Proliferating-cell nuclear antigen (PCNA), the auxiliary protein for DNA polymerase delta, is one of the key factors for both PCNA-dependent DNA synthesis and cell-cycle progression. Plant PCNA genes have previously been cloned from rice, carrot, tobacco, and soybean cells by screening the cDNA libraries using similarity to the human or rat PCNA genes. We subcloned the relevant gene from the rice PCNA cDNA into an Escherichia coli expression vector pMAL, and the PCNA protein was expressed in the bacteria in the form of a fusion protein (70 kDa) with maltose-binding protein (MBP). Monoclonal antibody against human PCNA reacted with both purified fusion protein and a 32-kDa fragment, resulting from restriction protease (factor Xa) digestion of the fusion protein. The N-terminal amino acid sequence of the 32-kDa fragment was identical to that of rice PCNA sequence. Rice PCNA fusion protein was found to stimulate DNA synthesis catalyzed by DNA polymerase delta from human cells (although much less effectively), while having no effect on DNA polymerase alpha activity. The results indicate that plant PCNA functions as one of the cofactors of DNA synthesis as is the case with other eukaryotes.