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Fig. 4. Model of epigenetic memory at PRE/TREs during the cell cycle. A
model based on published findings
(Buchenau et al., 1998;
Schmitt et al., 2005;
Sanchez-Elsner et al., 2006).
(1) During interphase, PRE/TREs silence by default. Only those PRE/TREs
that are transcribed escape this silencing. (2,3) After
replication, transcription through the PRE/TRE continues to counteract
silencing. (4) At the onset of mitosis, the PcG proteins PSC, PH and PC
(Posterior sex combs, Polyhomeotic and Polycomb, respectively) dissociate
simultaneously from chromatin (Buchenau et
al., 1998). (5) During mitosis, there is a global shutdown
of transcription. Those PRE/TREs that were transcribed in the previous
interphase must somehow be marked. (6) The PcG proteins reassociate
with chromatin at different points during late mitosis. PSC returns during
anaphase, PH in telophase and PC at the beginning of interphase
(Buchenau et al., 1998). The
transcription of marked PRE/TREs must resume before a functional PcG complex
has assembled at PRE/TREs. This would prevent default silencing in the next
interphase.
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