GSBS Dissertations and Theses

Approval Date

July 1997

Document Type

Doctoral Dissertation

Department

Graduate School of Biomedical Sciences, Department of Biochemistry

Subjects

Saccharomyces cerevisiae; Saccharomyces cerevisiae Proteins; Transcription Factors; Chromatin Assembly and Disassembly; Nucleosomes; Academic Dissertations; Dissertations, UMMS

Abstract

Chromatin remodeling is crucial to transcriptional regulation in vivo and a number of protein complexes capable of altering genomic architecture in the budding yeast Saccaromyces cerevisiae have been identified. Among these, the SWI/SNF complex, a 2 MDa, eleven subunit protein assembly, has been the most extensively characterized. The SWI/SNF complex is required for the proper expression of a number of genes in yeast, although it is completely dispensable for the expression of others. Likewise, some, but not all, transcriptional activator proteins require SWI/SNF activity in order to function in vivo. The goal of this thesis work was to identify those components of the transcription process which dictate this dependence on SWI/SNF activity.

Using the well characterized UASGAL system, we have determined that one of these components is the nucleosome state of activator binding sites within a promoter. We find that while SWI/SNF activity is not required for the GAL4 activator to bind to and activate transcription from nucleosome-free binding sites, the complex is required for GAL4 to bind and function at low affinity, nucleosomal binding sites in vivo. The SWI/SNF -dependence of these nucleosomal binding sites can be overcome by 1) replacing the low affinity sites with higher affinity, consensus GAL4 binding sequences, or 2) placing the low affinity sites into a nucleosme-free region. These results provide the first in vivo evidence that the SWI/SNF complex can regulate gene expression by modulating the DNA binding of a transcriptional activator protein.

To determine whether specific components of the GAL4 protein are necessary in order for the SWI/SNF complex to modulate binding to nucleosomal sites in our model system, we tested the SWI/SNF-dependent DNA binding of various derivative GAL4 proteins. We find that a functional activation domain is not required for SWI/SNF to modulate GAL4 binding in vivo. Interestingly, like the full length protein, GAL4 derivatives in which the activation domain has been mutated are able to partially occupy nucleosomal sites in the absence of SWI/SNF (binding in the absence of SWI/SNF is at least forty percent lower than in the presence of SWI/SNF), indicating the activation domain is also not required for SWI/SNF-independent DNA binding.

These results support a model in which the SWI/SNF-dependence of a gene reflects the nucleosomal context of its important regulatory sequences, e.g. binding sites for transcriptional regulatory proteins. Although nucleosomal promoter regions have been correlated with SWI/SNF-dependence in the past, there has of yet been no gene at which nucleosome location has correlated with a specific genetic function. In the final part of this thesis work, we initiated a search for an endogenous SWI/SNF-dependent gene for which the nucleosome state of activator binding sites could be determined.

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