Selective excision of the centromere chromatin complex from Saccharomyces cerevisiae

Histone H3 localizes to the centromeric DNA in budding yeast

During cell division, segregation of sister chromatids to daughter cells is achieved by the poleward pulling force of microtubules, which attach to the chromatids by means of a multiprotein complex, the kinetochore. Kinetochores assemble at the centromeric DNA organized by specialized centromeric nucleosomes. In contrast to other eukaryotes, which typically have large repetitive centromeric regions, budding yeast CEN DNA is defined by a 125 bp sequence and assembles a single centromeric nucleosome. In budding yeast, as well as in other eukaryotes, the Cse4 histone variant (known in vertebrates as CENP-A) is believed to substitute for histone H3 at the centromeric nucleosome. However, the exact composition of the CEN nucleosome remains a subject of debate. We report the use of a novel ChIP approach to reveal the composition of the centromeric nucleosome and its localization on CEN DNA in budding yeast. Surprisingly, we observed a strong interaction of H3, as well as Cse4, H4, H2A, and H2B, but not histone chaperone Scm3 (HJURP in human) with the centromeric DNA. H3 localizes to centromeric DNA at all stages of the cell cycle. Using a sequential ChIP approach, we could demonstrate the co-occupancy of H3 and Cse4 at the CEN DNA. Our results favor a H3-Cse4 heterotypic octamer at the budding yeast centromere. Whether or not our model is correct, any future model will have to account for the stable association of histone H3 with the centromeric DNA.

During cell division, replicated DNA molecules are pulled to daughter cells by microtubules, which originate at the spindle poles and attach to a multiprotein complex, the kinetochore. The kinetochore assembles at a special region of the chromosome, termed the centromere. The kinetochore is comprised of more than 50 different proteins whose precise functions are far from being fully understood. The kinetochore assembles on the foundation of a specialized centromeric nucleosome. A nucleosome is a complex of eight subunits, termed histones, which compacts the DNA by wrapping it around itself in 1.7 turns of a superhelix. The centromeric nucleosome is very special, and its stoichiometry and structure are a subject of intense debate. It is believed that the centromeric nucleosome is devoid of histone H3 and instead contains its variant, termed CENP-A in vertebrates or Cse4 in budding yeast. Here we report that in budding yeast both CENP-A and histone H3 localize to a small centromeric DNA fragment that, due to its size, cannot accommodate more than a single nucleosome. Our results necessitate a revision of what is known about the structure of the inner kinetochore and the role of CENP-A in its assembly.

Mechanisms of chromosome number evolution in yeast

The whole-genome duplication (WGD) that occurred during yeast evolution changed the basal number of chromosomes from 8 to 16. However, the number of chromosomes in post-WGD species now ranges between 10 and 16, and the number in non-WGD species (Zygosaccharomyces, Kluyveromyces, Lachancea, and Ashbya) ranges between 6 and 8. To study the mechanism by which chromosome number changes, we traced the ancestry of centromeres and telomeres in each species. We observe only two mechanisms by which the number of chromosomes has decreased, as indicated by the loss of a centromere. The most frequent mechanism, seen 8 times, is telomere-to-telomere fusion between two chromosomes with the concomitant death of one centromere. The other mechanism, seen once, involves the breakage of a chromosome at its centromere, followed by the fusion of the two arms to the telomeres of two other chromosomes. The only mechanism by which chromosome number has increased in these species is WGD. Translocations and inversions have cycled telomere locations, internalizing some previously telomeric genes and creating novel telomeric locations. Comparison of centromere structures shows that the length of the CDEII region is variable between species but uniform within species. We trace the complete rearrangement history of the Lachancea kluyveri genome since its common ancestor with Saccharomyces and propose that its exceptionally low level of rearrangement is a consequence of the loss of the non-homologous end joining (NHEJ) DNA repair pathway in this species.

The number of chromosomes in organisms often changes over evolutionary time. To study how the number changes, we compare several related species of yeast that share a common ancestor roughly 150 million years ago and have varying numbers of chromosomes. By inferring ancestral genome structures, we examine the changes in location of centromeres and telomeres, key elements that biologically define chromosomes. Their locations change over time by rearrangements of chromosome segments. By following these rearrangements, we trace an evolutionary path between existing centromeres and telomeres to those in the ancestral genomes, allowing us to identify the specific evolutionary events that caused changes in chromosome number. We show that, in these yeasts, chromosome number has generally decreased over time except for one notable exception: an event in an ancestor of several species where the whole genome was duplicated. Chromosome number reduction occurs by the simultaneous removal of a centromere from a chromosome and fusion of the rest of the chromosome to another that contains a working centromere. This process also results in telomere removal and the movement of genes from the ends of chromosomes to new locations in the middle of chromosomes.

Saccharomyces telomeres assume a non-nucleosomal chromatin structure

The chromatin structures of the telomeric and subtelomeric regions on chromosomal DNA molecules in Saccharomyces cerevisiae were analyzed using micrococcal nuclease and DNAse I. The subtelomeric repeats X and Y' were assembled in nucleosomes. However, the terminal tracts of C1-3A repeats were protein protected in a particle larger than a nucleosome herein called a telosome. The proximal boundary of the telosome was a DNase I hypersensitive site. This boundary between the telosome and adjacent nucleosomes was completely accessible to Escherichia coli dam methylase when this enzyme was expressed in yeast, whereas a site 250 bp internal to the telomeric repeats was relatively inaccessible. Telosomes could be cleaved from chromosome ends with nuclease and solubilized as protein-DNA complexes. Immunoprecipitation of chromosomal telosomes with antiserum to the RAP1 protein indicated that RAP1 was one component of isolated telosomes. Thus, the termini of chromosomal DNA molecules in yeast are assembled in a non-nucleosomal structure encompassing the entire terminal C1-3A tract. This structure is separated from adjacent nucleosomes by a region of DNA that is highly accessible to enzymes.

Rapid kidney changes resulting from glycosphingolipid depletion by treatment with a glucosyltransferase inhibitor

The ceramide analog, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, inhibits the glucosylation of ceramide and thus, by virtue of the normal catabolism of the higher glucosphingolipids, leads to a general depletion of cellular glucolipids. In a previous study with chronic administration of this inhibitor in mice, it was found that the kidneys and liver, particularly the former, grew more poorly than the organs of control mice. This study shows that the inhibitor produces rapid decreases in glucolipid concentration in kidney which are maintained for at least 5 days without noticeable harm. The changes were enhanced by inclusion of L-cycloserine in the injection scheme. Cycloserine blocks ketosphinganine synthase and thus slows the synthesis of all sphingolipids. However, sphingomyelin levels did not drop significantly in this study. The glucosyltransferase inhibitor also produced a small decrease in kidney beta-D-glucuronidase and distinct increases in the levels of glucocerebrosidase, galactocerebrosidase and sphingomyelinase. It also produced a small but distinct decrease in the level of glucosyltransferase, after a delay of a few hours, possibly because the inhibitor was metabolized to a covalently inactivating product. Comparison with kidney, liver and brain showed that the kidney was more sensitive to the action of the morpholino inhibitor.

Nucleosome depletion alters the chromatin structure of Saccharomyces cerevisiae centromeres

Saccharomyces cerevisiae centromeric DNA is packaged into a highly nuclease-resistant chromatin core of approximately 200 base pairs of DNA. The structure of the centromere in chromosome III is somewhat larger than a 160-base-pair nucleosomal core and encompasses the conserved centromere DNA elements (CDE I, II, and III). Extensive mutational analysis has revealed the sequence requirements for centromere function. Mutations affecting the segregation properties of centromeres also exhibit altered chromatin structures in vivo. Thus the structure, as delineated by nuclease digestion, correlated with functional centromeres. We have determined the contribution of histone proteins to this unique structural organization. Nucleosome depletion by repression of either histone H2B or H4 rendered the cell incapable of chromosome segregation. Histone repression resulted in increased nuclease sensitivity of centromere DNA, with up to 40% of CEN3 DNA molecules becoming accessible to nucleolytic attack. Nucleosome depletion also resulted in an alteration in the distribution of nuclease cutting sites in the DNA surrounding CEN3. These data provide the first indication that authentic nucleosomal subunits flank the centromere and suggest that nucleosomes may be the central core of the centromere itself.

RAP1 protein interacts with yeast telomeres in vivo: overproduction alters telomere structure and decreases chromosome stability

The protein encoded by the RAP1 gene of S. cerevisiae binds in vitro to a consensus sequence occurring at a number of sites in the yeast genome, including the repeated sequence C2-3A(CA)1-6 found at yeast telomeres. We present two lines of evidence for the in vivo binding of RAP1 protein at telomeres: first, RAP1 is present in telomeric chromatin and second, alterations in the level of RAP1 protein affect telomere length. The length changes seen with under- and overexpression of RAP1 are consistent with the interpretation that RAP1 binding to telomeres protects them from degradation. Unexpectedly, overproduction of the RAP1 protein was also shown to decrease greatly chromosome stability, suggesting that RAP1 mediates interactions that have a more global effect on chromosome behavior than simply protecting telomeres from degradation. Such interactions may involve telomere associations both with other telomeres and/or with structural elements of the nucleus.

Nucleosome depletion alters the chromatin structure of Saccharomyces cerevisiae centromeres

Saccharomyces cerevisiae centromeric DNA is packaged into a highly nuclease-resistant chromatin core of approximately 200 base pairs of DNA. The structure of the centromere in chromosome III is somewhat larger than a 160-base-pair nucleosomal core and encompasses the conserved centromere DNA elements (CDE I, II, and III). Extensive mutational analysis has revealed the sequence requirements for centromere function. Mutations affecting the segregation properties of centromeres also exhibit altered chromatin structures in vivo. Thus the structure, as delineated by nuclease digestion, correlated with functional centromeres. We have determined the contribution of histone proteins to this unique structural organization. Nucleosome depletion by repression of either histone H2B or H4 rendered the cell incapable of chromosome segregation. Histone repression resulted in increased nuclease sensitivity of centromere DNA, with up to 40% of CEN3 DNA molecules becoming accessible to nucleolytic attack. Nucleosome depletion also resulted in an alteration in the distribution of nuclease cutting sites in the DNA surrounding CEN3. These data provide the first indication that authentic nucleosomal subunits flank the centromere and suggest that nucleosomes may be the central core of the centromere itself.

Heterogeneity and maintenance of centromere plasmid copy number in Saccharomyces cerevisiae

We developed a novel approach to quantitate the heterogeneity of centromere number in yeast, and the cellular capacity for excess centromeres. Small circular plasmids were constructed to contain the CUP1 metallothionein gene. ARS1 (autonomously replicating sequence) and a conditionally functional centromere (GAL1-GAL10 promoter controlled centromere). The CUP1 gene provided a gene dosage marker, and therefore a genetic determinant of plasmid copy number. Growth of cells on glucose is permissive for centromere function, while growth on galactose renders the centromere nonfunctional and the plasmids are segregated in an asymmetric fashion. We identified "lines" of cells containing increased numbers of plasmids after transformation. Cell lines containing as many as five to ten active centromeres are stably maintained in the absence of genetic selection. Thus haploid yeast cells can tolerate a 50% increase in their centromere number without affecting progression through the cell cycle. This system provides the opportunity to address issues of specific cellular controls on centromere copy number.

Chromatin digestion with restriction endonucleases reveals 150-160 bp of protected DNA in the centromere of chromosome XIV in Saccharomyces cerevisiae

Isolated nuclei of Saccharomyces cerevisiae were incubated with five restriction nucleases. Out of the twenty-one recognition sequences for these nucleases in the centromere region of chromosome XIV, only five are accessible to cleavage. These sites map 11 bp and 74 bp to the left and 27 bp, 41 bp and 290 bp to the right, respectively, of the boundaries of the 118 bp functional CEN14 DNA sequence. The distance between the sites accessible to cleavage and closest to CEN14 is 156 bp, suggesting this is the maximal size of DNA protected in CEN14 chromatin. The DNA in CEN14 chromatin protected against cleavage with DNase I and micrococcal nuclease overlaps almost completely with this region. Hypersensitive regions flanking both sides are approximately 60 bp long. Analyses of other S. cerevisiae centromeres with footprinting techniques in intact cells or nucleolytic cleavages in isolated nuclei are discussed in relation to our results. We conclude that structural data of chromatin obtained with restriction nucleases are reliable and that the structure of CEN14 chromatin is representative for S. cerevisiae centromeres.

The structure of a primitive kinetochore

The isolation of yeast centromeres has provided the opportunity to describe the molecular structure of chromosome attachments to the mitotic spindle. Nucleolytic probes of chromatin structure and construction of conditional mutants in centromere function have been used to study the regulation and assembly of centromeres throughout the cell cycle in Saccharomyces cerevisiae.