The Exploration Of Two Mechanisms Of RNA Mediated Epigenetic Stability In Fission Yeast

Epigenetic inheritance fidelity is relatively stable.Variation of gene expression among individuals in a same genetic background indicate epigenetic inheritance instability.To study what factors modulate epigenetic inheritance instability is crucial to understand how cells control gene expression to fulfill various physiological needs.My thesis work is to explore two interrelated but different mechanisms of RNA mediated epigenetic stability in fission yeast.Previous studies and our experiments have detected facultative heterochromatin on several genes.One group of these genes are meiosis genes.A common feature of these meiosis genes is that their message RNAs contain DSR motif,a signature RNA element that causes RNA degradation.Using an artificial gene GFP-nxDSR to mimic DSR-containing meiotic genes,I observed green and dark sectoring patterns on the surface of colonies,indicating that the epigenetic inheritance of GFP-n×DSR is unstable.I detected H3K9me2/3 on GFP-nxDSR,but deleting clr4,the only methyltransferase in fission yeast,did not affect the sectoring patterns.By introducing histone H3 mutation,affecting H3 dimerization within the histone octamer core,and deleting clr5,a gene encoding a deacetylase which regulate transcription through deacetylation,I observed significant alteration in colony sectoring patterns.These results suggest that the variation of GFP-n×DSR expression is mediated via nucleosome and probably acetylation.Another group of facultative heterochromatin loci is ncRNA.We detected significant H3K9me2/3 signal on ncRNA.394 in Agafl cells treated with caffeine and rapamycin,which block the TORC1 signaling and mimic starvation conditions.I found that,in conjunction with heterochromatin formation on ncRNA.394,the expression level of ncRNA.394 was increased significantly.By deleting clr4,the expression level of ncRNA.394 and its neighbor gene enol increased even further.More evidence showed that the same RNA degradation pathway mentioned above mediates this type of heterochromatin formation.These results suggest that ncRNA.394 represses enol expression by introducing local heterochromatin formation via the RNA degradation pathway.Together,these results showed that via the RNA degradation pathway,some RNA could regulate histone modifications and then regulate gene expression,help cells adapt variant of environments.