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• W2024204233 abstract "We have defined the in vivoheterochromatin structure of the left telomere of Saccharomyces cerevisiae chromosome III (LIII). Analysis of heterochromatin of a single telomere was so far lacking, due to the difficulties intrinsic to the highly repetitive nature of telomeric sequences. In LIII, the terminal (C1–3A)n repetitive sequences are followed by a complete X element and by the single copy Ty5-1 retrotransposon. Both the telosome and the X element exhibit overall resistance to micrococcal nuclease digestion reflecting their tight chromatin structure organization. The X element contains protein complexes and irregularly distributed but well localized nucleosomes. In contrast, a regular array of phased nucleosomes is associated with the promoter region of Ty5-1 and with the more centromere-proximal sequences. The lack of a structural component of yeast telomeres, the SIR3 protein, does not alter the overall tight organization of the X element but causes a nucleosome rearrangement within the promoter region of Ty5-1 and releases Ty5-1 silencing. Thus, Sir3p links the modification of the heterochromatin structure with loss of transcriptional silencing. We have defined the in vivoheterochromatin structure of the left telomere of Saccharomyces cerevisiae chromosome III (LIII). Analysis of heterochromatin of a single telomere was so far lacking, due to the difficulties intrinsic to the highly repetitive nature of telomeric sequences. In LIII, the terminal (C1–3A)n repetitive sequences are followed by a complete X element and by the single copy Ty5-1 retrotransposon. Both the telosome and the X element exhibit overall resistance to micrococcal nuclease digestion reflecting their tight chromatin structure organization. The X element contains protein complexes and irregularly distributed but well localized nucleosomes. In contrast, a regular array of phased nucleosomes is associated with the promoter region of Ty5-1 and with the more centromere-proximal sequences. The lack of a structural component of yeast telomeres, the SIR3 protein, does not alter the overall tight organization of the X element but causes a nucleosome rearrangement within the promoter region of Ty5-1 and releases Ty5-1 silencing. Thus, Sir3p links the modification of the heterochromatin structure with loss of transcriptional silencing. Telomeres play an essential role in cell biology in stabilizing chromosomes and facilitating complete replication of chromosomal termini. Telomeric DNA usually contains tandem repetitions of a short motif flanked by subtelomeric middle repetitive sequences (1Zakian V. Annu. Rev. Genet. 1989; 23: 579-604Crossref PubMed Scopus (530) Google Scholar). In yeast, telomeric sequences are composed of about 350 base pairs containing the (C1–3A)n repeats and are followed by two main subtelomeric sequences: the Y′ and Xelements (2Louis E. Yeast. 1995; 11: 1553-1573Crossref PubMed Scopus (188) Google Scholar). Y elements are highly conserved and are found in about 70% of the telomeres (2Louis E. Yeast. 1995; 11: 1553-1573Crossref PubMed Scopus (188) Google Scholar, 3Walmsley R. Chan C. Tye B. Petes T. Nature. 1984; 310: 157-160Crossref PubMed Scopus (132) Google Scholar, 4Chan C. Tye B. J. Mol. Biol. 1983; 168: 505-523Crossref PubMed Scopus (71) Google Scholar, 5Chan C. Tye B.-K. Cell. 1983; 33: 563-573Abstract Full Text PDF PubMed Scopus (196) Google Scholar). X elements are present in all telomeres and can exist in two main forms: a complete form containing the X core and the STR-A,B,C,D (6Louis E. Naumova E. Lee A. Naumov G. Haber J. Genetics. 1994; 136: 789-802Crossref PubMed Google Scholar) elements or a short form containing essentially the X core or part of it (2Louis E. Yeast. 1995; 11: 1553-1573Crossref PubMed Scopus (188) Google Scholar, 4Chan C. Tye B. J. Mol. Biol. 1983; 168: 505-523Crossref PubMed Scopus (71) Google Scholar, 5Chan C. Tye B.-K. Cell. 1983; 33: 563-573Abstract Full Text PDF PubMed Scopus (196) Google Scholar, 6Louis E. Naumova E. Lee A. Naumov G. Haber J. Genetics. 1994; 136: 789-802Crossref PubMed Google Scholar). The complete X is found in about 80% of the telomeres, whereas uncomplete forms are found in the remaining 20%.Previous reports have referred to the chromatin structure of yeast telomeres as heterochromatin. This denomination is based on structural and functional similarities that yeast telomeres share withDrosophila heterochromatin (7Laurenson P. Rine J. Microbiol. Rev. 1992; 56: 543-560Crossref PubMed Google Scholar, 8Weiler K. Wakimoto B. Annu. Rev. Genet. 1995; 29: 577-605Crossref PubMed Scopus (455) Google Scholar, 9Henikoff S. Trends Genet. 1990; 6: 422-426Abstract Full Text PDF PubMed Scopus (229) Google Scholar). In Saccharomyces cerevisiae the terminal (C1–3A)n repeats are organized into a nuclease-resistant structure called telosome (10Wright J. Gottschling D. Zakian V. Genes Dev. 1992; 6: 197-210Crossref PubMed Scopus (228) Google Scholar, 11Wright J. Zakian V. Nucleic Acids Res. 1995; 23: 1454-1460Crossref PubMed Scopus (49) Google Scholar) that does not contain nucleosomes and is associated with the protein RAP1 (10Wright J. Gottschling D. Zakian V. Genes Dev. 1992; 6: 197-210Crossref PubMed Scopus (228) Google Scholar, 11Wright J. Zakian V. Nucleic Acids Res. 1995; 23: 1454-1460Crossref PubMed Scopus (49) Google Scholar, 12Klein F. Laroche T. Cardenas M. Hofman J. Schweizer D. Gasser S. J. Cell Biol. 1992; 117: 935-948Crossref PubMed Scopus (236) Google Scholar, 13Conrad M. Wright J. Wolf A. Zakian V. Cell. 1990; 63: 739-750Abstract Full Text PDF PubMed Scopus (354) Google Scholar). This protein binds to the repetitive (C1–3A)n sequences (14Longtime M. Wilson N. Petracek M. Berman J. Curr. Genet. 1989; 16: 225-240Crossref PubMed Scopus (130) Google Scholar, 15Buchman A. Kimmerly W. Rine J. Kornberg R. Mol. Cell. Biol. 1988; 8: 210-225Crossref PubMed Scopus (351) Google Scholar) and interacts with other proteins including RIF1, RIF2, SIR3, and SIR4 (16Wotton D. Shore D. Genes Dev. 1997; 11: 748-760Crossref PubMed Scopus (355) Google Scholar, 17Moretti P. Freeman K. Coodly L. Shore D. Genes Dev. 1994; 8: 2257-2269Crossref PubMed Scopus (461) Google Scholar, 18Hardy C. Sussel L. Shore D. Genes Dev. 1992; 6: 801-814Crossref PubMed Scopus (399) Google Scholar). The proteins SIR3 and SIR4 interact with each other, RAP1, and the amino terminus of the histones H3 and H4. Thus, the building of the telomeric heterochromatic structures in yeast involves complex homotypic and heterotypic interactions.Although the chromatin structure of yeast telomeres is probably the best known among all eukaryotes, its specific organization is still poorly understood. It is known that both Y′ and Xelements contain nucleosomes (10Wright J. Gottschling D. Zakian V. Genes Dev. 1992; 6: 197-210Crossref PubMed Scopus (228) Google Scholar). However, their distribution within both elements has not been described previously.A previous report has described that the Ty5-1 retrotransposon, a subtelomeric transcriptional unit located in S. cerevisiaeLIII, undergoes telomeric silencing (19Vega-Palas M. Venditti S. Di Mauro E. Nat. Genet. 1997; 15: 232-233Crossref PubMed Scopus (33) Google Scholar). This retrotransposon is silenced in wild-type strains but is derepressed in a sir3mutant. We describe here the chromatin structure of LIII and its influence on the silencing of Ty5-1. A link between Ty5-1 silencing and a specific Sir3p-dependent heterochromatin structure is established.DISCUSSIONThe URA3 reporter gene and other reporter genes placed immediately adjacent to terminal (C1–3A)n repeats in the absence of subtelomeric sequences are silenced (22Gottschling D. Aparicio O. Billington B. Zakian V. Cell. 1990; 63: 751-762Abstract Full Text PDF PubMed Scopus (1126) Google Scholar). Therefore, the special heterochromatic organization of the X elements is not required for the spreading of transcriptional telomeric silencing.Yeast telomeres repress the expression of adjacent genes (7Laurenson P. Rine J. Microbiol. Rev. 1992; 56: 543-560Crossref PubMed Google Scholar). This repression, referred to as telomere position effect, requires the integrity of telomeric proteins like RAP1, SIR2, SIR3, SIR4, and the amino termini of histones H3 and H4 (23Kyrion G. Boakye K. Lustig A. Mol. Cell. Biol. 1992; 12: 5159-5173Crossref PubMed Google Scholar, 24Thompson J. Ling X. Grunstein M. Nature. 1994; 369: 245-247Crossref PubMed Scopus (205) Google Scholar, 25Aparicio O. Billington B. Gottschling D. Cell. 1991; 66: 1279-1287Abstract Full Text PDF PubMed Scopus (606) Google Scholar). SIR3 and SIR4 interact among them, with RAP1 and with the amino terminus of the core histones. Thus, they have been proposed to physically connect telosomes and the nucleosomes located in the telomeric regions that undergo silencing (17Moretti P. Freeman K. Coodly L. Shore D. Genes Dev. 1994; 8: 2257-2269Crossref PubMed Scopus (461) Google Scholar, 26Hecht A. Laroche T. Strahl-Bolsinger S. Gasser S. Grunstein M. Cell. 1995; 80: 583-592Abstract Full Text PDF PubMed Scopus (692) Google Scholar, 27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar, 28Moazed D. Kistler A. Axelrod A. Rine J. Johnson A. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2186-2191Crossref PubMed Scopus (177) Google Scholar). Genetic and biochemical data support this notion. SIR3 and SIR4 require RAP1 and the amino termini of histones H3 and H4 to silence telomeric genes (23Kyrion G. Boakye K. Lustig A. Mol. Cell. Biol. 1992; 12: 5159-5173Crossref PubMed Google Scholar, 24Thompson J. Ling X. Grunstein M. Nature. 1994; 369: 245-247Crossref PubMed Scopus (205) Google Scholar, 25Aparicio O. Billington B. Gottschling D. Cell. 1991; 66: 1279-1287Abstract Full Text PDF PubMed Scopus (606) Google Scholar). SIR3, SIR4, and RAP1 localize by immunofluorescence to a number of foci near the nuclear periphery, being the amino termini of histones H3 and H4 required for the perinuclear localization of SIR3 and SIR4. The positioning of these foci inside the nuclei is coincident with hybridization signals of subtelomeric repeats (26Hecht A. Laroche T. Strahl-Bolsinger S. Gasser S. Grunstein M. Cell. 1995; 80: 583-592Abstract Full Text PDF PubMed Scopus (692) Google Scholar, 29Gotta M. Laroche T. Formenton A. Maillet L. Shertan H. Gasser S. J. Cell Biol. 1996; 134: 1349-1363Crossref PubMed Scopus (390) Google Scholar, 30Palladino F. Palladino F. Laroche T. Gilson E. Axelrod A. Pillus L. Gasser S. Cell. 1993; 75: 543-555Abstract Full Text PDF PubMed Scopus (340) Google Scholar). In addition, SIR3, SIR4, RAP1, and histone proteins coimmunoprecipitate and are found at the same distance from the telomeres (27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar, 31Strahl-Bolsinger S. Hecht A. Luo K. Grunstein M. Genes Dev. 1997; 11: 83-93Crossref PubMed Scopus (591) Google Scholar).We have shown in this report that the lack of Sir3p affects the chromatin structure of the X element and of the adjacent Ty5-1 retrotransposon in LIII. These results provide evidence forin vivo interactions between Sir3p and nucleosomes in the X element and in the Ty5-1 retrotransposon.Ty5-1 has been previously described to be silenced in its natural context in a SIR3 dependent manner (19Vega-Palas M. Venditti S. Di Mauro E. Nat. Genet. 1997; 15: 232-233Crossref PubMed Scopus (33) Google Scholar). The promoter region of the retrotransposon is located at about 1.3 kilobases from the end of LIII. This result is in agreement with previous studies showing that Sir3p can be immunolocalized up to 2–3 kilobases from the right telomere of S. cerevisiae chromosome VI (27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar, 31Strahl-Bolsinger S. Hecht A. Luo K. Grunstein M. Genes Dev. 1997; 11: 83-93Crossref PubMed Scopus (591) Google Scholar). Since the chromatin structure of the Ty5-1 promoter region is altered in asir3 mutant, Sir3p establishes a link between chromatin remodeling and transcriptional telomeric silencing. The SIR complex could function as a stapler joining nucleosomes from the Xelement and from the Ty5-1 retrotransposon and impair the access of the transcriptional machinery to the Ty5-1 promoter.In wild-type strains, the 5′ region of Ty5-1 shows overall sensitivity to MNase similar to the sensitivity of bulk DNA (data not shown) and contains a normal array of nucleosomes. Thus, this heterochromatic region does not show specific structural features. However, according to previous results (27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar), the 5′ region of Ty5-1 should associate with proteins like SIR2, SIR3, or SIR4. Therefore, these proteins should interact among them and with the amino termini of histones H3 and H4 without affecting the accessibility of MNase to the linker internucleosomal regions. Telomeres play an essential role in cell biology in stabilizing chromosomes and facilitating complete replication of chromosomal termini. Telomeric DNA usually contains tandem repetitions of a short motif flanked by subtelomeric middle repetitive sequences (1Zakian V. Annu. Rev. Genet. 1989; 23: 579-604Crossref PubMed Scopus (530) Google Scholar). In yeast, telomeric sequences are composed of about 350 base pairs containing the (C1–3A)n repeats and are followed by two main subtelomeric sequences: the Y′ and Xelements (2Louis E. Yeast. 1995; 11: 1553-1573Crossref PubMed Scopus (188) Google Scholar). Y elements are highly conserved and are found in about 70% of the telomeres (2Louis E. Yeast. 1995; 11: 1553-1573Crossref PubMed Scopus (188) Google Scholar, 3Walmsley R. Chan C. Tye B. Petes T. Nature. 1984; 310: 157-160Crossref PubMed Scopus (132) Google Scholar, 4Chan C. Tye B. J. Mol. Biol. 1983; 168: 505-523Crossref PubMed Scopus (71) Google Scholar, 5Chan C. Tye B.-K. Cell. 1983; 33: 563-573Abstract Full Text PDF PubMed Scopus (196) Google Scholar). X elements are present in all telomeres and can exist in two main forms: a complete form containing the X core and the STR-A,B,C,D (6Louis E. Naumova E. Lee A. Naumov G. Haber J. Genetics. 1994; 136: 789-802Crossref PubMed Google Scholar) elements or a short form containing essentially the X core or part of it (2Louis E. Yeast. 1995; 11: 1553-1573Crossref PubMed Scopus (188) Google Scholar, 4Chan C. Tye B. J. Mol. Biol. 1983; 168: 505-523Crossref PubMed Scopus (71) Google Scholar, 5Chan C. Tye B.-K. Cell. 1983; 33: 563-573Abstract Full Text PDF PubMed Scopus (196) Google Scholar, 6Louis E. Naumova E. Lee A. Naumov G. Haber J. Genetics. 1994; 136: 789-802Crossref PubMed Google Scholar). The complete X is found in about 80% of the telomeres, whereas uncomplete forms are found in the remaining 20%. Previous reports have referred to the chromatin structure of yeast telomeres as heterochromatin. This denomination is based on structural and functional similarities that yeast telomeres share withDrosophila heterochromatin (7Laurenson P. Rine J. Microbiol. Rev. 1992; 56: 543-560Crossref PubMed Google Scholar, 8Weiler K. Wakimoto B. Annu. Rev. Genet. 1995; 29: 577-605Crossref PubMed Scopus (455) Google Scholar, 9Henikoff S. Trends Genet. 1990; 6: 422-426Abstract Full Text PDF PubMed Scopus (229) Google Scholar). In Saccharomyces cerevisiae the terminal (C1–3A)n repeats are organized into a nuclease-resistant structure called telosome (10Wright J. Gottschling D. Zakian V. Genes Dev. 1992; 6: 197-210Crossref PubMed Scopus (228) Google Scholar, 11Wright J. Zakian V. Nucleic Acids Res. 1995; 23: 1454-1460Crossref PubMed Scopus (49) Google Scholar) that does not contain nucleosomes and is associated with the protein RAP1 (10Wright J. Gottschling D. Zakian V. Genes Dev. 1992; 6: 197-210Crossref PubMed Scopus (228) Google Scholar, 11Wright J. Zakian V. Nucleic Acids Res. 1995; 23: 1454-1460Crossref PubMed Scopus (49) Google Scholar, 12Klein F. Laroche T. Cardenas M. Hofman J. Schweizer D. Gasser S. J. Cell Biol. 1992; 117: 935-948Crossref PubMed Scopus (236) Google Scholar, 13Conrad M. Wright J. Wolf A. Zakian V. Cell. 1990; 63: 739-750Abstract Full Text PDF PubMed Scopus (354) Google Scholar). This protein binds to the repetitive (C1–3A)n sequences (14Longtime M. Wilson N. Petracek M. Berman J. Curr. Genet. 1989; 16: 225-240Crossref PubMed Scopus (130) Google Scholar, 15Buchman A. Kimmerly W. Rine J. Kornberg R. Mol. Cell. Biol. 1988; 8: 210-225Crossref PubMed Scopus (351) Google Scholar) and interacts with other proteins including RIF1, RIF2, SIR3, and SIR4 (16Wotton D. Shore D. Genes Dev. 1997; 11: 748-760Crossref PubMed Scopus (355) Google Scholar, 17Moretti P. Freeman K. Coodly L. Shore D. Genes Dev. 1994; 8: 2257-2269Crossref PubMed Scopus (461) Google Scholar, 18Hardy C. Sussel L. Shore D. Genes Dev. 1992; 6: 801-814Crossref PubMed Scopus (399) Google Scholar). The proteins SIR3 and SIR4 interact with each other, RAP1, and the amino terminus of the histones H3 and H4. Thus, the building of the telomeric heterochromatic structures in yeast involves complex homotypic and heterotypic interactions. Although the chromatin structure of yeast telomeres is probably the best known among all eukaryotes, its specific organization is still poorly understood. It is known that both Y′ and Xelements contain nucleosomes (10Wright J. Gottschling D. Zakian V. Genes Dev. 1992; 6: 197-210Crossref PubMed Scopus (228) Google Scholar). However, their distribution within both elements has not been described previously. A previous report has described that the Ty5-1 retrotransposon, a subtelomeric transcriptional unit located in S. cerevisiaeLIII, undergoes telomeric silencing (19Vega-Palas M. Venditti S. Di Mauro E. Nat. Genet. 1997; 15: 232-233Crossref PubMed Scopus (33) Google Scholar). This retrotransposon is silenced in wild-type strains but is derepressed in a sir3mutant. We describe here the chromatin structure of LIII and its influence on the silencing of Ty5-1. A link between Ty5-1 silencing and a specific Sir3p-dependent heterochromatin structure is established. DISCUSSIONThe URA3 reporter gene and other reporter genes placed immediately adjacent to terminal (C1–3A)n repeats in the absence of subtelomeric sequences are silenced (22Gottschling D. Aparicio O. Billington B. Zakian V. Cell. 1990; 63: 751-762Abstract Full Text PDF PubMed Scopus (1126) Google Scholar). Therefore, the special heterochromatic organization of the X elements is not required for the spreading of transcriptional telomeric silencing.Yeast telomeres repress the expression of adjacent genes (7Laurenson P. Rine J. Microbiol. Rev. 1992; 56: 543-560Crossref PubMed Google Scholar). This repression, referred to as telomere position effect, requires the integrity of telomeric proteins like RAP1, SIR2, SIR3, SIR4, and the amino termini of histones H3 and H4 (23Kyrion G. Boakye K. Lustig A. Mol. Cell. Biol. 1992; 12: 5159-5173Crossref PubMed Google Scholar, 24Thompson J. Ling X. Grunstein M. Nature. 1994; 369: 245-247Crossref PubMed Scopus (205) Google Scholar, 25Aparicio O. Billington B. Gottschling D. Cell. 1991; 66: 1279-1287Abstract Full Text PDF PubMed Scopus (606) Google Scholar). SIR3 and SIR4 interact among them, with RAP1 and with the amino terminus of the core histones. Thus, they have been proposed to physically connect telosomes and the nucleosomes located in the telomeric regions that undergo silencing (17Moretti P. Freeman K. Coodly L. Shore D. Genes Dev. 1994; 8: 2257-2269Crossref PubMed Scopus (461) Google Scholar, 26Hecht A. Laroche T. Strahl-Bolsinger S. Gasser S. Grunstein M. Cell. 1995; 80: 583-592Abstract Full Text PDF PubMed Scopus (692) Google Scholar, 27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar, 28Moazed D. Kistler A. Axelrod A. Rine J. Johnson A. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2186-2191Crossref PubMed Scopus (177) Google Scholar). Genetic and biochemical data support this notion. SIR3 and SIR4 require RAP1 and the amino termini of histones H3 and H4 to silence telomeric genes (23Kyrion G. Boakye K. Lustig A. Mol. Cell. Biol. 1992; 12: 5159-5173Crossref PubMed Google Scholar, 24Thompson J. Ling X. Grunstein M. Nature. 1994; 369: 245-247Crossref PubMed Scopus (205) Google Scholar, 25Aparicio O. Billington B. Gottschling D. Cell. 1991; 66: 1279-1287Abstract Full Text PDF PubMed Scopus (606) Google Scholar). SIR3, SIR4, and RAP1 localize by immunofluorescence to a number of foci near the nuclear periphery, being the amino termini of histones H3 and H4 required for the perinuclear localization of SIR3 and SIR4. The positioning of these foci inside the nuclei is coincident with hybridization signals of subtelomeric repeats (26Hecht A. Laroche T. Strahl-Bolsinger S. Gasser S. Grunstein M. Cell. 1995; 80: 583-592Abstract Full Text PDF PubMed Scopus (692) Google Scholar, 29Gotta M. Laroche T. Formenton A. Maillet L. Shertan H. Gasser S. J. Cell Biol. 1996; 134: 1349-1363Crossref PubMed Scopus (390) Google Scholar, 30Palladino F. Palladino F. Laroche T. Gilson E. Axelrod A. Pillus L. Gasser S. Cell. 1993; 75: 543-555Abstract Full Text PDF PubMed Scopus (340) Google Scholar). In addition, SIR3, SIR4, RAP1, and histone proteins coimmunoprecipitate and are found at the same distance from the telomeres (27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar, 31Strahl-Bolsinger S. Hecht A. Luo K. Grunstein M. Genes Dev. 1997; 11: 83-93Crossref PubMed Scopus (591) Google Scholar).We have shown in this report that the lack of Sir3p affects the chromatin structure of the X element and of the adjacent Ty5-1 retrotransposon in LIII. These results provide evidence forin vivo interactions between Sir3p and nucleosomes in the X element and in the Ty5-1 retrotransposon.Ty5-1 has been previously described to be silenced in its natural context in a SIR3 dependent manner (19Vega-Palas M. Venditti S. Di Mauro E. Nat. Genet. 1997; 15: 232-233Crossref PubMed Scopus (33) Google Scholar). The promoter region of the retrotransposon is located at about 1.3 kilobases from the end of LIII. This result is in agreement with previous studies showing that Sir3p can be immunolocalized up to 2–3 kilobases from the right telomere of S. cerevisiae chromosome VI (27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar, 31Strahl-Bolsinger S. Hecht A. Luo K. Grunstein M. Genes Dev. 1997; 11: 83-93Crossref PubMed Scopus (591) Google Scholar). Since the chromatin structure of the Ty5-1 promoter region is altered in asir3 mutant, Sir3p establishes a link between chromatin remodeling and transcriptional telomeric silencing. The SIR complex could function as a stapler joining nucleosomes from the Xelement and from the Ty5-1 retrotransposon and impair the access of the transcriptional machinery to the Ty5-1 promoter.In wild-type strains, the 5′ region of Ty5-1 shows overall sensitivity to MNase similar to the sensitivity of bulk DNA (data not shown) and contains a normal array of nucleosomes. Thus, this heterochromatic region does not show specific structural features. However, according to previous results (27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar), the 5′ region of Ty5-1 should associate with proteins like SIR2, SIR3, or SIR4. Therefore, these proteins should interact among them and with the amino termini of histones H3 and H4 without affecting the accessibility of MNase to the linker internucleosomal regions. The URA3 reporter gene and other reporter genes placed immediately adjacent to terminal (C1–3A)n repeats in the absence of subtelomeric sequences are silenced (22Gottschling D. Aparicio O. Billington B. Zakian V. Cell. 1990; 63: 751-762Abstract Full Text PDF PubMed Scopus (1126) Google Scholar). Therefore, the special heterochromatic organization of the X elements is not required for the spreading of transcriptional telomeric silencing. Yeast telomeres repress the expression of adjacent genes (7Laurenson P. Rine J. Microbiol. Rev. 1992; 56: 543-560Crossref PubMed Google Scholar). This repression, referred to as telomere position effect, requires the integrity of telomeric proteins like RAP1, SIR2, SIR3, SIR4, and the amino termini of histones H3 and H4 (23Kyrion G. Boakye K. Lustig A. Mol. Cell. Biol. 1992; 12: 5159-5173Crossref PubMed Google Scholar, 24Thompson J. Ling X. Grunstein M. Nature. 1994; 369: 245-247Crossref PubMed Scopus (205) Google Scholar, 25Aparicio O. Billington B. Gottschling D. Cell. 1991; 66: 1279-1287Abstract Full Text PDF PubMed Scopus (606) Google Scholar). SIR3 and SIR4 interact among them, with RAP1 and with the amino terminus of the core histones. Thus, they have been proposed to physically connect telosomes and the nucleosomes located in the telomeric regions that undergo silencing (17Moretti P. Freeman K. Coodly L. Shore D. Genes Dev. 1994; 8: 2257-2269Crossref PubMed Scopus (461) Google Scholar, 26Hecht A. Laroche T. Strahl-Bolsinger S. Gasser S. Grunstein M. Cell. 1995; 80: 583-592Abstract Full Text PDF PubMed Scopus (692) Google Scholar, 27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar, 28Moazed D. Kistler A. Axelrod A. Rine J. Johnson A. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 2186-2191Crossref PubMed Scopus (177) Google Scholar). Genetic and biochemical data support this notion. SIR3 and SIR4 require RAP1 and the amino termini of histones H3 and H4 to silence telomeric genes (23Kyrion G. Boakye K. Lustig A. Mol. Cell. Biol. 1992; 12: 5159-5173Crossref PubMed Google Scholar, 24Thompson J. Ling X. Grunstein M. Nature. 1994; 369: 245-247Crossref PubMed Scopus (205) Google Scholar, 25Aparicio O. Billington B. Gottschling D. Cell. 1991; 66: 1279-1287Abstract Full Text PDF PubMed Scopus (606) Google Scholar). SIR3, SIR4, and RAP1 localize by immunofluorescence to a number of foci near the nuclear periphery, being the amino termini of histones H3 and H4 required for the perinuclear localization of SIR3 and SIR4. The positioning of these foci inside the nuclei is coincident with hybridization signals of subtelomeric repeats (26Hecht A. Laroche T. Strahl-Bolsinger S. Gasser S. Grunstein M. Cell. 1995; 80: 583-592Abstract Full Text PDF PubMed Scopus (692) Google Scholar, 29Gotta M. Laroche T. Formenton A. Maillet L. Shertan H. Gasser S. J. Cell Biol. 1996; 134: 1349-1363Crossref PubMed Scopus (390) Google Scholar, 30Palladino F. Palladino F. Laroche T. Gilson E. Axelrod A. Pillus L. Gasser S. Cell. 1993; 75: 543-555Abstract Full Text PDF PubMed Scopus (340) Google Scholar). In addition, SIR3, SIR4, RAP1, and histone proteins coimmunoprecipitate and are found at the same distance from the telomeres (27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar, 31Strahl-Bolsinger S. Hecht A. Luo K. Grunstein M. Genes Dev. 1997; 11: 83-93Crossref PubMed Scopus (591) Google Scholar). We have shown in this report that the lack of Sir3p affects the chromatin structure of the X element and of the adjacent Ty5-1 retrotransposon in LIII. These results provide evidence forin vivo interactions between Sir3p and nucleosomes in the X element and in the Ty5-1 retrotransposon. Ty5-1 has been previously described to be silenced in its natural context in a SIR3 dependent manner (19Vega-Palas M. Venditti S. Di Mauro E. Nat. Genet. 1997; 15: 232-233Crossref PubMed Scopus (33) Google Scholar). The promoter region of the retrotransposon is located at about 1.3 kilobases from the end of LIII. This result is in agreement with previous studies showing that Sir3p can be immunolocalized up to 2–3 kilobases from the right telomere of S. cerevisiae chromosome VI (27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar, 31Strahl-Bolsinger S. Hecht A. Luo K. Grunstein M. Genes Dev. 1997; 11: 83-93Crossref PubMed Scopus (591) Google Scholar). Since the chromatin structure of the Ty5-1 promoter region is altered in asir3 mutant, Sir3p establishes a link between chromatin remodeling and transcriptional telomeric silencing. The SIR complex could function as a stapler joining nucleosomes from the Xelement and from the Ty5-1 retrotransposon and impair the access of the transcriptional machinery to the Ty5-1 promoter. In wild-type strains, the 5′ region of Ty5-1 shows overall sensitivity to MNase similar to the sensitivity of bulk DNA (data not shown) and contains a normal array of nucleosomes. Thus, this heterochromatic region does not show specific structural features. However, according to previous results (27Hecht A. Strahl-Bolsinger S. Grunstein M. Nature. 1996; 383: 92-96Crossref PubMed Scopus (447) Google Scholar), the 5′ region of Ty5-1 should associate with proteins like SIR2, SIR3, or SIR4. Therefore, these proteins should interact among them and with the amino termini of histones H3 and H4 without affecting the accessibility of MNase to the linker internucleosomal regions. The support of G. P. Ateneo, La Sapienza is acknowledged. We are indebted to M. Grunstein for encouragement and the generous gift of all yeast strains used in this study." @default.
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