing the major cell cycle control mechanism protecting against loss of a chromosome (aneuploidy).

Chromosome cohesion and separation: from men and molecules

Sister chromatid cohesion and separation are fundamental for accurate genome inheritance over cell generations. Work over recent years has established the existence of a chromosomal protein complex, cohesin, that connects sister chromatids from the time they are generated in S phase onwards, and which is destroyed at the onset of anaphase through cleavage by the protease separase. Over the last year, the function of cohesin has been investigated in higher eukaryotes, including humans, with results that have uncovered important new aspects of this process. The first structural views of cohesin have become available, and significant steps been made towards a mechanistic understanding of chromosome cohesion. Studies on separase have revealed new levels of regulation of chromosome segregation.

SMC protein complexes and the maintenance of chromosome integrity

Structural maintenance of chromosomes (SMC) family proteins have attracted much attention for their unique protein structure and critical roles in mitotic chromosome organization. Elegant genetic and biochemical studies in yeast and Xenopus identified two different SMC heterodimers in two conserved multiprotein complexes termed 'condensin' and 'cohesin'. These complexes are required for mitotic chromosome condensation and sister chromatid cohesion, respectively, both of which are prerequisite to accurate segregation of chromosomes. Although structurally similar, the SMC proteins in condensin and cohesin appear to have distinct functions, whose specificity and cell cycle regulation are critically determined by their interactions with unique sets of associated proteins. Recent studies of subcellular localization of SMC proteins and SMC-containing complexes, identification of their interactions with other cellular factors, and discovery of new SMC family members have uncovered unexpected roles for SMC proteins and SMC-containing complexes in different aspects of genome functions and chromosome organization beyond mitosis, all of which are critical for the maintenance of chromosome integrity.

Chromatin proteins are determinants of centromere function

Recent advances in the identification of molecular components of centromeres have demonstrated a crucial role for chromatin proteins in determining both centromere identity and the stability of kinetochore-microtubule attachments. Although we are far from a complete understanding of the establishment and propagation of centromeres, this review seeks to highlight the contribution of histones, histone deposition factors, histone modifying enzymes, and heterochromatin proteins to the assembly of this sophisticated, highly specialized chromatin structure. First, an overview of DNA sequence elements at centromeric regions will be presented. We will then discuss the contribution of chromatin to kinetochore function in budding yeast, and pericentric heterochromatin domains in other eukaryotic systems. We will conclude with discussion of specialized nucleosomes that direct kinetochore assembly and propagation of centromere-defining chromatin domains.

RNA polymerase I propagates unidirectional spreading of rDNA silent chromatin

The ribosomal DNA (rDNA) tandem array in Saccharomyces cerevisiae induces transcriptional silencing of RNA polymerase II-transcribed genes. This SIR2-dependent form of repression (rDNA silencing) also functions to limit rDNA recombination and is involved in life span control. In this report, we demonstrate that rDNA silencing spreads into the centromere-proximal unique sequence located downstream of RNA polymerase I (Pol I) transcription, but fails to enter the upstream telomere-proximal sequences. The spreading of silencing correlates with SIR2-dependent histone H3 and H4 deacetylation and can be extended by SIR2 overexpression. Surprisingly, rDNA silencing required transcription by RNA polymerase I and the direction of spreading was controlled by the direction of Pol I transcription.

Physical and functional interactions among basic chromosome organizational features govern early steps of meiotic chiasma formation

Analysis of meiotic recombination by functional genomic approaches reveals prominent spatial and functional interactions among diverse organizational determinants. Recombination occurs between chromatin loop sequences; however, these sequences are spatially tethered to underlying chromosome axes via their recombinosomes. Meiotic chromosomal protein, Red1, localizes to chromosome axes; however, Red1 loading is modulated by R/G-bands isochores and thus by bulk chromatin state. Recombination is also modulated by isochore determinants: R-bands differentially favor double-strand break (DSB) formation but disfavor subsequent loading of meiotic RecA homolog, Dmc1. Red1 promotes DSB formation in both R- and G-bands and then promotes Dmc1 loading, specifically counteracting disfavoring R-band effects. These complexities are discussed in the context of chiasma formation as a series of coordinated local changes at the DNA and chromosome-axis levels.

Cell-cycle control of the establishment of mating-type silencing in S. cerevisiae

Transcriptional silencing in Saccharomyces cerevisiae involves the assembly of a heterochromatic domain that is heritable from generation to generation. To maintain the silenced state, the propagation of silent heterochromatin must be coordinated with the events of chromosome duplication and segregation. Here we present an in vivo analysis of the cell-cycle events required for the establishment of the silenced state at the HMR silent mating-type locus. We show that Sir protein recruitment to and spreading from the HMRE silencer is poor during S phase, but is robust during G2 and subsequent phases. Despite abundant Sir protein association in cells arrested in G2/M phase, silencing is not fully established by this stage of the cell cycle. Rather, robust silencing is not observed until telophase. Interestingly, the elimination of the cohesin subunit Scc1/Mcd1p allows the full establishment of silencing to occur in G2/M phase. Furthermore, expression of a noncleavable allele of Scc1/Mcd1p inhibits the establishment of silencing. Our findings reveal both S- and M-phase requirements for the establishment of silencing and implicate the loss of sister-chromatid cohesion as a critical event in this process.

Subcellular localization of the Bacillus subtilis structural maintenance of chromosomes (SMC) protein

The Bacillus subtilis structural maintenance of chromosomes (SMC) protein is a member of a large family of proteins involved in chromosome organization. We found that SMC is a moderately abundant protein ( approximately 1000 dimers per cell). In vivo cross-linking and immunoprecipitation assays revealed that SMC binds to many regions on the chromosome. Visualization of SMC in live cells using a fusion to the green fluorescent protein (GFP) and in fixed cells using immunofluorescence microscopy indicated that a portion of SMC localizes as discrete foci in positions similar to that of the DNA replication machinery (replisome). When visualized simultaneously, SMC and the replisome were often in similar regions of the cell but did not always co-localize. Persistence of SMC foci did not depend on ongoing replication, but did depend on ScpA and ScpB, two proteins thought to interact with SMC. Our results indicate that SMC is bound to many sites on the chromosome and a concentration of SMC is localized near replication forks, perhaps there to bind and organize newly replicated DNA.

Structure, function and DNA composition of Saccharomyces cerevisiae chromatin loops

Recent localization of cohesin association regions along the yeast chromatin fibre suggests that compositional variability of DNA in yeast is related to the function and organization of the chromosomal loops. The bases of the loops, where the chromatin fibre is attached to the chromosomal axis, are AT-rich, bind cohesin, and are flanked by genes transcribed convergently. The hotspots of meiotic recombination are mainly found in the GC-rich parts of the loops, 'external' with respect to the chromosomal axis, frequently in the vicinity of the promoters of divergently transcribed genes. There are two possible reasons why the regions of the hotspots of recombination were enriched in GC content during evolution. One is a biased repair of recombination intermediates, and the second is a selective advantage due to an increased chromatin accessibility, which may have the carriers of GC-enriched alleles over the carriers of AT-rich alleles.

Chromosome cohesion: ring around the sisters?

Sister chromatid cohesion is a key aspect of accurate chromosome transmission during mitosis, yet little is known about the structure of cohesin, the protein complex that links the two sister chromatids. Recent studies shed light on the structure of the cohesin complex, leading to intriguing models that could explain how sister chromatids are held together.

The many functions of SMC proteins in chromosome dynamics

Members of the structural maintenance of chromosomes (SMC) family share a characteristic design and configuration of protein domains that provides the molecular basis for the various functions of this family in chromosome dynamics. SMC proteins have a role in chromosome condensation, sister-chromatid cohesion, DNA repair and recombination, and gene dosage compensation, and they function in somatic and meiotic cells. As more is learned about how their unique design affects their function, a picture of a dynamic and varied protein family is emerging.

Centromeres become unstuck without heterochromatin

In most if not all eukaryotes, sister-chromatid cohesion, which is mediated by the chromosomal complex Cohesin, is destroyed by proteolysis at the transition from metaphase to anaphase. In metazoans, Cohesin is removed from chromosomes in two steps, and the centromere and its associated pericentric heterochromatin constitute the last point of linkage between sister chromatids at metaphase. Mechanistic insight is now emerging on the way in which cells distinguish cohesion at the centromere from cohesion along chromosome arms. We discuss recent advances in our understanding of the role of centromeric heterochromatin in sister-chromatid cohesion and propose a causal relationship between this specialized type of chromatin and the removal by proteolysis of Cohesins that are associated with it.

The 2 micron plasmid purloins the yeast cohesin complex: a mechanism for coupling plasmid partitioning and chromosome segregation?

The yeast 2 micron plasmid achieves high fidelity segregation by coupling its partitioning pathway to that of the chromosomes. Mutations affecting distinct steps of chromosome segregation cause the plasmid to missegregate in tandem with the chromosomes. In the absence of the plasmid stability system, consisting of the Rep1 and Rep2 proteins and the STB DNA, plasmid and chromosome segregations are uncoupled. The Rep proteins, acting in concert, recruit the yeast cohesin complex to the STB locus. The periodicity of cohesin association and dissociation is nearly identical for the plasmid and the chromosomes. The timely disassembly of cohesin is a prerequisite for plasmid segregation. Cohesin-mediated pairing and unpairing likely provides a counting mechanism for evenly partitioning plasmids either in association with or independently of the chromosomes.

Expression-state boundaries in the mating-type region of fission yeast

A transcriptionally silent chromosomal domain is found in the mating-type region of fission yeast. Here we show that this domain is delimited by 2-kb inverted repeats, IR-L and IR-R. IR-L and IR-R prevent the expansion of transcription-permissive chromatin into the silenced region and that of silenced chromatin into the expressed region. Their insulator activity is partially orientation dependent. The silencing defects that follow deletion or inversion of IR-R are suppressed by high dosage of the chromodomain protein Swi6. Combining chromosomal deletions and Swi6 overexpression shows that IR-L and IR-R provide firm borders in a region where competition between silencing and transcriptional competence occurs. IR-R possesses autonomously replicating sequence (ARS) activity, leading to a model where replication factors, or replication itself, participate in boundary formation.

The SUMO-1 isopeptidase Smt4 is linked to centromeric cohesion through SUMO-1 modification of DNA topoisomerase II

In S. cerevisiae, posttranslational modification by the ubiquitin-like Smt3/SUMO-1 protein is essential for survival, but functions and cellular targets for this modification are largely unknown. We find that one function associated with the Smt3/SUMO-1 isopeptidase Smt4 is to control chromosome cohesion at centromeric regions and that a key Smt3/SUMO-1 substrate underlying this function is Top2, DNA Topoisomerase II. Top2 modification by Smt3/SUMO-1 is misregulated in smt4 strains, and top2 mutants resistant to Smt3/SUMO-1 modification suppress the smt4 cohesion defect. top2 mutants display aberrant chromatid stretching at the centromere in response to mitotic spindle tension and altered chromatid reassociation following microtubule depolymerization. These results suggest Top2 modification by Smt3/SUMO-1 regulates a component of chromatin structure or topology required for centromeric cohesion.

Beyond the ABCs of CKC and SCC. Do centromeres orchestrate sister chromatid cohesion or vice versa?

The centromere-kinetochore complex is a highly specialized chromatin domain that both mediates and monitors chromosome-spindle interactions responsible for accurate partitioning of sister chromatids to daughter cells. Centromeres are distinguished from adjacent chromatin by specific patterns of histone modification and the presence of a centromere-specific histone H3 variant (e.g. CENP-A). Centromere-proximal regions usually correspond to sites of avid and persistent sister chromatid cohesion mediated by the conserved cohesin complex. In budding yeast, there is a substantial body of evidence indicating centromeres direct formation and/or stabilization of centromere-proximal cohesion. In other organisms, the dependency of cohesion on centromere function is not as clear. Indeed, it appears that pericentromeric heterochromatin recruits cohesion proteins independent of centromere function. Nonetheless, aspects of centromere function are impaired in the absence of sister chromatid cohesion, suggesting the two are interdependent. Here we review the nature of centromeric chromatin, the dynamics and regulation of sister chromatid cohesion, and the relationship between the two.

Braking the silence: how heterochromatic gene repression is stopped in its tracks

Eukaryotic DNA is assembled into nucleosomes, which are further packaged into higher order chromatin structures containing many non-histone chromosomal proteins. The details of this packaging have profound effects on gene expression and other cellular processes involving the genetic material. Heterochromatic domains of the genome are usually transcriptionally repressed, while euchromatic regions are transcriptionally competent. Current models of gene activation postulate the existence of boundary elements that either prevent inappropriate activation of genes by distal enhancers (enhancer blockers), or sequences that block the propagation of heterochromatin into euchromatic regions (barriers). While numerous boundary sequences have been identified, little is known with regard to the molecular mechanisms used to punctuate the genome. This review will focus on recent data that provide insight into the mode of action of barrier elements.

In vivo dissection of the chromosome condensation machinery: reversibility of condensation distinguishes contributions of condensin and cohesin

The machinery mediating chromosome condensation is poorly understood. To begin to dissect the in vivo function(s) of individual components, we monitored mitotic chromosome structure in mutants of condensin, cohesin, histone H3, and topoisomerase II (topo II). In budding yeast, both condensation establishment and maintenance require all of the condensin subunits, but not topo II activity or phospho-histone H3. Structural maintenance of chromosome (SMC) protein 2, as well as each of the three non-SMC proteins (Ycg1p, Ycs4p, and Brn1p), was required for chromatin binding of the condensin complex in vivo. Using reversible condensin alleles, we show that chromosome condensation does not involve an irreversible modification of condensin or chromosomes. Finally, we provide the first evidence of a mechanistic link between condensin and cohesin function. A model discussing the functional interplay between cohesin and condensin is presented.

Capturing chromosome conformation

We describe an approach to detect the frequency of interaction between any two genomic loci. Generation of a matrix of interaction frequencies between sites on the same or different chromosomes reveals their relative spatial disposition and provides information about the physical properties of the chromatin fiber. This methodology can be applied to the spatial organization of entire genomes in organisms from bacteria to human. Using the yeast Saccharomyces cerevisiae, we could confirm known qualitative features of chromosome organization within the nucleus and dynamic changes in that organization during meiosis. We also analyzed yeast chromosome III at the G1 stage of the cell cycle. We found that chromatin is highly flexible throughout. Furthermore, functionally distinct AT- and GC-rich domains were found to exhibit different conformations, and a population-average 3D model of chromosome III could be determined. Chromosome III emerges as a contorted ring.

The molecular basis of sister-chromatid cohesion

The replicated copies of each chromosome, the sister chromatids, are attached prior to their segregation in mitosis and meiosis. This association or cohesion is critical for each sister chromatid to bind to microtubules from opposite spindle poles and thus segregate away from each other at anaphase of mitosis or meiosis II. The cohesin protein complex is essential for cohesion in both mitosis and meiosis, and cleavage of one of the subunits is sufficient for loss of cohesion at anaphase. The localization of the cohesin complex and other cohesion proteins permits evaluation of the positions of sister-chromatid associations within the chromosome structure, as well as the relationship between cohesion and condensation. Recently, two key riddles in the mechanism of meiotic chromosome segregation have yielded to molecular answers. First, analysis of the cohesin complex in meiosis provides molecular support for the long-standing hypothesis that sister-chromatid cohesion links homologs in meiosis I by stabilizing chiasmata. Second, the isolation of the monopolin protein that controls kinetochore behavior in meiosis I defines a functional basis by which sister kinetochores are directed toward the same pole in meiosis I.

Requirement of heterochromatin for cohesion at centromeres

Centromeres are heterochromatic in many organisms, but the mitotic function of this silent chromatin remains unknown. During cell division, newly replicated sister chromatids must cohere until anaphase when Scc1/Rad21-mediated cohesion is destroyed. In metazoans, chromosome arm cohesins dissociate during prophase, leaving centromeres as the only linkage before anaphase. It is not known what distinguishes centromere cohesion from arm cohesion. Fission yeast Swi6 (a Heterochromatin protein 1 counterpart) is a component of silent heterochromatin. Here we show that this heterochromatin is specifically required for cohesion between sister centromeres. Swi6 is required for association of Rad21-cohesin with centromeres but not along chromosome arms and, thus, acts to distinguish centromere from arm cohesion. Therefore, one function of centromeric heterochromatin is to attract cohesin, thereby ensuring sister centromere cohesion and proper chromosome segregation.

Chromosome cohesion and segregation in mitosis and meiosis

The faithful segregation of the genetic material into daughter cells during cell division is crucial for the production of healthy progeny. Sister chromatid cohesion and separation are fundamental to this process. Progress has been made in our molecular understanding of cohesion and mechanisms for the dissolution of cohesion have been uncovered.

Identification of a conserved erythroid specific domain of histone acetylation across the alpha-globin gene cluster

We have analyzed the pattern of core histone acetylation across 250 kb of the telomeric region of the short arm of human chromosome 16. This gene-dense region, which includes the alpha-globin genes and their regulatory elements embedded within widely expressed genes, shows marked differences in histone acetylation between erythroid and non-erythroid cells. In non-erythroid cells, there was a uniform 2- to 3-fold enrichment of acetylated histones, compared with heterochromatin, across the entire region. In erythroid cells, an approximately 100-kb segment of chromatin encompassing the alpha genes and their remote major regulatory element was highly enriched in histone H4 acetylated at Lys-5. Other lysines in the N-terminal tail of histone H4 showed intermediate and variable levels of enrichment. Similar broad segments of erythroid-specific histone acetylation were found in the corresponding syntenic regions containing the mouse and chicken alpha-globin gene clusters. The borders of these regions of acetylation are located in similar positions in all three species, and a sharply defined 3' boundary coincides with the previously identified breakpoint in conserved synteny between these species. We have therefore demonstrated that an erythroid-specific domain of acetylation has been conserved across several species, encompassing not only the alpha-globin genes but also a neighboring widely expressed gene. These results contrast with those at other clusters and demonstrate that not all genes are organized into discrete regulatory domains.

Shaping the metaphase chromosome: coordination of cohesion and condensation

Recent progress in our understanding of mitotic chromosome dynamics has been accelerated by the identification of two essential protein complexes, cohesin and condensin. Cohesin is required for holding sister chromatids (duplicated chromosomes) together from S phase until the metaphase-to-anaphase transition. Condensin is a central player in chromosome condensation, a process that initiates at the onset of mitosis. The main focus of this review is to discuss how the mitotic metaphase chromosome is assembled and shaped by a precise balance between the cohesion and condensation machineries. We argue that, in different eukaryotic organisms, the balance of cohesion and condensation is adjusted in such a way that the size and shape of the resulting chromosomes are best suited for their accurate segregation.

Evidence that replication fork components catalyze establishment of cohesion between sister chromatids

Accurate chromosome segregation requires that replicated sister chromatids are held together until anaphase, when their "cohesion" is dissolved, and they are pulled to opposite spindle poles by microtubules. Establishment of new cohesion between sister chromatids in the next cell cycle is coincident with replication fork passage. Emerging evidence suggests that this temporal coupling is not just a coincident timing of independent events, but rather that the establishment of cohesion is likely to involve the active participation of replication-related activities. These include PCNA, a processivity clamp for some DNA polymerases, Trf4/Pol final sigma (formerly Trf4/Pol kappa), a novel and essential DNA polymerase, and a modified Replication Factor C clamp--loader complex. Here we describe recent advances in how cohesion establishment is linked to replication, highlight important unanswered questions in this new field, and describe a "polymerase switch" model for how cohesion establishment is coupled to replication fork progression. Building the bridges between newly synthesized sister chromatids appears to be a fundamental but previously unrecognized function of the eukaryotic replication machinery.

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

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

Chromosomal boundaries in S. cerevisiae

Chromatin boundary elements or insulators in metazoans delimit distinct chromosomal domains of gene expression. Recently, DNA sequences with properties similar to boundary elements were also discovered in Saccharomyces cerevisiae. These sequences block the spread of transcriptionally silent chromatin, the yeast equivalent of metazoan heterochromatin, and are referred to as 'heterochromatin barriers'. These barriers share no sequence homology but all consist of multiple binding sites for various regulatory proteins. Current data suggest that barriers may function in yeast by recruiting a protein complex that precludes nucleosome assembly and thereby disrupts a contiguous array of nucleosomes required for the spread of silent chromatin.