Elimination of shelterin components bypasses RNAi for pericentric heterochromatin assembly.

The RNAi pathway is required for heterochromatin assembly at repetitive DNA elements in diverse organisms. In fission yeast, loss of RNAi causes pericentric heterochromatin defects, compromising gene silencing and chromosome segregation. Here we show that deletion of telomere shelterin components restores pericentric heterochromatin and its functions in RNAi mutants. We further isolated a separation-of-function mutant of Poz1 and revealed that defective telomere silencing, but not telomere length control, is critical for bypassing RNAi. Further analyses demonstrated that compromising shelterin-mediated heterochromatin assembly in RNAi mutants releases heterochromatin protein Swi6, which is redistributed to pericentric regions through RNAi-independent heterochromatin assembly pathways. Given the high mobility of Swi6 protein and that increased levels of Swi6 facilitates heterochromatin spreading as well as ectopic heterochromatin assembly, our results suggest that constitutive heterochromatin domains use multiple pathways to form high-affinity platforms to restrain Swi6, thus limiting its availability and avoiding promiscuous heterochromatin formation.

Elimination of a specific histone H3K14 acetyltransferase complex bypasses the RNAi pathway to regulate pericentric heterochromatin functions.

In Schizosaccharomyces pombe, the RNAi pathway is required for the formation of pericentric heterochromatin, proper chromosome segregation, and repression of pericentric meiotic recombination. Here we demonstrate that, when the activity of the histone H3 Lys 14 (H3K14) acetyltransferase Mst2 is eliminated, the RNAi machinery is no longer required for pericentric heterochromatin functions. We further reveal that reducing RNA polymerase II recruitment to pericentric regions is essential for maintaining heterochromatin in the absence of RNAi.

Histone H3 lysine 14 acetylation is required for activation of a DNA damage checkpoint in fission yeast.

Histone lysine acetylation has emerged as a key regulator of genome organization. However, with a few exceptions, the contribution of each acetylated lysine to cellular functions is not well understood because of the limited specificity of most histone acetyltransferases and histone deacetylases. Here we show that the Mst2 complex in Schizosaccharomyces pombe is a highly specific H3 lysine 14 (H3K14) acetyltransferase that functions together with Gcn5 to regulate global levels of H3K14 acetylation (H3K14ac). By analyzing the effect of H3K14ac loss through both enzymatic inactivation and histone mutations, we found that H3K14ac is critical for DNA damage checkpoint activation by directly regulating the compaction of chromatin and by recruiting chromatin remodeling protein complex RSC.

Rapid epigenetic adaptation to uncontrolled heterochromatin spreading.

Heterochromatin, a highly compact chromatin state characterized by histone H3K9 methylation and HP1 protein binding, silences the underlying DNA and influences the expression of neighboring genes. However, the mechanisms that regulate heterochromatin spreading are not well understood. In this study, we show that the conserved Mst2 histone acetyltransferase complex in fission yeast regulates histone turnover at heterochromatin regions to control heterochromatin spreading and prevents ectopic heterochromatin assembly. The combined loss of Mst2 and the JmjC domain protein Epe1 results in uncontrolled heterochromatin spreading and massive ectopic heterochromatin, leading to severe growth defects due to the inactivation of essential genes. Interestingly, these cells quickly recover by accumulating heterochromatin at genes essential for heterochromatin assembly, leading to their reduced expression to restrain heterochromatin spreading. Our studies discover redundant pathways that control heterochromatin spreading and prevent ectopic heterochromatin assembly and reveal a fast epigenetic adaptation response to changes in heterochromatin landscape.