Mechanisms of Heterochromatic Transcriptional Gene Silencing
Project Summary / Abstract
Heterochromatin plays a crucial role in the repression of transposons, genome stability, and cell differentiation
through its ability to silence DNA transcription. How heterochromatin represses transcription by RNA polymerase
II (Pol II) is an open question. In Schizosaccharomyces pombe, histone three lysine 9 di- and tri-methyl (H3K9me)
bearing heterochromatic loci accomplish transcriptional silencing by recruiting the Heterochromatin Protein 1
(HP1) homologs Swi6 and Chp2. Together, HP1swi6/chp2 proteins induce chromatin compaction, recruit
downstream factors such as histone deacetylases (HDAC) and chromatin remodelers, and promote nascent
RNA decay. Previous studies have unraveled the mechanisms responsible for processing and degrading
nascent RNA transcripts. However, the mechanism(s) by which heterochromatin inhibits Pol II activity, referred
to as transcriptional gene silencing (TGS), is not fully understood. In the proposed work, I explore the following
questions: Which HP1swi6/chp2 functions are sufficient for TGS and what is the mechanism of HP1swi6/chp2 mediated
TGS? I hypothesize that HP1swi6/chp2 mediates TGS through distinct activities: intrinsic nonspecific DNA binding,
liquid-liquid phase separation formation, and the downstream recruitment of histone deacetylase and remodeling
activities. To test this hypothesis, I will 1) identify which Swi6 functions are necessary for heterochromatic TGS
using HP1swi6/chp2 mutant strains, 2) identify activities downstream of HP1swi6/chp2 that are sufficient for
heterochromatic TGS by direct recruitment of proteins to H3K9me loci in cells lacking recruitment of downstream
factors, and 3) Identify the specific transcriptional stage inhibited by HP1swi6/chp2 by characterizing the proteome
of heterochromatic and euchromatic ectopic locus. Achieving these aims would contribute to understanding the
mechanisms of gene regulation and transposon repression. In addition, mechanistic knowledge of
heterochromatic TGS could help treat diseases associated with heterochromatin defects.