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Abstract Telomere elongation is coupled with genome replication, raising the question of the repair of short telomeres in post-mitotic cells. We investigated the fate of a telomere-repeat capped end that mimics a single short telomere in quiescent fission yeast cells. We show that telomerase is able to elongate this single short telomere during quiescence despite the binding of Ku to the proto-telomere. While Taz1 and Rap1 repress telomerase in vegetative cells, both shelterin proteins are required for efficient telomere extension in quiescent cells, underscoring a distinct mode of telomerase control. We further show that Rad3ATR and Tel1ATM are redundantly required for telomere elongation in quiescence through the phosphorylation of Ccq1 and that Rif1 and its associated-PP1 phosphatases negatively regulate telomerase activity by opposing Ccq1 phosphorylation. The distinct mode of telomerase regulation in quiescent fission yeast cells may be relevant to that in human stem and progenitor cells.
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Ccq1 restrains Mre11-mediated degradation of short telomeres
Telomeres cap chromosome ends with specialized chromatin composed of DNA repeats bound by a multiprotein complex called shelterin. Fission yeast telomeres can be formed by cleaving a “proto-telomere” bearing 48 bp of telomere repeats to form a new stable chromosomal end that prevents the rapid degradation seen at similar DNA double-strand breaks (DSBs). This end-protection was investigated in viable mutants lacking telomere-associated proteins. Telomerase, the shelterin components Taz1, Rap1, or Poz1 or the telomere-associated protein Rif1 were not required to form a stable chromosome end after cleavage of the proto-telomere. However, cells lacking the fission yeast shelterin component Ccq1 converted the cleaved telomere repeat-capped end to a rapidly degraded DSB. Degradation was greatly reduced by eliminating the nuclease activity of Mre11, a component of the Mre11-Rad50-Nbs1/Xrs2 complex that processes DSBs. These results demonstrate a novel function for Ccq1 to effect end-protection by restraining Mre11-dependent degradation.
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- Award ID(s):
- 1908875
- PAR ID:
- 10345135
- Date Published:
- Journal Name:
- Research square
- ISSN:
- 2693-5015
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Zappulla, David C (Ed.)TRF2 is an essential and conserved double-strand telomere binding protein that stabilizes chromosome ends by suppressing DNA damage response and aberrant DNA repair. Herein we investigated the mechanisms and functions of the Trf2 ortholog in the basidiomycete fungusUstilago maydis, which manifests strong resemblances to metazoans with regards to the telomere and DNA repair machinery. We showed thatUmTrf2 binds to Blmin vitroand inhibits Blm-mediated unwinding of telomeric DNA substrates. Consistent with a similar inhibitory activityin vivo, over-expression of Trf2 induces telomere shortening, just like deletion ofblm, which is required for efficient telomere replication. While the loss of Trf2 engenders growth arrest and multiple telomere aberrations, these defects are fully suppressed by the concurrent deletion ofblmormre11(but not other DNA repair factors). Over-expression of Blm alone triggers aberrant telomere recombination and the accumulation of aberrant telomere structures, which are blocked by concurrent Trf2 over-expression. Together, these findings highlight the suppression of Blm as a key protective mechanism of Trf2. Notably,U.maydisharbors another double-strand telomere-binding protein (Tay1), which promotes Blm activity to ensure efficient replication. We found that deletion oftay1partially suppresses the telomere aberration of Trf2-depleted cells. Our results thus point to opposing regulation of Blm helicase by telomere proteins as a strategy for optimizing both telomere maintenance and protection. We also show that aberrant transcription of both telomere G- and C-strand is a recurrent phenotype of telomere mutants, underscoring another potential similarity between double strand breaks and de-protected telomeres.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.21203/rs.3.rs-1456395
