DNA replication and post replication DNA repair affect the genomic instability and cancer suppression

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In order to study how cells establish fidelity DNA replication during the lagging strand DNA replication, we isolated active mutants in Saccharomyces cerevisiae pol alpha that have a defect in error discrimination. Among them, L868F pol alpha has a spontaneous error frequency of 3 in 100 nucleotides and 570-fold lower replication fidelity than wild type. In vivo, L868F pol alpha confers a mutator phenotype. For this result, we suggest accurate DNA synthesis by pol alpha is important to suppress genomic instability in cells. We also found L868F pol alpha catalyzed relatively efficient bypass of a cis-syn cyclobutane pyrimidine dimer, extending the 3'-T 26000-fold more efficiently than wild type. This type of activity is characteristic to translesion DNA polymerases, which are classified in another family of DNA polymerases. Interestingly, leucine and phenylalanine residues are conserved in replicative and translesion DNA polymerases, respectively. In a translesion DNA polymerase eta mutant of S. cerevisiae F34L pol eta showed reduced translesion DNA synthesis activity in vitro. The corresponding mutants in human DNA polymerases also showed the similar biochemical features. These data suggest that the phenylalanine and leucine residues may have played a role in the functional evolution of these enzyme classes.The observed high fidelity of chromosomal replication is achieved not only through mechanisms that enhance correct nucleotide insertion by the DNA polymerase but also through ones that correct errors after they occur. We are under investigation of the downstream pathways that may either suppress or promote genomic instability.