In most eukaryotes methylation of histone H3 on lysine 9 (H3K9me) is the key post-translational
modification required for the assembly of constitutive heterochromatin at
centromeres and other chromosomal regions. H3K9me is bound by the
chromodomain proteins HP1/Swi6 and the Suv39/Clr4 H3K9 methyltransferase itself
suggesting that, once established, H3K9me might act as an epigenetic mark that can
transmit the chromatin state independently of the initiator signal. However, it has not
been demonstrated that H3K9me does indeed act as an epigenetic mark. Fission
yeast represents an excellent system to address this question since S. pombe lacks
DNA methylation and H3K9me is catalysed by the unique, non-essential H3K9
methyltransferase Clr4. To determine whether H3K9me carries epigenetic properties
it is important to uncouple H3K9me from genomic domains that have the intrinsic
ability to recruit the heterochromatin machinery. One way to solve this problem is to
isolate H3K9me from its original context and investigate whether at an ectopic site
H3K9me can self-propagate through cell division. To accomplish this, we tethered
regulatable TetR-Clr4 fusion protein at euchromatic loci in fission yeast. This resulted
in the assembly of an extensive domain of H3K9me-dependent heterochromatin that
is rapidly disassembled following TetR-Clr4 release. Strikingly, the inactivation of
Epe1, a putative histone demethylase, is sufficient to maintain the silent H3K9me-dependent
heterochromatin at the tethering sites through mitotic and meiotic cell
divisions in absence of TetR-Clr4. These results indicate that H3K9me acts as an
epigenetic mark to maintain heterochromatin domains; however, a regulatory
mechanism dependent on Epe1 exists to actively remove H3K9me and thus prevent
heterochromatin from being transmitted when assembled at inappropriate regions of
the genome