The role of single-stranded DNA and polymerase alpha in establishing the ATR, Hus1 DNA replication checkpoint.

Using a nucleus-free DNA replication system we have investigated the roles of Xenopus ATR (XATR) and Hus1 (Xhus1) as the DNA replication checkpoint sensors. Like XATR, Xhus1 is required for the checkpoint-dependent phosphorylation of Xchk1 and associates with chromatin in an initiation-dependent manner. While removal of replication protein A inhibits chromatin association of both XATR and Xhus1, removal of polymerase alpha only disrupts chromatin association of Xhus1. In addition, chromatin association of XATR and Xhus1 are independent of each other. Finally, like XATR, Xhus1 associates with chromatin in unperturbed S phase and dissociates from chromatin following completion of DNA replication.