Chl12 (Ctf18) forms a novel replication factor C-related complex and functions redundantly with Rad24 in the DNA replication checkpoint pathway

RAD24 has been identified as a gene essential for the DNA damage checkpoint in budding yeast. Rad24 is structurally related to subunits of the replication factor C (RFC) complex, and forms an PFC-related complex with Rfc2, Rfc3, Rfe4, and Rfc5. The rad24 Delta mutation enhances the defect of rfc5-1 in the DNA replication block checkpoint, implicating RAD24 in this checkpoint. CHL12 (also called CTF18) encodes a protein that is structurally related to the Rad24 and RFC proteins. We show here that although neither chl12 Delta nor rad24 Delta single mutants are defective, chl12 Delta rad24 Delta double mutants become defective in the replication block checkpoint. We also show that ChI12 interacts physically with Rfc2, Rfc3, Rfc4, and Rfc5 and forms an PFC-related complex which is distinct from the RFC and RAD24 complexes. Our results suggest that ChI12 forms a novel REC-related complex and functions redundantly with Rad24 in the DNA replication block checkpoint.