Title: Regulation of heterochromatin formation by the JmjC-domain protein Epe1
Description: In eukaryotic cells, DNA wraps around histones to form nucleosomes, which are the basic units of chromatin. Chromatin is classified as active euchromatin or repressive heterochromatin, depending on the modifications on histones and DNA. Heterochromatin, which is defined by the presence of histone modifications such as H3K9 methylation, serves important functions in cells such as silencing transposable elements, preventing aberrant recombination, and regulating gene expression.The fission yeast, which shares basic chromatin modification pathways with higher eukaryotes, is a premier model system for study heterochromatin formation. One important heterochromatin regulator is the JmjC-domain protein Epe1. It contains a conserved JmjC domain, which is commonly found in active demethylases. Despite that no in vitro demethylase activity has been demonstrated, Epe1 has been regarded as an H3K9 demethylase based on genetic evidence. However, the mechanism of its regulation is unclear at the beginning of my studies.

In this thesis, I investigated the regulation of Epe1 through an unbiased genetic screen to identify factors important for Epe1 functions. From the screen, I identified multiple subunits within a transcriptional coactivator SAGA complex. I determined that Epe1 physically recruits SAGA to heterochromatin to promote histone acetylation and transcription, which provides a mechanism for a long-standing paradox regarding heterochromatin at repetitive DNA elements: heterochromatin normally represses transcription but the formation of heterochromatin requires transcription of the repeats. While past results suggest a role of Epe1 in promoting transcription of repeats, our results demonstrate how Epe1 promotes transcription. From this screen, I also identified multiple genes in the cAMP signaling pathway that are important for Epe1 function. I demonstrated that the cAMP signaling pathway regulates Epe1 protein levels post-transcriptionally, and this effect was also seen in cells experiencing glucose starvation, which dampens the cAMP signaling.

This study uncovers another layer of control of Epe1 and provides a critical link between nutrient conditions and heterochromatin regulation. Altogether, my studies identified both a mechanism by which Epe1 promotes transcription within heterochromatin and a layer of Epe1 regulation by the glucose-sensing cAMP signaling pathway. These results will help future studies on Epe1 functions and how it is involved in epigenetic adaptation to changing nutrient conditions.