The adenine misincorporated by replicative DNA polymerases (pols) opposite 7,8-dihydro-8-oxoguanine (8-oxo-G) is removed by a specific glycosylase, leaving the lesion on the DNA. Subsequent incorporation of C opposite 8-oxo-G on the resulting 1-nt gapped DNA is essential for the removal of the 8-oxo-G to prevent G-C to T-A transversion mutations. By using model DNA templates, purified DNA pols ␤ and and knockout cell extracts, we show here that the auxiliary proteins replication protein A and proliferating cell nuclear antigen act as molecular switches to activate the DNA pol -dependent highly efficient and faithful repair of A:8-oxo-G mismatches in human cells and to repress DNA pol ␤ activity. By using an immortalized human fibroblast cell line that has the potential to induce cancer in mice, we show that the development of a tumoral phenotype in these cells correlated with a differential expression of DNA pols and ␤. R eactive oxygen species (ROS) are produced during normal cell metabolism and through the action of exogenous agents (1). When ROS react with DNA, the most frequently generated lesion (10 3 to 10 4 per cell per day) is 7,8-dihydro-8-oxoguanine (8-oxo-G) (2), whose mutagenic potential in aging, tumor transformation, and neurodegenerative diseases is well established. The presence of 8-oxo-G in the replicating strand can lead to frequent misincorporation of A opposite the lesion by the human replicative DNA polymerases (pols) ␣, ␦, and (3). Full repair of 8-oxo-G lesion is guaranteed by 2 different base excision repair (BER) systems: (i) an OGG1-dependent, which targets C:8-oxo-G mispairs, removes the lesion and leaves an intact DNA strand to act as template for the resynthesis step (4); and (ii) a MUTYH-dependent pathway, which targets the A:8-oxo-G base pair and removes the adenine (5-7). Subsequent error-free bypass of the lesion requires a specialized DNA pol that can catalyze the correct incorporation of C opposite 8-oxo-G during the resynthesis step, reconstituting a C:8-oxo-G base pair that could subsequently be repaired by the OGG1-dependent BER. However, the majority of human DNA pols insert an adenine opposite 8-oxo-G on the template strand with high frequencies (10-75% of the time). Thus, the molecular mechanism ensuring correct and efficient repair of A:8-oxo-G mismatches in human cells is currently undetermined.We have recently shown (8) that the BER enzyme DNA pol , which belongs to DNA pol family X (9), is very efficient in performing error-free translesion synthesis past the 2 major oxidative lesions 8-oxo-G (8) and 2-hydroxy-adenine (2-OH-A) (10). Moreover, its fidelity and efficiency is enhanced 2 orders of magnitude by the auxiliary proteins proliferating cell nuclear antigen (PCNA) and replication protein A (RP-A), both for normal and translesion synthesis, resulting in dATP incorporation frequencies opposite 8-oxo-G as low as 10 Ϫ3 . On the other hand, the other major BER enzyme DNA pol ␤ shows a relaxed nucleotide insertion specificity opposite 8-oxo-G, with erroneous (i.e., d...

The oxidized base 7,8-oxoguanine (8-oxo-G) is the most common DNA lesion generated by reactive oxygen species. This lesion is highly mutagenic due to the frequent misincorporation of A opposite 8-oxo-G during DNA replication. In mammalian cells, the DNA polymerase (pol) family X enzyme DNA pol l catalyzes the correct incorporation of C opposite 8-oxo-G, together with the auxiliary factor proliferating cell nuclear antigen (PCNA). Here, we show that Arabidopsis thaliana DNA pol l, the only member of the X family in plants, is as efficient in performing error-free translesion synthesis past 8-oxo-G as its mammalian homolog. Arabidopsis, in contrast with animal cells, possesses two genes for PCNA. Using in vitro and in vivo approaches, we observed that PCNA2, but not PCNA1, physically interacts with DNA pol l, enhancing its fidelity and efficiency in translesion synthesis. The levels of DNA pol l in transgenic plantlets characterized by overexpression or silencing of Arabidopsis POLL correlate with the ability of cell extracts to perform error-free translesion synthesis. The important role of DNA pol l is corroborated by the observation that the promoter of POLL is activated by UV and that both overexpressing and silenced plants show altered growth phenotypes.

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Terminal deoxynucletidyl transferase (TdT) is overexpressed in some cancer types, where it might compete with pol μ during the mutagenic repair of double strand breaks (DSBs) through the nonhomologous end joining (NHEJ) pathway. Here we report the discovery and characterization of pyrrolyl and indolyl diketo acids that specifically target TdT and behave as nucleotide-competitive inhibitors. These compounds show a selective toxicity toward MOLT-4 compared to HeLa cells that correlate well with in vitro selectivity for TdT. The binding site of two of these inhibitors was determined by cocrystallization with TdT, explaining why these compounds are competitive inhibitors of the deoxynucleotide triphosphate (dNTP). In addition, because of the observed dual localization of the phenyl substituent, these studies open the possibility of rationally designing more potent compounds.