The dynamic regulation of chromatin structure, essential for the expression of eukaryotic genes, is achieved in part by the combined activity of histone modifying and chromatin remodeling complexes. Our long term goal is to understand how these complexes act coordinately to regulate transcription. RSC (Remodels the Structure of Chromatin) is a chromatin remodeling complex conserved from yeast to human that is required for viability. Although RSC is known to regulate the transcription of genes involved in important biological processes such as cell division and the responses to DNA damage and stress, how RSC is recruited to its target genes as well as how it functions to regulate their expression remains to be defined. The RSC complex possesses multiple subunits that can recognize and bind acetylated histones, which supports the prevailing view that the recognition of acetylated histone residues is important for RSC recruitment and function. We recently showed that acetylation in coding sequences is inversely correlated with histone occupancy, which further suggests that RSC recognizes and binds acetylated histones to remove them. Interestingly, RSC also interacts with RNA Polymerase II (Pol II), which suggests that the polymerase may target RSC to transcribed genes. In support of this hypothesis, our preliminary data shows that the interaction of RSC with Pol II is lost in a kin28-ts (C-terminal domain (CTD) kinase) mutant, indicating that phosphorylation of the Pol II CTD by Kin28 may promote the recruitment of RSC to coding sequences during transcription elongation. We thus propose a two-step model in which RSC is initially recruited to coding sequences by elongating polymerases, and subsequently recognizes particular patterns of histone acetylation to target nucleosomes for remodeling or eviction. To understand the mechanism involved in targeting RSC to transcribed coding regions, we will perform genome-wide localization assays in wild type cells and cells mutant for Pol II CTD kinases and histone acetyltransferases. Using a novel assay that we have recently developed, we will identify and characterize critical histone lysine residues important for the interaction of RSC with chromatin. We will then analyze the effect of depleting RSC on transcription elongation to define the mechanism by which RSC is able to facilitate Pol II movement through coding regions. These contributions will be significant in that they are expected to lend insight into the mechanism of how RSC is recruited to chromatin, as well as how it functions to regulate the transcription of its target genes in a healthy organism. This information will be valuable to our understanding of how mutations in the RSC complex lead to the initiation and progression of diseases such as cancer, and could potentially aid in identifying new therapeutic targets.
Alterations in histone modification patterns and mutations in chromatin remodeling complexes are linked to developmental defects and disease, including the onset of various cancers. The proposed study is expected to provide valuable mechanistic insight as to how an essential chromatin remodeling complex is recruited to its target genes to regulate their expression. An understanding of how such complexes regulate precise changes in chromatin structure during the growth and development of a healthy organism will ultimately allow us to determine how mutations in those complexes lead to a disease state.
