Yeast 9-1-1 complex acts as a sliding clamp for DNA synthesis by DNA polymerase ε

Abstract

Eukaryotic cells harbor two DNA binding clamps, PCNA, and another clamp commonly referred to as 9-1-1 clamp. In contrast to the essential role of PCNA in DNA replication as a sliding clamp for DNA polymerase (Pol) δ, no such role in DNA synthesis has been identified for the human 9-1-1 clamp or the orthologous yeast 17-3-1 clamp. The only role identified for either the 9-1-1 or 17-3-1 clamp is in the recruitment of signal transduction kinases which affect the activation of cell cycle checkpoints in response to DNA damage. However, unlike the loading of PCNA by the RFC clamp loader onto 3' recessed DNA junctions for processive DNA synthesis by Polδ, the 17-3-1 clamp or the 9-1-1 clamp is loaded by their respective clamp loader Rad24-RFC or RAD17-RFC onto the 5' recessed DNA junction of RPA coated DNA for the recruitment of signal transduction kinases. Here we identify a novel role of 17-3-1 clamp as a sliding clamp for DNA synthesis by Polε. We provide evidence that similar to the loading of PCNA by RFC, the 17-3-1 clamp is loaded by the Rad24-RFC clamp loader at the 3'-recessed DNA junction in an ATP dependent manner. However, unlike PCNA, the 17-3-1 clamp does not enhance the processivity of DNA synthesis by Polε; instead, it greatly increases the catalytic efficiency of Polε for correct nucleotide incorporation. Further, we show that the same PIP domain in the Pol2 catalytic subunit mediates Polε interaction with the 17-3-1 clamp and with PCNA.