Telomere binding protein Taz1 establishes Swi6 heterochromatin independently of RNAi at telomeres.
BACKGROUND
The telomere is a specialized heterochromatin conserved among eukaryotes. However, it remains unknown how heterochromatin protein 1 (HP1) is recruited to telomeres and how telomere heterochromatin is formed. In fission yeast, the RNAi (RNA interference)-RITS (RNA-induced initiation of transcriptional silencing) pathway initiates heterochromatin formation at the centromeres and the silent mat locus by using common DNA sequences, the dg and dh repeats, as the templates for small interfering RNA (siRNA).


RESULTS
We found that telomeric repeats are sufficient for the establishment of Swi6 (a fission-yeast HP1 homolog) heterochromatin, and the establishment requires Taz1, a telomere binding protein of the TRF family. Additionally, Swi6 heterochromatin is established by a part of the subtelomere that contains sequences highly homologous to that of the dh repeat, and it is strikingly destabilized by the deletion of both Taz1 and RNAi-RITS. Transcripts from the telomeric dh-homologous region were specifically associated with RITS, and deletion of the telomeric dh-homologous region showed the phenotype similar to that of the rnai mutant in terms of the telomeric silencing, indicating that the RNAi-RITS pathway acts at the telomeric dh-homologous region to establish Swi6 heterochromatin. Furthermore, we found that Taz1 establishes Swi6 heterochromatin independently of the telomeric repeats and the RNAi-RITS pathway at the subtelomeres.


CONCLUSIONS
The telomere heterochromatin is regulated by at least two factors: One is Taz1, which is telomere specific, and the other is RNAi-RITS, which is commonly used at the constitutive heterochromatin regions.
Kanoh
Junko
J
Department of Gene Mechanisms, Graduate School of Biostudies, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan.
Sadaie
Mahito
M
Urano
Takeshi
T
Ishikawa
Fuyuki
F
D047369:Chromatin Immunoprecipitation; D002868:Chromosomal Proteins, Non-Histone / Q000378:metabolism*; D003001:Cloning, Molecular; D017353:Gene Deletion; D006570:Heterochromatin / Q000378:metabolism*; D017404:In Situ Hybridization, Fluorescence; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D012568:Schizosaccharomyces / Q000235:genetics / Q000378:metabolism*; D029702:Schizosaccharomyces pombe Proteins / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000235:genetics*; D016615:Telomere / Q000235:genetics / Q000378:metabolism*; D034501:Telomere-Binding Proteins / Q000378:metabolism*; D020079:Terminal Repeat Sequences / Q000235:genetics
D002868:Chromosomal Proteins, Non-Histone; D006570:Heterochromatin; D034741:RNA, Small Interfering; D029702:Schizosaccharomyces pombe Proteins; C465958:Swi6 protein, S pombe; D034501:Telomere-Binding Proteins; C104344:taz1 protein, S pombe; C400810:ura4 protein, S pombe