Phosphorylated Rad 18 directs DNA Polymerase {eta} to sites of stalled replication.

The E3 ubiquitin ligase Rad 18 guides DNA Polymerase eta (Poleta) to sites of replication fork stalling and mono-ubiquitinates proliferating cell nuclear antigen (PCNA) to facilitate binding of Y family trans-lesion synthesis (TLS) DNA polymerases during TLS. However, it is unclear exactly how Rad 18 is regulated in response to DNA damage and how Rad 18 activity is coordinated with progression through different phases of the cell cycle. Here we identify Rad18 as a novel substrate of the essential protein kinase Cdc7 (also termed Dbf4/Drf1-dependent Cdc7 kinase [DDK]). A serine cluster in the Poleta-binding motif of Rad 18 is phosphorylated by DDK. Efficient association of Rad 18 with Poleta is dependent on DDK and is necessary for redistribution of Poleta to sites of replication fork stalling. This is the first demonstration of Rad 18 regulation by direct phosphorylation and provides a novel mechanism for integration of S phase progression with postreplication DNA repair to maintain genome stability.