Gene for proliferating-cell nuclear antigen (DNA polymerase delta auxiliary protein) is present in both mammalian and higher plant genomes.

Proliferating-cell nuclear antigen (PCNA; also called cyclin) was originally described in proliferating mammalian cells as a nuclear protein with an apparent Mr of 33,000-36,000 and recently was found to be a DNA polymerase delta auxiliary protein. To elucidate whether PCNA/cyclin is a universal protein necessary for proliferation of eukaryotes, a search was conducted for PCNA/cyclin homologues in higher plants. In Southern blot-hybridization analysis, a rat PCNA/cyclin cDNA probe hybridized with homologous sequences in genomic DNAs from rice, soybean, and tobacco. A PCNA/cyclin-related molecular clone (pCJ-1) was isolated from rice DNA and was partially sequenced. The pCJ-1 probe hybridized with a 1.2-kilobase transcript in RNA from rice root tips and shoots. Immunoblot analysis of the soluble extract of soybean root tips with monospecific anti-PCNA/cyclin identified an immunoreactive protein with an apparent Mr of 34,000. Immunohistochemical analysis revealed the presence of an immunoreactive PCNA/cyclin protein in the nuclei of cells in the meristem of soybean root tips. The highly homologous nature of the gene for PCNA/cyclin throughout the animal and plant kingdoms suggests that the product of the gene plays an essential role in DNA replication in eukaryotes.