The Roles of Splicing and H2A.Z in Chromatin Assembly

Eukaryotic nuclear DNA is folded with histone and non-histone proteins into chromatin, a nucleoprotein structure regulated by histone post-translational modifications and substitution with histone variants. Chromatin mediates processes such as DNA damage repair, cell differentiation, gene silencing, and centromere specification. Mistaken inheritance of chromatin-mediated gene silencing, for instance, can cause both aberrant development and cancer. Gene silencing at pericentromeres and centromeres, which can be attained through obstruction of transcription as well as through recruitment of specific RNA-degrading proteins, is essential for centromere specification. However, the molecular mechanisms of these processes are not yet thoroughly understood, and therefore they will be the focus of this thesis. A structure termed heterochromatin, for which the essential hallmark is histone H3 lysine 9 methylation (H3K9me), preferentially assembles at repetitive DNA such as pericentric regions, playing roles in transcriptional silencing, recombination suppression, and chromosome segregation. The RNA interference (RNAi) machinery is required for heterochromatin assembly over DNA repeats in diverse organisms by targeting histone-modifying activities. Surprisingly, RNA splicing factors are also required for this process. A widely-held model derived from studies in fission yeast is that splicing factors provide a platform for siRNA generation independently of their splicing activity. Here, we discovered the requirement of four non-essential splicing factors for pericentric heterochromatin assembly, allowing us to more clearly address the role of splicing in heterochromatin assembly. Sequencing total cellular RNA from the strongest of these mutants, cwf14Δ, showed intron retention in mRNAs of several RNAi factors, which correspond to strong reduction in levels of a central RNAi protein, Argonaute. Moreover, introducing cDNA versions of RNAi factors significantly restores pericentric heterochromatin in splicing mutants. We also found that mutation of splicing factors affects telomeric heterochromatin, and replacement of mis-spliced factor tpz1+ with its cDNA partially rescued heterochromatin defects at telomeres in splicing mutants. Thus proper splicing of RNAi and shelterin factors contributes to heterochromatin assembly at pericentric regions and telomeres. In addition to post-translational modifications, chromatin silencing can be regulated by histone variants such as H2A.Z. The incorporation of H2A.Z into chromatin regulates chromatin structure and gene expression. The Swr1 chromatin remodeling complex deposits H2A.Z in budding yeast and mammals. Here we characterize a novel component of the fission yeast Swr1 complex, Msc1, which is a Jumonji domain protein frequently associated with histone demethylation. We found that Msc1 is required for Swr1-mediated incorporation of H2A.Z into chromatin at gene promoters. We demonstrated that H2A.Z is required for the expression of CENP-C, which in turn regulates centromere silencing and chromosome segregation. Together, these results show that chromatin silencing at pericentromeres and centromeres is mediated by splicing factors and H2A.Z, respectively, to promote proper regulation of other chromatin factors, thus ensuring faithful chromosome segregation