Mediator Influences Telomeric Silencing and Cellular Life Span

Abstract

The Mediator complex is required for the regulated transcription of nearly all RNA polymerase II-dependent genes. Here we demonstrate a new role for Mediator which appears to be separate from its function as a transcriptional coactivator. Mediator associates directly with heterochromatin at telomeres and influences the exact boundary between active and inactive chromatin. Loss of the Mediator Med5 subunit or mutations in Med7 cause a depletion of the complex from regions located near subtelomeric X elements, which leads to a change in the balance between the Sir2 and Sas2 proteins. These changes in turn result in increased levels of H4K16 acetylation near telomeres and in desilencing of subtelomeric genes. Increases in H4K16 acetylation have been observed at telomeres in aging cells. In agreement with this observation, we found that the loss of MED5 leads to shortening of the Saccharomyces cerevisiae (budding yeast) replicative life span.In Saccharomyces cerevisiae (budding yeast), telomeric DNA consists of imperfect tandem repeats of the consensus sequence (TG 1-3 ) n , with a combined length of about 300 nucleotides (21). Next to the telomere is the subtelomeric region, which often contains two types of repeats, the YЈ and X elements. The YЈ elements are between 4 and 12 kb long and are located next to the telomeres at many chromosome ends (26). The sizes of X elements vary, but they always contain a "core X" repeat region that is found at nearly all telomeres. Depending on how the YЈ and X elements are distributed, S. cerevisiae chromosome ends can be divided into X and X-YЈ types (22,23).Genes situated close to telomeres undergo reversible silencing, a phenomenon that has been termed the telomere position effect (TPE) (28). This effect was first observed when a reporter gene was inserted next to a telomeric TG 1-3 tract of an artificial telomere. TPE can also be observed at native yeast telomeres, but the phenomenon appears to be a bit more complex at these locations, since TPE varies between telomeres and in different strain backgrounds (29,40,41). The molecular basis of TPE is believed to be the Rap1, Ku, and Sir protein-mediated spreading of heterochromatin-like structures from the telomeric DNA inwards, which represses genes located in the subtelomeric region (42). According to this model, the Rap1/Ku/Sir structures are formed at telomeres and propagate toward the subtelomeres via interactions between the Sir proteins and histone tails. Sir2 is an active histone deacetylase that removes the acetyl group on lysine 16 of histone H4 (H4K16), which allows Sir3 and Sir4 to bind the nonacetylated histone tails (11, 35).As mentioned, TPE varies between individual chromosome ends, and the exact repeat structure of the subtelomeric region may in fact influence the spread of heterochromatin. YЈ elements counteract the spread of Sir proteins, and the YЈ regions display high levels of H4K16 acetylation. Furthermore, YЈ elements are highly enriched in nucleosomes and are transcriptionally active. In contrast, even X ...

Mediator links transcription and DNA repair by facilitating Rad2/XPG recruitment

“…Recently, a role of Mediator on telomeres was proposed, since this complex influences telomeric silencing, cellular life span, and telomere heterochromatin maintenance (Zhu et al 2011;Peng and Zhou 2012). In yeast, Mediator is bound to the telomeric regions.…”

Differential Regulation of White-Opaque Switching by Individual Subunits of Candida albicans Mediator

“…Transcriptional profiling in vivo has shown that other subunits are essential for transcription of virtually all genes (20). Work in S. cerevisiae has also pointed to a role for several Mediator subunits in transcriptional repression and silencing that likely involves chromatin (21)(22)(23)(24)(25)(26)(27).…”

“…Although the Cdk8 module is clearly involved in transcription repression (15,28), this functionality appears to be separable from those regulating chromatin, silencing, and certain forms of repression (24,26,29). Mutations in med16 (sin4) and med14 (rgr1), which lead to derepression of a subset of genes potentially through an epigenetic mechanism (30), are accompanied by gross alterations in chromatin structure in vivo (22,31).…”

The Tlo Proteins Are Stoichiometric Components of Candida albicans Mediator Anchored via the Med3 Subunit

“…Transcriptional profiling in vivo shows that other Mediator subunits are essential for the transcription of virtually all genes in S. cerevisiae (18), suggesting that the complex was also a general transcription factor. Additional studies of S. cerevisiae have also pointed to a role for some Mediator subunits in transcriptional repression and silencing that likely involves chromatin (19,21,38,39,49,50).…”