Faithful propagation of DNA methylation patterns during DNA replication is 
critical for maintaining cellular phenotypes of individual differentiated cells. 
Although it is well established that Uhrf1 (ubiquitin-like with PHD and ring 
finger domains 1; also known as Np95 and ICBP90) specifically binds to 
hemi-methylated DNA through its SRA (SET and RING finger associated) domain and 
has an essential role in maintenance of DNA methylation by recruiting Dnmt1 to 
hemi-methylated DNA sites, the mechanism by which Uhrf1 coordinates the 
maintenance of DNA methylation and DNA replication is largely unknown. Here we 
show that Uhrf1-dependent histone H3 ubiquitylation has a prerequisite role in 
the maintenance DNA methylation. Using Xenopus egg extracts, we successfully 
reproduce maintenance DNA methylation in vitro. Dnmt1 depletion results in a 
marked accumulation of Uhrf1-dependent ubiquitylation of histone H3 at lysine 
23. Dnmt1 preferentially associates with ubiquitylated H3 in vitro though a 
region previously identified as a replication foci targeting sequence. The RING 
finger mutant of Uhrf1 fails to recruit Dnmt1 to DNA replication sites and 
maintain DNA methylation in mammalian cultured cells. Our findings represent the 
first evidence, to our knowledge, of the mechanistic link between DNA 
methylation and DNA replication through histone H3 ubiquitylation.