Processive searching ability varies among members of the gap-filling DNA polymerase X family

Molecular and Functional Characteristics of DNA Polymerase Beta-Like Enzymes From Trypanosomatids

Trypanosomatids are a group of primitive unicellular eukaryotes that can cause diseases in plants, insects, animals, and humans. Kinetoplast genome integrity is key to trypanosomatid cell survival and viability. Kinetoplast DNA (kDNA) is usually under attack by reactive oxygen and nitric species (ROS and RNS), damaging the DNA, and the cells must remove and repair those oxidatively generated lesions in order to survive and proliferate. Base excision repair (BER) is a well-conserved pathway for DNA repair after base damage, single-base loss, and single-strand breaks, which can arise from ROS, RSN, environmental genotoxic agents, and UV irradiation. A powerful BER system has been described in the T. cruzi kinetoplast and it is mainly carried out by DNA polymerase β (pol β) and DNA polymerase β-PAK (pol β-PAK), which are kinetoplast-located in T. cruzi as well as in other trypanosomatids. Both pol β and pol β-PAK belong to the X-family of DNA polymerases (pol X family), perform BER in trypanosomatids, and display intrinsic 5-deoxyribose phosphate (dRP) lyase and DNA polymerase activities. However, only Pol β-PAK is able to carry out trans-lesion synthesis (TLS) across 8oxoG lesions. T. cruzi cells overexpressing pol β are more resistant to ROS and are also more efficient to repair 8oxoG compared to control cells. Pol β seems to play a role in kDNA replication, since it associates with kinetoplast antipodal sites in those development stages in trypanosomatids which are competent for cell replication. ROS treatment of cells induces the overexpression of pol β, indicating that plays a role in kDNA repair. In this review, we will summarize the main features of trypanosomatid minicircle kDNA replication and the biochemical characteristics of pol β-like enzymes and their involvement in BER and kDNA replication. We also summarize key structural features of trypanosomatid pol β compared to their mammalian (human) counterpart.

Lysines in the lyase active site of DNA polymerase β destabilize nonspecific DNA binding, facilitating searching and DNA gap recognition

DNA polymerase (pol) β catalyzes two reactions at DNA gaps generated during base excision repair, gap-filling DNA synthesis and lyase-dependent 5´-end deoxyribose phosphate removal. The lyase domain of pol β has been proposed to function in DNA gap recognition and to facilitate DNA scanning during substrate search. However, the mechanisms and molecular interactions used by pol β for substrate search and recognition are not clear. To provide insight into this process, a comparison was made of the DNA binding affinities of WT pol β, pol λ, and pol μ, and several variants of pol β, for 1-nt-gap-containing and undamaged DNA. Surprisingly, this analysis revealed that mutation of three lysine residues in the lyase active site of pol β, 35, 68, and 72, to alanine (pol β KΔ3A) increased the binding affinity for nonspecific DNA ∼11-fold compared with that of the WT. WT pol μ, lacking homologous lysines, displayed nonspecific DNA binding behavior similar to that of pol β KΔ3A, in line with previous data demonstrating both enzymes were deficient in processive searching. In fluorescent microscopy experiments using mouse fibroblasts deficient in PARP-1, the ability of pol β KΔ3A to localize to sites of laser-induced DNA damage was strongly decreased compared with that of WT pol β. These data suggest that the three lysines in the lyase active site destabilize pol β when bound to DNA nonspecifically, promoting DNA scanning and providing binding specificity for gapped DNA.

WITHDRAWN: Requirements for PARP-1 covalent crosslinking to DNA (PARP-1 DPC)

The Publisher regrets that this article is an accidental duplication of an article that has already been published in DNA Repair, 90C (2020) 102850, https://doi.org/10.1016/j.dnarep.2020.102850. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal.

The deoxyribose phosphate lyase of DNA polymerase β suppresses a processive DNA synthesis to prevent trinucleotide repeat instability

Trinucleotide repeat (TNR) instability is associated with over 42 neurodegenerative diseases and cancer, for which the molecular mechanisms remain to be elucidated. We have shown that the DNA base excision repair (BER) pathway and its central component, DNA polymerase β (pol β), in particular, its polymerase activity plays an active role in regulating somatic TNR instability. Herein, we revealed a unique role of the pol β dRP lyase in preventing somatic TNR instability. We found that deficiency of pol β deoxyribose phosphate (dRP) lyase activity locked the pol β dRP lyase domain to a dRP group, and this ‘tethered’ pol β to its template forcing the polymerase to perform a processive DNA synthesis. This subsequently promoted DNA strand slippage allowing pol β to skip over a template loop and causing TNR deletion. We showed that the effects were eliminated by complementation of the dRP lyase deficiency with wild-type pol β protein. The results indicate that pol β dRP lyase activity restrained the pol β-dRP interaction to suppress a pol β processive DNA synthesis, thereby preventing TNR deletion. This further implicates a potential of pol β dRP lyase inhibition as a novel treatment of TNR-expansion diseases.

Eukaryotic Base Excision Repair: New Approaches Shine Light on Mechanism

Genomic DNA is susceptible to endogenous and environmental stresses that modify DNA structure and its coding potential. Correspondingly, cells have evolved intricate DNA repair systems to deter changes to their genetic material. Base excision DNA repair involves a number of enzymes and protein cofactors that hasten repair of damaged DNA bases. Recent advances have identified macromolecular complexes that assemble at the DNA lesion and mediate repair. The repair of base lesions generally requires five enzymatic activities: glycosylase, endonuclease, lyase, polymerase, and ligase. The protein cofactors and mechanisms for coordinating the sequential enzymatic steps of repair are being revealed through a range of experimental approaches. We discuss the enzymes and protein cofactors involved in eukaryotic base excision repair, emphasizing the challenge of integrating findings from multiple methodologies. The results provide an opportunity to assimilate biochemical findings with cell-based assays to uncover new insights into this deceptively complex repair pathway.

Molecular basis for the faithful replication of 5-methylcytosine and its oxidized forms by DNA polymerase β

DNA methylation is an epigenetic mark that regulates gene expression in mammals. One method of methylation removal is through ten-eleven translocation-catalyzed oxidation and the base excision repair pathway. The iterative oxidation of 5-methylcytosine catalyzed by ten-eleven translocation enzymes produces three oxidized forms of cytosine: 5-hydroxmethylcytosine, 5-formylcytosine, and 5-carboxycytosine. The effect these modifications have on the efficiency and fidelity of the base excision repair pathway during the repair of opposing base damage, and in particular DNA polymerization, remains to be elucidated. Using kinetic assays, we show that the catalytic efficiency for the incorporation of dGTP catalyzed by human DNA polymerase β is not affected when 5-methylcytosine, 5-hydroxmethylcytosine, and 5-formylcytosine are in the DNA template. In contrast, the catalytic efficiency of dGTP insertion decreases ∼20-fold when 5-carboxycytosine is in the templating position, as compared with unmodified cytosine. However, DNA polymerase fidelity is unaltered when these modifications are in the templating position. Structural analysis reveals that the methyl, hydroxymethyl, and formyl modifications are easily accommodated within the polymerase active site. However, to accommodate the carboxyl modification, the phosphate backbone on the templating nucleotide shifts ∼2.5 Å to avoid a potential steric/repulsive clash. This altered conformation is stabilized by lysine 280, which makes a direct interaction with the carboxyl modification and the phosphate backbone of the templating strand. This work provides the molecular basis for the accommodation of epigenetic base modifications in a polymerase active site and suggests that these modifications are not mutagenically copied during base excision repair.

DNA polymerase beta and other gap-filling enzymes in mammalian base excision repair

DNA polymerase β plays a central role in the base excision DNA repair pathway that cleanses the genome of apurinic/apyrimidinic (AP) sites. AP sites arise in DNA from spontaneous base loss and DNA damage-specific glycosylases that hydrolyze the N-glycosidic bond between the deoxyribose and damaged base. AP sites are deleterious lesions because they can be mutagenic and/or cytotoxic. DNA polymerase β contributes two enzymatic activities, DNA synthesis and lyase, during the repair of AP sites; these activities reside on carboxyl- and amino-terminal domains, respectively. Accordingly, its cellular, structural, and kinetic attributes have been extensively characterized and it serves as model enzyme for the nucleotidyl transferase reaction utilized by other replicative, repair, and trans-lesion DNA polymerases.

DNA scanning by base excision repair enzymes and implications for pathway coordination

Site-specific DNA binding proteins must search the genome to locate their target sites, and many DNA modifying enzymes have the ability to scan along DNA in search of their substrates. This process is termed processive searching, and it serves to decrease the search time by effectively increasing the DNA binding footprint of a protein. The repertoire of proteins capable of processive searching is expanding, highlighting the need to understand the governing principles behind this fundamental process. Many of the enzymes in the base excision DNA repair pathway are capable of processive searching. Here, we briefly summarize methodology for determining if a protein can scan DNA and highlight the discovery that the base excision repair DNA polymerase β performs a processive search. Elucidation of physical models for DNA searching has also provided a plausible mechanism for pathway coordination during repair. The ability of BER enzymes to transiently sample adjacent DNA sites while bound to their product confers accessibility to downstream enzymes and does not require protein-protein interactions for coordination.

Biochemical Basis of APOBEC3 Deoxycytidine Deaminase Activity on Diverse DNA Substrates

The Apolipoprotein B mRNA editing complex (APOBEC) family of enzymes contains single-stranded polynucleotide cytidine deaminases. These enzymes catalyze the deamination of cytidine in RNA or single-stranded DNA, which forms uracil. From this 11 member enzyme family in humans, the deamination of single-stranded DNA by the seven APOBEC3 family members is considered here. The APOBEC3 family has many roles, such as restricting endogenous and exogenous retrovirus replication and retrotransposon insertion events and reducing DNA-induced inflammation. Similar to other APOBEC family members, the APOBEC3 enzymes are a double-edged sword that can catalyze deamination of cytosine in genomic DNA, which results in potential genomic instability due to the many mutagenic fates of uracil in DNA. Here, we discuss how these enzymes find their single-stranded DNA substrate in different biological contexts such as during human immunodeficiency virus (HIV) proviral DNA synthesis, retrotransposition of the LINE-1 element, and the "off-target" genomic DNA substrate. The enzymes must be able to efficiently deaminate transiently available single-stranded DNA during reverse transcription, replication, or transcription. Specific biochemical characteristics promote deamination in each situation to increase enzyme efficiency through processivity, rapid enzyme cycling between substrates, or oligomerization state. The use of biochemical data to clarify biological functions and alignment with cellular data is discussed. Models to bridge knowledge from biochemical, structural, and single molecule experiments are presented.