Organization of chromatin by epigenetic mechanisms is essential for establishing 
and maintaining cellular identity in developing and adult organisms. A key 
question that remains unresolved about this process is how epigenetic marks are 
transmitted to the next cell generation during cell division. Here we provide a 
model to explain how trimethylated Lys 27 of histone 3 (H3K27me3), which is 
catalysed by the EZH2-containing Polycomb Repressive Complex 2 (PRC2), is 
maintained in proliferating cells. We show that the PRC2 complex binds to the 
H3K27me3 mark and colocalizes with this mark in G1 phase and with sites of 
ongoing DNA replication. Efficient binding requires an intact trimeric PRC2 
complex containing EZH2, EED and SUZ12, but is independent of the catalytic SET 
domain of EZH2. Using a heterologous reporter system, we show that transient 
recruitment of the PRC2 complex to chromatin, upstream of the transcriptional 
start site, is sufficient to maintain repression through endogenous PRC2 during 
subsequent cell divisions. Thus, we suggest that once the H3K27me3 is 
established, it recruits the PRC2 complex to maintain the mark at sites of DNA 
replication, leading to methylation of H3K27 on the daughter strands during 
incorporation of newly synthesized histones. This mechanism ensures maintenance 
of the H3K27me3 epigenetic mark in proliferating cells, not only during DNA 
replication when histones synthesized de novo are incorporated, but also outside 
S phase, thereby preserving chromatin structure and transcriptional programs.