scholarly journals Reinventing Heterochromatin in Budding Yeasts: Sir2 and the Origin Recognition Complex Take Center Stage

2011 ◽  
Vol 10 (9) ◽  
pp. 1183-1192 ◽  
Author(s):  
Meleah A. Hickman ◽  
Cara A. Froyd ◽  
Laura N. Rusche
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Schizosaccharomyces Pombe ◽  
Mating Type ◽  
Origin Recognition Complex ◽  
Transcriptional Silencing ◽  
Gene Duplications ◽  
Heterochromatin Protein 1 ◽  
Heterochromatin Protein ◽  
Content Type ◽  
Sir Proteins

ABSTRACTThe transcriptional silencing of the cryptic mating-type loci inSaccharomyces cerevisiaeis one of the best-studied models of repressive heterochromatin. However, this type of heterochromatin, which is mediated by the Sir proteins, has a distinct molecular composition compared to the more ubiquitous type of heterochromatin found inSchizosaccharomyces pombe, other fungi, animals, and plants and characterized by the presence of HP1 (heterochromatin protein 1). This review discusses how the loss of important heterochromatin proteins, including HP1, in the budding yeast lineage presented an evolutionary opportunity for the development and diversification of alternative varieties of heterochromatin, in which the conserved deacetylase Sir2 and the replication protein Orc1 play key roles. In addition, we highlight how this diversification has been facilitated by gene duplications and has contributed to adaptations in lifestyle.

scholarly journals SUM1-1, a dominant suppressor of SIR mutations in Saccharomyces cerevisiae, increases transcriptional silencing at telomeres and HM mating-type loci and decreases chromosome stability.

1996 ◽  
Vol 16 (8) ◽  
pp. 4281-4294 ◽  
Author(s):  
M H Chi ◽  
D Shore
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Mating Type ◽  
Transcriptional Silencing ◽  
Chromosome Loss ◽  
Yeast Saccharomyces Cerevisiae ◽  
Sir Proteins ◽  
Telomere Binding Protein ◽  
Single Allele ◽  
Dominant Suppressor

Transcriptional silencing in the yeast Saccharomyces cerevisiae occurs at HML and HMR mating-type loci and telomeres and requires the products of the silent information regulator (SIR) genes. Recent evidence suggests that the silencer- and telomere-binding protein Rap1p initiates silencing by recruiting a complex of Sir proteins to the chromosome, where they act in some way to modify chromatin structure or accessibility. A single allele of the SUM1gene (SUM1-1) which restores silencing at HM loci in strains mutant for any of the four SIR genes was identified a number of years ago. However, conflicting genetic results and the lack of other alleles of SUM1 made it difficult to surmise the wild-type function of SUM1 or the manner in which the SUM1-1 mutation restores silencing in sir mutant strains. Here we report the cloning and characterization of the SUM1 gene and the SUM1-1 mutant allele. Our results indicate that SUM1-1 is an unusual altered-function mutation that can bypass the need for SIR function in HM silencing and increase repression at telomeres. A sum1 deletion mutation has only minor effects on silencing in SIR strains and does not restore silencing in sir mutants. In addition to its effect on transcriptional silencing, the SUM1-1 mutation (but not a sum1 deletion) increases the rate of chromosome loss and cell death. We suggest several speculative models for the action of SUM1-1 in silencing based on these and other data.

scholarly journals Asymmetric Positioning of Nucleosomes and Directional Establishment of Transcriptionally Silent Chromatin by Saccharomyces cerevisiae Silencers

2006 ◽  
Vol 26 (20) ◽  
pp. 7806-7819 ◽  
Author(s):  
Yanfei Zou ◽  
Qun Yu ◽  
Xin Bi
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Origin Recognition Complex ◽  
Transcriptional Silencing ◽  
Nucleosome Positioning ◽  
Silent Chromatin ◽  
Genomic Context ◽  
Independent Manner ◽  
Sir Proteins ◽  
Sir Complex ◽  
Asymmetrically Distributed

ABSTRACT In Saccharomyces cerevisiae, silencers flanking the HML and HMR loci consist of various combinations of binding sites for Abf1p, Rap1p, and the origin recognition complex (ORC) that serve to recruit the Sir silencing complex, thereby initiating the establishment of transcriptionally silent chromatin. There have been seemingly conflicting reports concerning whether silencers function in an orientation-dependent or -independent manner, and what determines the directionality of a silencer has not been explored. We demonstrate that chromatin plays a key role in determining the potency and directionality of silencers. We show that nucleosomes are asymmetrically distributed around the HML-I or HMR-E silencer so that a nucleosome is positioned close to the Abf1p side but not the ORC side of the silencer. This coincides with preferential association of Sir proteins and transcriptional silencing on the Abf1p side of the silencer. Elimination of the asymmetry in nucleosome positioning at a silencer leads to comparable silencing on both sides. Asymmetric nucleosome positioning in the immediate vicinity of a silencer is independent of its orientation and genomic context, indicating that it is the inherent property of the silencer. Moreover, it is also independent of the Sir complex and thus precedes the formation of silent chromatin. Finally, we demonstrate that asymmetric positioning of nucleosomes and directional silencing by a silencer depend on ORC and Abf1p. We conclude that the HML-I and HMR-E silencers promote asymmetric positioning of nucleosomes, leading to unequal potentials of transcriptional silencing on their sides and, hence, directional silencing.

scholarly journals A Novel Form of Transcriptional Silencing by Sum1-1 Requires Hst1 and the Origin Recognition Complex

2001 ◽  
Vol 21 (10) ◽  
pp. 3514-3522 ◽  
Author(s):  
Ann Sutton ◽  
Ryan C. Heller ◽  
Joseph Landry ◽  
Jennifer S. Choy ◽  
Agnieszka Sirko ◽  
...  
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Origin Recognition Complex ◽  
Chromatin Condensation ◽  
Transcriptional Silencing ◽  
The Novel ◽  
Dominant Mutation ◽  
Sir Proteins ◽  
Associated Proteins ◽  
Type Information ◽  
Origin Recognition

ABSTRACT In the yeast Saccharomyces cerevisiae, a and α mating-type information is stored in transcriptionally silenced cassettes called HML and HMR. Silencing of these loci, maintained by the formation of a specialized type of heterochromatin, requires trans-acting proteins andcis-acting elements. Proteins required for silencing include the Sir2 NAD+-dependent deacetylase, Sir3, and Sir4. Factors that bind to the cis elements atHMR and HML and that are important for silencing include the origin recognition complex (ORC). Mutations of any of these Sir proteins or combinations of cis elements result in loss of silencing. SUM1-1 was previously identified as a dominant mutation that restores silencing toHMR in the absence of either the Sir proteins or some of the cis elements. We have investigated the novel mechanism whereby Sum1-1 causes Sir-independent silencing at HMR and present the following findings: Sum1-1 requires the Sir2 homolog, Hst1, for silencing and most probably requires the NAD+-dependent deacetylase activity of this protein. Sum1-1 interacts strongly with ORC, and this strong interaction is dependent on HMR DNA. Furthermore, ORC is required for Sum1-1-mediated silencing atHMR. These observations lead to a model for Sum1-1 silencing of HMR in which Sum1-1 is recruited toHMR by binding to ORC. Sum1-1, in turn, recruits Hst1. Hst1 then deacetylates histones or other chromatin-associated proteins to cause chromatin condensation and transcriptional silencing.

scholarly journals A Silencer Promotes the Assembly of Silenced Chromatin Independently of Recruitment

2008 ◽  
Vol 29 (1) ◽  
pp. 43-56 ◽  
Author(s):  
Patrick J. Lynch ◽  
Laura N. Rusche
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Mating Type ◽  
Chromatin Immunoprecipitation ◽  
Protein Assembly ◽  
The Other ◽  
Sir Proteins ◽  
Genomic Locations ◽  
Nonlinear Fashion ◽  
Silent Mating Type Loci

ABSTRACT In Saccharomyces cerevisiae, silenced chromatin occurs at telomeres and the silent mating-type loci HMR and HML. At these sites, the Sir proteins are recruited to a silencer and then associate with adjacent chromatin. We used chromatin immunoprecipitation to compare the rates of Sir protein assembly at different genomic locations and discovered that establishment of silenced chromatin was much more rapid at HMR than at the telomere VI-R. Silenced chromatin also assembled more quickly on one side of HMR-E than on the other. Despite differences in spreading, the Sir proteins were recruited to HMR-E and telomeric silencers at equivalent rates. Additionally, insertion of HMR-E adjacent to the telomere VI-R increased the rate of Sir2p association with the telomere. These data suggest that HMR-E functions to both recruit Sir proteins and promote their assembly across several kilobases. Observations that association of Sir2p occurs simultaneously throughout HMR and that silencing at HMR is insensitive to coexpression of catalytically inactive Sir2p suggest that HMR-E acts by enabling assembly to occur in a nonlinear fashion. The ability of silencers to promote assembly of silenced chromatin over several kilobases is likely an important mechanism for maintaining what would otherwise be unstable chromatin at the correct genomic locations.

scholarly journals The [URE3] Prion in Candida

2013 ◽  
Vol 12 (4) ◽  
pp. 551-558 ◽  
Author(s):  
Herman K. Edskes ◽  
Reed B. Wickner
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Candida Albicans ◽  
Mating Type ◽  
Test Bed ◽  
Type Locus ◽  
Content Type ◽  
Mating Type Locus ◽  
Domain Sequence

ABSTRACTUre2p, normally a regulator of nitrogen catabolism inSaccharomyces cerevisiae, can be a prion (infectious protein) by forming a folded in-register parallel amyloid called [URE3]. UsingS. cerevisiaeas a test bed, we previously showed that Ure2p ofCandida albicans(CaUre2p) can also form a prion, but that Ure2p ofC. glabrata(CgUre2p) cannot. Here, we constructedC. glabratastrains to test whether CgUre2p can form a prion in its native environment. We find that while CaUre2p can form a [URE3] inC. glabrata, CgUre2p cannot, although the latter has a prion domain sequence more similar to that of ScUre2p than that of CaUre2p. This supports the notion that prion formation is not a conserved property of Ure2p but is a pathology arising sporadically. We find that some [URE3albicans] variants are restricted in their transmissibility to certain recipient strains. In addition, we show that theC. glabrataHO can induce switching of theC. glabratamating type locus.

scholarly journals POL30 alleles in Saccharomyces cerevisiae reveal complexities of the cell cycle and ploidy on heterochromatin assembly

2019 ◽  
Author(s):  
Molly Brothers ◽  
Jasper Rine
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Cell Cycle ◽  
Mating Type ◽  
Genome Stability ◽  
Transcriptional Silencing ◽  
Nuclear Antigen ◽  
Histone Chaperones ◽  
Type Identity ◽  
Assembly Factor ◽  
Proliferating Cell

ABSTRACTIn Saccharomyces cerevisiae, transcriptional silencing at HML and HMR maintains mating-type identity. The repressive chromatin structure at these loci is replicated every cell cycle and must be re-established quickly to prevent transcription of the genes at these loci. Mutations in a component of the replisome, the Proliferating Cell Nuclear Antigen (PCNA), encoded by POL30, cause a loss of transcriptional silencing at HMR. We used an assay that captures transient losses of silencing at HML and HMR to perform extended genetic analyses of the pol30-6, pol30-8, and pol30-79 alleles. All three alleles destabilized silencing only transiently and only in cycling cells. Whereas pol30-8 caused loss of silencing by disrupting the function of Chromatin Assembly Factor 1 (CAF-I), pol30-6 and pol30-79 acted through a separate genetic pathway but one still dependent on histone chaperones. Surprisingly, the silencing-loss phenotypes depended on ploidy but not on POL30 dosage or mating-type identity. Separately from silencing loss, the pol30-6 and pol30-79 alleles also displayed high levels of mitotic recombination in diploids. These results established that histone trafficking involving PCNA at replication forks is crucial to the maintenance of chromatin state and genome stability during DNA replication. They also raised the possibility that increased ploidy may protect chromatin states when the replisome is perturbed.

scholarly journals Schizosaccharomyces pombe Disaggregation Machinery Chaperones Support Saccharomyces cerevisiae Growth and Prion Propagation

2013 ◽  
Vol 12 (5) ◽  
pp. 739-745 ◽  
Author(s):  
Michael Reidy ◽  
Ruchika Sharma ◽  
Daniel C. Masison
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Schizosaccharomyces Pombe ◽  
Similar Species ◽  
Nucleotide Exchange ◽  
Content Type ◽  
E Coli ◽  
Yeast Prions ◽  
Nucleotide Exchange Factor ◽  
Prion Propagation ◽  
Species Specific

ABSTRACT Hsp100 chaperones protect microorganisms and plants from environmental stress by cooperating with Hsp70 and its nucleotide exchange factor (NEF) and Hsp40 cochaperones to resolubilize proteins from aggregates. The Saccharomyces cerevisiae Hsp104 (Sc-Hsp104)-based disaggregation machinery also is essential for replication of amyloid-based prions. Escherichia coli ClpB can substitute for Hsp104 to propagate [ PSI + ] prions in yeast, but only if E. coli DnaK and GrpE (Hsp70 and NEF) are coexpressed. Here, we tested if the reported inability of Schizosaccharomyces pombe Hsp104 (Sp-Hsp104) to support [ PSI + ] propagation was due to similar species-specific chaperone requirements and find that Sp-Hsp104 alone supported propagation of three different yeast prions. Sp-Hsp70 and Sp-Fes1p (NEF) likewise functioned in place of their Sa. cerevisiae counterparts. Thus, chaperones of these long-diverged species possess conserved activities that function in processes essential for both cell growth and prion propagation, suggesting Sc. pombe can propagate its own prions. We show that curing by Hsp104 overexpression and inactivation can be distinguished and confirm the observation that, unlike Sc-Hsp104, Sp-Hsp104 cannot cure yeast of [ PSI + ] when it is overexpressed. These results are consistent with a view that mechanisms underlying prion replication and elimination are distinct.

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