Repetitive DNA sequences, which constitute half the genome in some organisms, often undergo homologous recombination. This can instigate genomic instability resulting from a gain or loss of DNA1. Assembly of DNA into silent chromatin is generally thought to serve as a mechanism ensuring repeat stability by limiting access to the recombination machinery2. Consistent with this notion is the observation, in the budding yeast Saccharomyces cerevisiae, that stability of the highly repetitive ribosomal DNA (rDNA) sequences requires a Sir2-containing chromatin silencing complex that also inhibits transcription from foreign promoters and transposons inserted within the repeats by a process called rDNA silencing2,3,4,5. Here we describe a protein network that stabilizes rDNA repeats of budding yeast by means of interactions between rDNA-associated silencing proteins and two proteins of the inner nuclear membrane (INM). Deletion of either the INM or silencing proteins reduces perinuclear rDNA positioning, disrupts the nucleolus–nucleoplasm boundary, induces the formation of recombination foci, and destabilizes the repeats. In addition, artificial targeting of rDNA repeats to the INM suppresses the instability observed in cells lacking an rDNA-associated silencing protein that is typically required for peripheral tethering of the repeats. Moreover, in contrast to Sir2 and its associated nucleolar factors, the INM proteins are not required for rDNA silencing, indicating that Sir2-dependent silencing is not sufficient to inhibit recombination within the rDNA locus. These findings demonstrate a role for INM proteins in the perinuclear localization of chromosomes and show that tethering to the nuclear periphery is required for the stability of rDNA repeats. The INM proteins studied here are conserved and have been implicated in chromosome organization in metazoans6,7. Our results therefore reveal an ancient mechanism in which interactions between INM proteins and chromosomal proteins ensure genome stability.
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We thank C. Yip, T. Walz, J. Huang, M. Bühler, D. Koshland, A. Palazzo, D. E. Libuda, F. Winston, L. Vasiljeva, S. Buratowski, J. E. Warner, C. Anderson, G. A. Beltz, M. Lisby, T. Daniel, L. Ding and the Harvard NeuroDiscovery Optical Imaging Center for technical assistance or materials, and T. Rapoport, T. Iida, M. Motamedi, A. Johnson, M. Onishi, E. Gerace, S. Buker, M. Halic and members of the Moazed laboratory for helpful discussions and comments. This work was supported by the National Institutes of Health and the Howard Hughes Medical Institute (D.M.), and the Canadian Institutes of Health Research Institute of Aging (K.M.). D.M. is a scholar of the Leukemia and Lymphoma Society.
Author Contributions K.M. and D.M. designed experiments and wrote the paper. K.M. and J.S. performed LC–MS/MS analyses. K.M. performed the other experiments. S.P.G. provided mass spectrometry expertise and equipment.
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Mekhail, K., Seebacher, J., Gygi, S. et al. Role for perinuclear chromosome tethering in maintenance of genome stability. Nature 456, 667–670 (2008). https://doi.org/10.1038/nature07460
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