Formation of facultative heterochromatin in fission yeast

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We have studied the molecular mechanism of facultative heterochromatin formation in fission yeast. We first demonstrated that the fission yeast meiotic genes, mei4 and ssm4 loci are heterochromatinated, as evidenced by the presence of histone H3 methylated at lysine 9 (H3K9me), in vegetative growth. This heterochromatin marker disappears upon the entry of cells into meiotic cell cycle. We then revealed that the Mmi1 -degradation machinery of mei4 and ssm4 mRNA transcript is required for the establishment of H3K9me when cells enters vegetative growth phase. Moreover, we found that DSRs, a set of mRNA regions included in mei4 and ssm4 transcripts, are required for the Mmi1 -dependent heterochromatin establishment. These results shed an insight into our understanding of how heterochromatin is formed in eukaryotes.