One of the sliding clamp subunits that acts as a moving platform for DNA processing. Responsible for tethering the catalytic subunit of DNA polymerase to DNA during high-speed replication. Heterotrimer stimulates the Holliday junction resolvase Hjc. DNA polymerase I, DNA ligase and the flap endonuclease may be constitutively associated with the PCNA heterotrimer forming a scanning complex able to couple DNA synthesis and Okazaki fragment maturation. Forms heterodimers with PCNA2, which then recruit PCNA3; does not form homotrimers (PubMed:12535540, PubMed:18703842). The heterodimers interact with RfcS homotetramers (PubMed:12535540). Heterotrimer which circularizes head-to-tail (head is at N-terminus, tail is at C-terminus) to form a toroid; DNA passes through its center. Replication factor C (RFC) is required to load the toroid on the DNA. Heterotrimer interacts, probably via this subunit, with flap endonuclease 1 (fen) (PubMed:12535540), Hjc (PubMed:17011573), Dpo4 (PubMed:19054331), and XPF (PubMed:12675797). Belongs to the PCNA family.