Little is known about the functions of DNA polymerase k (Pol k) recently identified in mammals. From the genomic sequence information of rice and Arabidopsis, we found that Pol k may be the only member of the X-family in higher plants. We have succeeded in isolating the cDNA and recombinant protein of Pol k in a higher plant, rice (Oryza sativa L. cv. Nipponbare) (OsPol k). OsPol k had activities of DNA polymerase, terminal deoxyribonucleotidyl transferase and deoxyribose phosphate lyase, a marker enzyme for base excision repair. It also interacted with rice proliferating cell nuclear antigen (OsPCNA) in a pull-down assay. OsPCNA increased the processivity of OsPol k. Northern blot analysis showed that the level of OsPol k expression correlated with cell proliferation in meristematic and meiotic tissues, and was induced by DNA-damaging treatments. These properties suggest that plant Pol k is a DNA repair enzyme which functions in plant meristematic and meiotic tissues, and that it can substitute for Pol b and terminal deoxyribonucleotidyl transferase.

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Replication protein A (RPA) complex has been shown, using both in vivo and in vitro approaches, to be required for most aspects of eukaryotic DNA metabolism: replication, repair, telomere maintenance and homologous recombination. Here, we review recent data concerning the function and biological importance of the multi‐RPA complex. There are distinct complexes of RPA found in the biological kingdoms, although for a long time only one type of RPA complex was believed to be present in eukaryotes. Each complex probably serves a different role. In higher plants, three distinct large and medium subunits are present, but only one species of the smallest subunit. Each of these protein subunits forms stable complexes with their respective partners. They are paralogs as complex. Humans possess two paralogs and one analog of RPA. The multi‐RPA system can be regarded as universal in eukaryotes. Among eukaryotic kingdoms, paralogs, orthologs, analogs and heterologs of many DNA synthesis‐related factors, including RPA, are ubiquitous. Convergent evolution seems to be ubiquitous in these processes. Using recent findings, we review the composition and biological functions of RPA complexes.

A novel DNA polymerase, designated as OsPolI-like, has been identified from the higher plant, rice (Oryza sativa L. cv. Nipponbare). The OsPolI-like cDNA was 3765 bp in length, and the open reading frame encoded a predicted product of 977 amino acid residues with a molecular weight of 100 kDa. The OsPolI-like gene has been mapped to chromosome 8 and contains 12 exons and 11 introns. The encoded protein showed a high degree of sequence and structural homology to Escherichia coli pol I protein, but differed from DNA polymerase gamma and theta. The DNA polymerase domain of OsPolI-like showed DNA polymerase activity. Subcellular fractionation analysis suggested that the protein is localized in the plastid. Northern and western blotting, and in situ hybridization analyses demonstrated preferential expression of OsPolI-like in meristematic tissues such as shoot apical meristem, root apical meristem, leaf primordia and the marginal meristem. Interestingly, no expression was detected in mature leaves, although they have a high chloroplast content. These properties indicated that OsPolI-like is a novel plant DNA polymerase. The function of OsPolI-like is discussed in relation to plastid maturation.

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