Vertebrate POLQ and POLbeta cooperate in base excision repair of oxidative DNA damage

Human DNA polymerase iota protects cells against oxidative stress

Human DNA polymerase iota (poliota) is a unique member of the Y-family of specialised polymerases that displays a 5'deoxyribose phosphate (dRP) lyase activity. Although poliota is well conserved in higher eukaryotes, its role in mammalian cells remains unclear. To investigate the biological importance of poliota in human cells, we generated fibroblasts stably downregulating poliota (MRC5-pol iota(KD)) and examined their response to several types of DNA-damaging agents. We show that cell lines downregulating poliota exhibit hypersensitivity to DNA damage induced by hydrogen peroxide (H(2)O(2)) or menadione but not to ethylmethane sulphonate (EMS), UVC or UVA. Interestingly, extracts from cells downregulating poliota show reduced base excision repair (BER) activity. In addition, poliota binds to chromatin after treatment of cells with H(2)O(2) and interacts with the BER factor XRCC1. Finally, green fluorescent protein-tagged poliota accumulates at the sites of oxidative DNA damage in living cells. This recruitment is partially mediated by its dRP lyase domain and ubiquitin-binding domains. These data reveal a novel role of human poliota in protecting cells from oxidative damage.

Caenorhabditis elegans POLQ-1 and HEL-308 function in two distinct DNA interstrand cross-link repair pathways

DNA interstrand cross-links (ICLs) are highly cytotoxic DNA lesions hindering DNA replication and transcription. Whereas in bacteria and yeast the molecular mechanisms involved in ICL repair are genetically well dissected, the scenario in multicellular organisms remains unclear. Here, we report that the two new mus308 genes, polq-1 and hel-308 are involved in ICL repair in Caenorhabditis elegans. After treatment with ICL agents, a decrease in survival and an increase in checkpoint-induced cell-cycle arrest and apoptosis of germ cells is observed in mutants of both genes. Although sensitive to ICL agents and to a minor extent to IR, cytological and epistatic analyses suggest that polq-1 and hel-308 are involved in different DNA repair pathways. While hel-308 functions in a Fanconi anemia-dependent pathway, polq-1 has a role in a novel distinct and brc-1 (CeBRCA1)-dependent ICL repair process in metazoans.

Low-fidelity DNA synthesis by human DNA polymerase theta

Human DNA polymerase theta (pol or POLQ) is a proofreading-deficient family A enzyme implicated in translesion synthesis (TLS) and perhaps in somatic hypermutation (SHM) of immunoglobulin genes. These proposed functions and kinetic studies imply that pol may synthesize DNA with low fidelity. Here, we show that when copying undamaged DNA, pol generates single base errors at rates 10- to more than 100-fold higher than for other family A members. Pol adds single nucleotides to homopolymeric runs at particularly high rates, exceeding 1% in certain sequence contexts, and generates single base substitutions at an average rate of 2.4 x 10(-3), comparable to inaccurate family Y human pol kappa (5.8 x 10(-3)) also implicated in TLS. Like pol kappa, pol is processive, implying that it may be tightly regulated to avoid deleterious mutagenesis. Pol also generates certain base substitutions at high rates within sequence contexts similar to those inferred to be copied by pol during SHM of immunoglobulin genes in mice. Thus, pol is an exception among family A polymerases, and its low fidelity is consistent with its proposed roles in TLS and SHM.

DNA damage response protein ASCIZ links base excision repair with immunoglobulin gene conversion

ASCIZ (ATMIN) was recently identified as a novel DNA damage response protein. Here we report that ASCIZ-deficient chicken DT40 B lymphocyte lines displayed markedly increased Ig gene conversion rates, whereas overexpression of human ASCIZ reduced Ig gene conversion below wild-type levels. However, neither the efficiency of double-strand break repair nor hypermutation was affected by ASCIZ levels, indicating that ASCIZ does not directly control homologous recombination or formation of abasic sites. Loss of ASCIZ led to mild sensitivity to the base damaging agent methylmethane sulfonate (MMS), yet remarkably, suppressed the dramatic MMS hypersensitivity of polbeta-deficient cells. These data suggest that ASCIZ may affect the choice between competing base repair pathways in a manner that reduces the amount of substrates available for Ig gene conversion.

Structure of the DNA repair helicase hel308 reveals DNA binding and autoinhibitory domains

Hel308 is a superfamily 2 helicase conserved in eukaryotes and archaea. It is thought to function in the early stages of recombination following replication fork arrest and has a specificity for removal of the lagging strand in model replication forks. A homologous helicase constitutes the N-terminal domain of human DNA polymerase Q. The Drosophila homologue mus301 is implicated in double strand break repair and meiotic recombination. We have solved the high resolution crystal structure of Hel308 from the crenarchaeon Sulfolobus solfataricus, revealing a five-domain structure with a central pore lined with essential DNA binding residues. The fifth domain is shown to act as an autoinhibitory domain or molecular brake, clamping the single-stranded DNA extruded through the central pore of the helicase structure to limit the helicase activity of the enzyme. This provides an elegant mechanism to tune the processivity of the enzyme to its functional role. Hel308 can displace streptavidin from a biotinylated DNA molecule, and this activity is only partially inhibited when the DNA is pre-bound with abundant DNA-binding proteins RPA or Alba1, whereas pre-binding with the recombinase RadA has no effect on activity. These data suggest that one function of the enzyme may be in the removal of bound proteins at stalled replication forks and recombination intermediates.

Archaeal Hel308 domain V couples DNA binding to ATP hydrolysis and positions DNA for unwinding over the helicase ratchet

Hel308 and PolQ are paralogues with roles promoting genome stability in archaea and higher eukaryotes. The context in which they act is not clear, although Hel308 helicase from archaea may interact with abnormal replication forks. The atomic structure of archaeal Hel308 from Archaeoglobus fulgidus in complex with DNA was recently reported and has given insights into the mechanisms of superfamily-2 helicases generally. An intriguing aspect of the structure was the positioning of a C-terminal domain V relative to single-stranded DNA and to the helicase ratchet domain IV. We have mutagenised a triplet of arginine residues in domain V of archaeal Hel308 to assess the effects on DNA binding, unwinding, and ATPase activities. Our observations can now be interpreted in light of the atomic structure. We describe crucial roles for domain V as a brake on ATP hydrolysis by coupling it to binding single-stranded DNA and in positioning DNA relative to the helicase ratchet domain IV for efficient unwinding of forked DNA.

Structural basis for DNA duplex separation by a superfamily-2 helicase

To reveal the mechanism of processive strand separation by superfamily-2 (SF2) 3'-->5' helicases, we determined apo and DNA-bound crystal structures of archaeal Hel308, a helicase that unwinds lagging strands and is related to human DNA polymerase theta. Our structure captures the duplex-unwinding reaction, shows that initial strand separation does not require ATP and identifies a prominent beta-hairpin loop as the unwinding element. Similar loops in hepatitis C virus NS3 helicase and RNA-decay factors support the idea that this duplex-unwinding mechanism is applicable to a broad subset of SF2 helicases. Comparison with ATP-bound SF2 enzymes suggests that ATP promotes processive unwinding of 1 base pair by ratchet-like transport of the 3' product strand. Our results provide a first structural framework for strand separation by processive SF2 3'-->5' helicases and reveal important mechanistic differences from SF1 helicases.

Interplay between DNA polymerases beta and lambda in repair of oxidation DNA damage in chicken DT40 cells

DNA polymerase lambda (Pol lambda) is a DNA polymerase beta (Pol beta)-like enzyme with both DNA synthetic and 5'-deoxyribose-5'-phosphate lyase domains. Recent biochemical studies implicated Pol lambda as a backup enzyme to Pol beta in the mammalian base excision repair (BER) pathway. To examine the interrelationship between Pol lambda and Pol beta in BER of DNA damage in living cells, we disrupted the genes for both enzymes either singly or in combination in the chicken DT40 cell line and then characterized BER phenotypes. Disruption of the genes for both polymerases caused hypersensitivity to H(2)O(2)-induced cytotoxicity, whereas the effect of disruption of either polymerase alone was only modest. Similarly, BER capacity in cells after H(2)O(2) exposure was lower in Pol beta(-/-)/Pol lambda(-/-) cells than in Pol beta(-/-), wild-type, and Pol lambda(-/-) cells, which were equivalent. These results suggest that these polymerases can complement for one another in counteracting oxidative DNA damage. Similar results were obtained in assays for in vitro BER capacity using cell extracts. With MMS-induced cytotoxicity, there was no significant effect on either survival or BER capacity from Pol lambda gene disruption. A strong hypersensitivity and reduction in BER capacity was observed for Pol beta(-/-)/Pol lambda(-/-) and Pol beta(-/-) cells, suggesting that Pol beta had a dominant role in counteracting alkylation DNA damage in this cell system.

Multiple functions of DNA polymerases

The primary role of DNA polymerases is to accurately and efficiently replicate the genome in order to ensure the maintenance of the genetic information and its faithful transmission through generations. This is not a simple task considering the size of the genome and its constant exposure to endogenous and environmental DNA damaging agents. Thus, a number of DNA repair pathways operate in cells to protect the integrity of the genome. In addition to their role in replication, DNA polymerases play a central role in most of these pathways. Given the multitude and the complexity of DNA transactions that depend on DNA polymerase activity, it is not surprising that cells in all organisms contain multiple highly specialized DNA polymerases, the majority of which have only recently been discovered. Five DNA polymerases are now recognized in Escherichia coli, 8 in Saccharomyces cerevisiae, and at least 15 in humans. While polymerases in bacteria, yeast and mammalian cells have been extensively studied much less is known about their counterparts in plants. For example, the plant model organism Arabidopsis thaliana is thought to contain 12 DNA polymerases, whose functions are mostly unknown. Here we review the properties and functions of DNA polymerases focusing on yeast and mammalian cells but paying special attention to the plant enzymes and the special circumstances of replication and repair in plant cells.