Mechanisms of Chromatin Remodeling at Yeast Promoters

DESCRIPTION (provided by applicant): The timing and the extent of gene expression ultimately control normal cell proliferation and cellular differentiation in humans and other organisms. The packaging of eukaryotic DNA with histone proteins into chromatin inhibits virtually all steps in transcription. As such, chromatin functions not only as a platform to regulae transcription, but also as a filter to prevent aberrant transcription. The remodeling of chromatin structure is integral to all stages of transcription. The biological importance of chromatin remodeling in humans is manifested by the fact that mutations in genes important for chromatin remodeling are associated with numerous cancers. The goal of this project is to understand how gene expression occurs in the context of chromatin by elucidating the molecular steps that remodel chromatin structure. The histone variant H2A.Z is a key player in chromatin remodeling. H2A.Z marks gene promoters and has been proposed to help poise genes for transcription by forming nucleosomes that are predisposed for disassembly. However, the mechanism by which H2A.Z nucleosomes are disassembled remains obscure. Previously, I used a combinatorial approach that involves yeast genetics, genomics and biochemistry to dissect the mechanism of H2A.Z deposition. In this project, I will extend the approach to study the disassembly pathway of H2A.Z nucleosomes. One aim is to identify the genes involved by screening mutants that are defective in H2A.Z eviction. These mutants will then be used to study the role of H2A.Z eviction in transcriptional activation. In the second aim, the proteins encoded by these genes will be characterized biochemically in nucleosome eviction assays. Finally, a kinetic approach will be applied to understand how opposing pathways of assembly and disassembly of H2A.Z nucleosomes are maintained in a dynamic equilibrium to prepare genes for transcription. In humans, H2A.Z controls the expression of developmental and cell cycle regulators, and the overexpression of the gene encoding H2A.Z is linked to cancer progression. Given the conserved nature of chromatin structure and its remodeling pathways, the principles developed for the yeast system are likely translatable to humans. This study could lead to the identification of new targets for anticancer drug development.