BACKGROUND: The INK4/ARF locus encodes three tumor suppressor genes (p15(Ink4b), 
Arf and p16(Ink4a)) and is frequently inactivated in a large number of human 
cancers. Mechanisms regulating INK4/ARF expression are not fully characterized.
PRINCIPAL FINDINGS: Here we show that in young proliferating embryonic 
fibroblasts (MEFs) the Polycomb Repressive Complex 2 (PRC2) member EZH2 together 
with PRC1 members BMI1 and M33 are strongly expressed and localized at the 
INK4/ARF regulatory domain (RD) identified as a DNA replication origin. When 
cells enter senescence the binding to RD of both PRC1 and PRC2 complexes is lost 
leading to a decreased level of histone H3K27 trimethylation (H3K27me3). This 
loss is accompanied with an increased expression of the histone demethylase 
Jmjd3 and with the recruitment of the MLL1 protein, and correlates with the 
expression of the Ink4a/Arf genes. Moreover, we show that the Polycomb protein 
BMI1 interacts with CDC6, an essential regulator of DNA replication in 
eukaryotic cells. Finally, we demonstrate that Polycomb proteins and associated 
epigenetic marks are crucial for the control of the replication timing of the 
INK4a/ARF locus during senescence.
CONCLUSIONS: We identified the replication licencing factor CDC6 as a new 
partner of the Polycomb group member BMI1. Our results suggest that in young 
cells Polycomb proteins are recruited to the INK4/ARF locus through CDC6 and the 
resulting silent locus is replicated during late S-phase. Upon senescence, Jmjd3 
is overexpressed and the MLL1 protein is recruited to the locus provoking the 
dissociation of Polycomb from the INK4/ARF locus, its transcriptional activation 
and its replication during early S-phase. Together, these results provide a 
unified model that integrates replication, transcription and epigenetics at the 
INK4/ARF locus.