Suppression of pea nuclear topoisomerase I enzyme activity by pea PCNA

Proliferating cell nuclear antigen (PCNA), a highly conserved DNA polymerase accessory protein of eukary- otic kingdom, has not been studied thoroughly in bio- chemical terms in plants. We describe the isolation of the cDNA encoding PCNA from the pea cDNA library using the PCR approach. The cDNA was used for expression of pea PCNA in bacteria as a fusion protein (GST.PCNA) with the GST tag at the amino terminal end. The GST.PCNA stimulated the partially purified pea DNA polymerases approximately 30-fold. The stimulation was due to the oligomeric form of GST.PCNA. The pea PCNA interacted with the recombinant type I pea topoiso- merase as well as the native pea nuclear topoisomerase I and repressed the DNA relaxation activities. However, the DNA binding activity of Topo I remained undisturbed in the presence of high amounts of PCNA, thereby signify- ing that the catalysis of Topo I was probably affected by PCNA.