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Gajjalaiahvari, U. R. (2012). Checkpoint Kinase Mediated Regulation of Histone Gene Expression in Response to Genotoxic Stress. Retrieved from http://purl.flvc.org/fsu/fd/FSU_migr_etd-5354
The faithful maintenance of structure and function of chromatin is crucial for maintaining genomic stability. In proliferating cells, nucleosome assembly occurs largely during S-phase with a strict coordination between DNA and histone synthesis. Histone gene transcription is induced during the G1/S transition of cell cycle and shuts off before DNA replication ends. When S-phase cells are treated with genotoxic agents that impede DNA replication, an evolutionary conserved response leads to a rapid drop in histone transcript levels. The objective of this thesis was to understand how cells achieve this regulation by unraveling the underlying molecular mechanisms. We have chosen the budding yeast as a model system to study this because of the simple organization of histone genes in this unicellular eukaryote. In particular, we have focused on the transcriptional regulation of histone genes as this is the major form of regulation for histone genes in budding yeast. A novel surveillance mechanism has been described in the budding yeast Sacccharomyces cerevisiae that monitors the accumulation of non-chromatin-bound histones and promotes their rapid degradation by the proteasome dependent on the checkpoint kinase Rad53. This led us to explore the role of DNA Damage Response kinases in histone gene regulation in response to DNA damage and replication arrest. We have uncovered that the transcriptional repression of histone genes upon DNA damage and replication arrest is dependent on Rad53 and its upstream activators Mec1/Tel1. Further, based on the identification of Hpc2, one of the components of the Hir complex as a novel target of DDR kinases, we identified the phosphorylation sites on Hpc2 that regulate histone gene transcription under replication stress. Additionally, we find that the phosphorylation at these specific sites is crucial for the recruitment of the Rsc chromatin remodeling complex to the histone promoters. Interestingly the DDR kinase dependent phosphorylation of Hpc2 leads to the stabilization of the Hpc2 protein itself upon Hydroxyurea (HU) treatment. This may provide a potential mechanism for how the Hir mediated repression is relieved at G1/S or after checkpoint recovery. Putting all the information together, we propose that DDR kinase dependent phosphorylation stabilizes Hpc2. Its stability plays a regulatory role in the Hir-dependent repression of histone genes upon replication inhibition. It remains to be determined what triggers the instability of Hpc2. Lastly, we explored the role of histone H3K56 acetylation, a mark that persists under replication stress, in the regulation of histone genes. We find histone H3 acetylation at Lysine 56 is indispensable for histone gene repression under replication stress and its removal after recovery is crucial for histone gene transcription to resume. Together the findings in this thesis highlight the important contribution of checkpoint kinases in the maintenance of genomic stability via novel mechanisms that coordinate DNA and histone synthesis.
A Dissertation submitted to the Department of Biomedical sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Bibliography Note
Includes bibliographical references.
Advisory Committee
Johanna Paik, Professor Directing Dissertation; Hong-Guo Yu, University Representative; Akash Gunjan, Committee Member; Myra Hurt, Committee Member; Yamchang Wang, Committee Member.
Publisher
Florida State University
Identifier
FSU_migr_etd-5354
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Gajjalaiahvari, U. R. (2012). Checkpoint Kinase Mediated Regulation of Histone Gene Expression in Response to Genotoxic Stress. Retrieved from http://purl.flvc.org/fsu/fd/FSU_migr_etd-5354