Highly conserved structure of proliferating cell nuclear antigen (DNA polymerase δ auxiliary protein) gene in plants

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Abstract
In a previous paper [Suzuka, I., Daidoji, H., Matsuoka, M., Kadowaki, K., Takasaki, Y., Nakane, P. K. & Moriuchi, T. (1989) Proc. Natl Acad. Sci. USA 86, 3189-3193], we demonstrated the presence of a rice genomic-DNA fragment which hybridized with a rat proliferating-cell nuclear antigen (PCNA) cDNA probe, and proposed that plants possess the homolog of the PCNA gene which plays an essential role in DNA replication in mammals. Here we show that the sequence of the rice PCNA gene supports this proposal. The rice gene consists of four exons. The 5'-flanking region contains a variety of sequences similar to several consensus sequences believed to be important for the regulation of gene expression in eukaryotes. Determination of the transcription-start site and the structure of the 3'-flanking region of the gene indicated that the coded mRNA is 1073 bases. The rice PCNA genomic and cDNA clones are composed of 789 nucleotides; they encode a putative protein of 263 amino acids having a molecular mass of 29275 Da. Comparison of the coding regions of rice PCNA and rat PCNA revealed about 64% similarity at the nucleotide level and about 62% similarity at the amino acid level. Furthermore, comparison of the predicted amino acid sequence of the soybean PCNA with the rice PCNA sequence indicated a high degree of similarity (88%). The structural similarity between PCNA from plants and animals, despite the distant evolutionary relationship, indicates a strong selection pressure for conservation of this structure and suggests the presence of conserved DNA-replication mechanisms throughout the eukaryotes.