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dc.description.abstractAlterations of chromatin structure have been shown to be crucial for gene expression and silencing mechanism. Posttranscriptional histone modifications control the chromatin structure and histone binding proteins, such as other histone modification enzymes and chromatin regulators. Regulation of histone modifications is important for transcriptional regulation and gene silencing by interacting with transcription factors or chromatin associated proteins. Histone modifications show a crosstalk between other modifications. In S. cerevisiae, histone H2B ubiquitylation regulates the histone methylation at H3 lysine 4 and 79 during transcriptional elongation. This trans-tail regulation is conserved in metazoan, including human. Typically, covalent histone modification and its removal show the opposite effect. However, H2B of ubiquitylation and deubiquitylation are both required for gene activation. H2B ubiquitylation is a reversible covalent modification. One of deubiquitylases is Ubp8, which is a component of Spt-Ada-Gcn5-acetyltransferase (SAGA) complex and associates transcription initiation. Another deubiquitylase is Ubp10/Dot4, which is involved in telomeric associated gene silencing. Therefore, histone H2B ubiquitylation is not involved in only the transcription, but also the telomeric silencing. First, we show that Large1 (Lge1) controls the recruitment of Bre1, an ubiquitin ligase, and Ubp8, a deubiquitylase, to promote ubiquitylationin the early steps of transcriptional elongation in yeast. Chromatin immunoprecipitation experiments showed that Lge1 is recruited to the promoter and coding regions of actively transcribed genes. Loss of Lge1 abolished both histone H2B ubiquitylation and H3 lysine 4 and 79 methylation and showed a significant sensitivity to 6-azauracil and mycophenolic acid. Disruption of Lge1 decreases Bre1 recruitment, whereas increasing Ubp8 recruitment near the promoter region. Moreover, these changes alter an interaction between Bre1/Ubp8 and serine 5 phosphorylation of RNA polymerase II of CTD. These results indicate that Lge1 has important for the balance between the recruitment of Bre1 and Ubp8, during transcriptional elongation. Second, Histone modification is also involved in the telomeric silencing and lifespan regulation. Heterochromatin regions are inaccessible to DNA binding factors and transcriptionally silenced. Large block of heterochromatin structures contains telomere and centromere, whereas smaller heterochromatin are separated throughout the chromosome. In each heterochromatin region, there are mechanisms to initiate the silencing process as a unique sequence and then to spread the silencing machinery from the initiation site throughout the entire region. The formation of heterochromatin has been well characterized and is consist of the recruitment of Sir complex, containing Sir2 deacetylase.Acetylation at H4 lysine 16 regulated by antagonizing activities of Sas2 and Sir2 at telomeric proximal regions. But, less is known about other histone modifications in aging regulation. We investigated that histone ubiquitylation provides a functional link between aging and telomeric silencing. H2B ubiqtuiylation regulates Sir2 and Sir2-regulated H4 lysine 16 acetylation by histone trans-tail modifications in the telomeric proximal region. We also observed an increase in H2B ubiquitylation and H3 lysine 4 and79 methylations and a decrease in Sir2 recruitment to the specific telomeric regions in old cells. Moreover, loss of the Rad6/Bre1 complex reduced the recruitment of Sir2 and slightly increased the H4 lysine 16 acetylation in the native telomeric region and the URA3-based TEL07L region. These results propose that the histone trans-tail modifications also regulate a telomeric silencing through the regulation of age-associated Sir2 pathway at telomere-proximal regions. Thus, these data reveal that low levels of histone ubiquitylation are important both transcriptional elongation and age-associated telomeric silencing. Balance of histone H2B ubiquitylation and deubiquitylationis important to transcriptional elongation and maintenance of telomeric silencing in old cells.-
dc.title히스톤 H2B 유비퀴틴화에 의한 전사 신장 및 텔로미어 침묵 조절 연구-
dc.title.alternativeRegulation of transcription elongation and telomeric silencing through histone H2B ubiquitylation-
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