Dionne, I., Nookala, R. K., Jackson, S. P., Doherty, A. J. and Bell, S. D. (2003) A heterotrimeric PCNA in the hyperthermophilic archaeon Sulfolobus solfataricus. Mol. Cell 11:275-282.  

The sliding clamp, PCNA, of the archaeon Sulfolobus solfataricus P2 is a heterotrimer of three distinct subunits (PCNA1, 2 and 3) that assembles in a defined manner. The PCNA heterotrimer, but not individual subunits, stimulates the activities of the DNA polymerase, DNA ligase I and the flap endonuclease (FEN1) of S. solfataricus. Distinct PCNA subunits contact DNA polymerase, DNA ligase or FEN1, imposing a defined architecture at the lagging-strand fork and suggesting the existence of a pre-formed scanning complex at the fork. This provides a mechanism to tightly couple DNA synthesis and Okazaki fragment maturation. Additionally, unique subunit-specific interactions between components of the clamp loader, RFC, suggest a model for clamp loading of PCNA.