Sliding clamp subunit that acts as a moving platform for DNA processing. Responsible for tethering the catalytic subunit of DNA polymerase to DNA during high-speed replication (By similarity). In conjunction with replication factor C (RFC) stimulates DNA synthesis by PolB, relieving inhibition by replication protein A (RPA). Homotrimer. The subunits circularize to form a toroid; DNA passes through its center. Replication factor C (RFC) is required to load the toroid on the DNA. Belongs to the PCNA family.