A Heterochromatin Domain Forms Gradually at a New Telomere and Is Dynamic at Stable Telomeres.

Jinyu Wang,Kurt W. Runge,Kathleen L. Berkner,Jessica R. Eisenstatt,Matthew Shaughnessy,Kristen Cornelius,Julien Audry

doi:10.1128/mcb.00393-17

Abstract

Heterochromatin domains play important roles in chromosome biology, organismal development, and aging, including centromere function, mammalian female X chromosome inactivation, and senescence-associated heterochromatin foci. In the fission yeast Schizosaccharomyces pombe and metazoans, heterochromatin contains histone H3 that is dimethylated at lysine 9. While factors required for heterochromatin have been identified, the dynamics of heterochromatin formation are poorly understood. Telomeres convert adjacent chromatin into heterochromatin. To form a new heterochromatic region in S. pombe, an inducible DNA double-strand break (DSB) was engineered next to 48 bp of telomere repeats in euchromatin, which caused formation of a new telomere and the establishment and gradual spreading of a new heterochromatin domain. However, spreading was dynamic even after the telomere had reached its stable length, with reporter genes within the heterochromatin domain showing variegated expression. The system also revealed the presence of repeats located near the boundaries of euchromatin and heterochromatin that are oriented to allow the efficient healing of a euchromatic DSB to cap the chromosome end with a new telomere. Telomere formation in S. pombe therefore reveals novel aspects of heterochromatin dynamics and fail-safe mechanisms to repair subtelomeric breaks, with implications for similar processes in metazoan genomes.

Highlights

Heterochromatin domains play important roles in chromosome biology, organismal development, and aging, including centromere function, mammalian female X chromosome inactivation, and senescence-associated heterochromatin foci
Heterochromatin plays an important role at centromeres, the chromosomal structure required for chromosome segregation at mitosis, as centromeric chromatin is flanked by heterochromatin domains that are required for complete
Telomeres have been formed in S. pombe by integrating in vitro-constructed telomeric DNA into genomic sites in vivo, but 30 population doublings (PDs) or more must pass between the formation of the new telomere and the production of enough cells for chromatin and phenotypic analysis [21]

Summary

Introduction

Heterochromatin domains play important roles in chromosome biology, organismal development, and aging, including centromere function, mammalian female X chromosome inactivation, and senescence-associated heterochromatin foci. To form a new heterochromatic region in S. pombe, an inducible DNA double-strand break (DSB) was engineered next to 48 bp of telomere repeats in euchromatin, which caused formation of a new telomere and the establishment and gradual spreading of a new heterochromatin domain. Similar approaches requiring many PDs have followed heterochromatin formation at centromeres and other loci by introducing wild-type genes into mutants defective for heterochromatin assembly [26,27,28,29] This approach converts a mutant cell to a wild-type one, so the levels of cellular chromatin proteins during domain formation are initially different than in wild-type cells. The formation of the initiation site, e.g., a functional telomere, may allow spreading over many cell divisions, with the size of the heterochromatin domain gradually increasing over time to form the final state (as suggested for several histone modifications [30] and in S. pombe [14])

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Results

Conclusion