A cDNA encoding the proliferating cell nuclear antigen (PCNA) from Brassica napus (oilseed rape) was shown to complement the lethal deletion mutation in the PCNA gene (delta POL30) of Saccharomyces cerevisiae. We provide unequivocal evidence that the B. napus PCNA can perform all the essential functions of the yeast PCNA in DNA replication, although some species-specific differences may exist. In addition, the B. napus PCNA expressed as a fusion polypeptide with glutathione S-transferase (GST) was shown to stimulate the activity and processivity of two delta-like DNA polymerases from wheat in vitro. These experiments provide direct biochemical evidence that the B. napus PCNA may function as an auxiliary factor in plant cell DNA replication.