Direct interaction between uracil-DNA glycosylase and a proliferating cell nuclear antigen homolog in the crenarchaeon Pyrobaculum aerophilum

Structure and function of the uracil DNA glycosylases from hyperthermophiles: Elucidating DNA uracil repair mechanisms: A review

Base deamination can lead to DNA base damage, among which cytosine deamination to uracil occurs frequently. Before repair, replication of uracil in DNA will generate GC → AT transversion mutation. Since base deamination is accelerated by high temperature, genomic DNA stability of hyperthermophiles, which grow optimally above 75 °C, is facing a severe threat by the elevated base deamination created by their living high temperature environments. To counteract its potentially harmful effect, cells have employed several pathways for DNA uracil repair, among which base excision repair (BER) is one of major pathways. Uracil DNA glycosylase (UDG) is the first enzyme that initiates BER by excising uracil from DNA. Based on their sequence similarities, UDGs have been divided into six families, among which families IV and V members are predominantly found in hyperthermophiles. Besides, two novel UDGs have been reported from hyperthermophiles. Generally, UDGs from hyperthermophiles exhibit biochemical and structural characteristics distinct from other family UDG members, thereby enriching functional diversity of UDGs. Herein, we have reviewed structure and function of UDGs from hyperthermophiles to provide insights into DNA uracil repair mechanisms, focusing on difference between UDGs from various hyperthermophiles, and difference between archaeal UDGs and bacterial homologs.

The mesophilic archaeon Methanosarcina acetivorans counteracts uracil in DNA with multiple enzymes: EndoQ, ExoIII, and UDG

Cytosine deamination into uracil is one of the most prevalent and pro-mutagenic forms of damage to DNA. Base excision repair is a well-known process of uracil removal in DNA, which is achieved by uracil DNA glycosylase (UDG) that is found in all three domains of life. However, other strategies for uracil removal seem to have been evolved in Archaea. Exonuclease III (ExoIII) from the euryarchaeon Methanothermobacter thermautotrophicus has been described to exhibit endonuclease activity toward uracil-containing DNA. Another uracil-acting protein, endonuclease Q (EndoQ), was recently identified from the euryarchaeon Pyrococcus furiosus. Here, we describe the uracil-counteracting system in the mesophilic euryarchaeon Methanosarcina acetivorans through genomic sequence analyses and biochemical characterizations. Three enzymes, UDG, ExoIII, and EndoQ, from M. acetivorans exhibited uracil cleavage activities in DNA with a distinct range of substrate specificities in vitro, and the transcripts for these three enzymes were detected in the M. acetivorans cells. Thus, this organism appears to conduct uracil repair using at least three distinct pathways. Distribution of the homologs of these uracil-targeting proteins in Archaea showed that this tendency is not restricted to M. acetivorans, but is prevalent and diverse in most Archaea. This work further underscores the importance of uracil-removal systems to maintain genome integrity in Archaea, including 'UDG lacking' organisms.

Three proliferating cell nuclear antigen homologues from Metallosphaera sedula form a head-to-tail heterotrimer

Proliferating cell nuclear antigen (PCNA) is a sliding clamp that plays a key role in DNA metabolism. Genome sequence analysis has revealed that some crenarchaea possess three PCNA genes in their genome, but it has been reported that three PCNAs do not always form a unique heterotrimer composed of one of each molecule. The thermoacidophilic archaeon, Metallosphaera sedula, has three PCNA homologue genes. Here, we demonstrated that the three PCNA homologues, MsePCNA1, MsePCNA2 and MsePCNA3, exclusively form a heterotrimer in a stepwise fashion; MsePCNA1 and MsePCNA2 form a heterodimer, and then MsePCNA3 binds to the heterodimer. We determined that the dissociation constants between MsePCNA1 and MsePCNA2, and between MsePCNA3 and the MsePCNA1:MsePCNA2 heterodimer are 0.29 and 43 nM, respectively. Moreover, the MsePCNA1, MsePCNA2 and MsePCNA3 heterotrimer stimulated M. sedula DNA ligase 1 activity, suggesting that the heterotrimer works as a DNA sliding clamp in the organism. The stable and stepwise heterotrimerization of M. sedula PCNA homologues would be useful to generate functional protein-based materials such as artificial multi-enzyme complexes, functional hydrogels and protein fibres, which have recently been achieved by protein self-assembly.

PCNA is involved in the EndoQ-mediated DNA repair process in Thermococcales

To maintain genome integrity for transfer to their offspring, and to maintain order in cellular processes, all living organisms have DNA repair systems. Besides the well-conserved DNA repair machineries, organisms thriving in extreme environments are expected to have developed efficient repair systems. We recently discovered a novel endonuclease, which cleaves the 5′ side of deoxyinosine, from the hyperthermophilic archaeon, Pyrococcus furiosus. The novel endonuclease, designated as Endonulcease Q (EndoQ), recognizes uracil, abasic site and xanthine, as well as hypoxanthine, and cuts the phosphodiester bond at their 5′ sides. To understand the functional process involving EndoQ, we searched for interacting partners of EndoQ and identified Proliferating Cell Nuclear Angigen (PCNA). The EndoQ activity was clearly enhanced by addition of PCNA in vitro. The physical interaction between the two proteins through a PIP-motif of EndoQ and the toroidal structure of PCNA are critical for the stimulation of the endonuclease activity. These findings provide us a clue to elucidate a unique DNA repair system in Archaea.

Comparative analyses of the two proliferating cell nuclear antigens from the hyperthermophilic archaeon, Thermococcus kodakarensis

The DNA sliding clamp is a multifunctional protein involved in cellular DNA transactions. In Archaea and Eukaryota, proliferating cell nuclear antigen (PCNA) is the sliding clamp. The ring-shaped PCNA encircles double-stranded DNA within its central hole and tethers other proteins on DNA. The majority of Crenarchaeota, a subdomain of Archaea, have multiple PCNA homologues, and they are capable of forming heterotrimeric rings for their functions. In contrast, most organisms in Euryarchaeota, the other major subdomain, have a single PCNA forming a homotrimeric ring structure. Among the Euryarchaeota whose genome is sequenced, Thermococcus kodakarensis is the only species with two genes encoding PCNA homologues on its genome. We cloned the two genes from the T. kodakarensis genome, and the gene products, PCNA1 and PCNA2, were characterized. PCNA1 stimulated the DNA synthesis reactions of the two DNA polymerases, PolB and PolD, from T. kodakarensis in vitro. PCNA2, however, only had an effect on PolB. We were able to disrupt the gene for PCNA2, whereas gene disruption for PCNA1 was not possible, suggesting that PCNA1 is essential for DNA replication. The sensitivities of the Δpcna2 mutant strain to ultraviolet irradiation (UV), methyl methanesulfonate (MMS) and mitomycin C (MMC) were indistinguishable from those of the wild-type strain.

Uracil-DNA glycosylase of Thermoplasma acidophilum directs long-patch base excision repair, which is promoted by deoxynucleoside triphosphates and ATP/ADP, into short-patch repair

Hydrolytic deamination of cytosine to uracil in DNA is increased in organisms adapted to high temperatures. Hitherto, the uracil base excision repair (BER) pathway has only been described in two archaeons, the crenarchaeon Pyrobaculum aerophilum and the euryarchaeon Archaeoglobus fulgidus, which are hyperthermophiles and use single-nucleotide replacement. In the former the apurinic/apyrimidinic (AP) site intermediate is removed by the sequential action of a 5'-acting AP endonuclease and a 5'-deoxyribose phosphate lyase, whereas in the latter the AP site is primarily removed by a 3'-acting AP lyase, followed by a 3'-phosphodiesterase. We describe here uracil BER by a cell extract of the thermoacidophilic euryarchaeon Thermoplasma acidophilum, which prefers a similar short-patch repair mode as A. fulgidus. Importantly, T. acidophilumcell extract also efficiently executes ATP/ADP-stimulated long-patch BER in the presence of deoxynucleoside triphosphates, with a repair track of ∼15 nucleotides. Supplementation of recombinant uracil-DNA glycosylase (rTaUDG; ORF Ta0477) increased the formation of short-patch at the expense of long-patch repair intermediates, and additional supplementation of recombinant DNA ligase (rTalig; Ta1148) greatly enhanced repair product formation. TaUDG seems to recruit AP-incising and -excising functions to prepare for rapid single-nucleotide insertion and ligation, thus excluding slower and energy-costly long-patch BER.

Revealing the essentiality of multiple archaeal pcna genes using a mutant propagation assay based on an improved knockout method

Organisms belonging to the Crenarchaeota lineage contain three proliferating cell nuclear antigen (PCNA) subunits, while those in the Euryarchaeota have only one, as for Eukarya. To study the mechanism of archaeal sliding clamps, we sought to generate knockouts for each pcna gene in Sulfolobus islandicus, a hyperthermophilic crenarchaeon, but failed with two conventional knockout methods. Then, a new knockout scheme, known as marker insertion and target gene deletion (MID), was developed, with which transformants were obtained for each pMID-pcna plasmid. We found that mutant cells persisted in transformant cultures during incubation of pMID-pcna3 and pMID-araS-pcna1 transformants under counter selection. Studying the propagation of mutant cells by semiquantitative PCR analysis of the deleted target gene allele (Δpcna1 or Δpcna3) revealed that mutant cells could no longer be propagated, demonstrating that these pcna genes are absolutely required for host cell viability. Because the only prerequisite for this assay is the generation of a MID transformant, this approach can be applied generally to any micro-organisms proficient in homologous recombination.

The hyperthermophilic euryarchaeon Archaeoglobus fulgidus repairs uracil by single-nucleotide replacement

Hydrolytic deamination of cytosine to uracil in cellular DNA is a major source of C-to-T transition mutations if uracil is not repaired by the DNA base excision repair (BER) pathway. Since deamination increases rapidly with temperature, hyperthermophiles, in particular, are expected to succumb to such damage. There has been only one report of crenarchaeotic BER showing strong similarities to that in most eukaryotes and bacteria for hyperthermophilic Archaea. Here we report a different type of BER performed by extract prepared from cells of the euryarchaeon Archaeoglobus fulgidus. Although immunodepletion showed that the monofunctional family 4 type of uracil-DNA glycosylase (UDG) is the principal and probably only UDG in this organism, a β-elimination mechanism rather than a hydrolytic mechanism is employed for incision of the abasic site following uracil removal. The resulting 3' remnant is removed by efficient 3'-phosphodiesterase activity followed by single-nucleotide insertion and ligation. The finding that repair product formation is stimulated similarly by ATP and ADP in vitro raises the question of whether ADP is more important in vivo because of its higher heat stability.

MutS and MutL are dispensable for maintenance of the genomic mutation rate in the halophilic archaeon Halobacterium salinarum NRC-1

Background

The genome of the halophilic archaeon Halobacterium salinarum NRC-1 encodes for homologs of MutS and MutL, which are key proteins of a DNA mismatch repair pathway conserved in Bacteria and Eukarya. Mismatch repair is essential for retaining the fidelity of genetic information and defects in this pathway result in the deleterious accumulation of mutations and in hereditary diseases in humans.

Methodology/Principal Findings

We calculated the spontaneous genomic mutation rate of H. salinarum NRC-1 using fluctuation tests targeting genes of the uracil monophosphate biosynthesis pathway. We found that H. salinarum NRC-1 has a low incidence of mutation suggesting the presence of active mechanisms to control spontaneous mutations during replication. The spectrum of mutational changes found in H. salinarum NRC-1, and in other archaea, appears to be unique to this domain of life and might be a consequence of their adaption to extreme environmental conditions. In-frame targeted gene deletions of H. salinarum NRC-1 mismatch repair genes and phenotypic characterization of the mutants demonstrated that the mutS and mutL genes are not required for maintenance of the observed mutation rate.

Conclusions/Significance

We established that H. salinarum NRC-1 mutS and mutL genes are redundant to an alternative system that limits spontaneous mutation in this organism. This finding leads to the puzzling question of what mechanism is responsible for maintenance of the low genomic mutation rates observed in the Archaea, which for the most part do not have MutS and MutL homologs.

Studies on the base excision repair (BER) complex in Pyrococcus furiosus

We have been studying the functions of PCNA (proliferating-cell nuclear antigen) for the assembly and reassembly of the replisome during replication fork progression. We have identified the functional interactions between PCNA and several proteins involved in DNA replication and repair from Pyrococcus furiosus. We recently reported that the activity of UDG (uracil–DNA glycosylase) in P. furiosus (PfuUDG) is stimulated by PCNA (PfuPCNA) in vitro, and identified an atypical PCNA-binding site, AKTLF, in the PfuUDG protein. To understand further the function of the complex in the BER (base excision repair) process, we investigated the AP (apurinic/apyrimidinic) endonuclease, which can process the BER pathway after uracil removal by UDG. Interestingly, one candidate ORF (open reading frame) for the AP endonuclease was found in the operon containing the gene encoding UDG in the P. furiosus genome. However, this ORF did not exhibit any activity. Instead, we identified the AP endonuclease activity from the other candidate gene products, and designated the protein as PfuAP. We discovered a physical interaction between PfuAP and PfuPCNA, suggesting the formation of a BER complex in one of the repair systems in P. furiosus.

Physical and functional interactions between uracil-DNA glycosylase and proliferating cell nuclear antigen from the euryarchaeon Pyrococcus furiosus

Uracil-DNA glycosylase (UDG) is an important repair enzyme in all organisms to remove uracil bases from DNA. Recent biochemical studies have revealed that human nuclear UDG (UNG2) forms a multiprotein complex in replication foci and initiates the base excision repair pathway by interacting with proliferating cell nuclear antigen (PCNA). Here, we show the physical and functional interactions between UDG and PCNA from the hyperthermophilic euryarchaeon, Pyrococcus furiosus. The physical interaction between the two proteins was identified by a surface plasmon resonance analysis. Furthermore, the uracil glycosylase activity of P. furiosus UDG is stimulated by P. furiosus PCNA (PfuPCNA) in vitro. This stimulatory effect was observed only when wild type PfuPCNA, but not a monomeric PCNA mutant, was present in the reaction. Mutational analyses revealed that our predicted PCNA-binding region (AKTLF) in P. furiosus UDG is actually important for the interaction with PfuPCNA. This is the first report describing the functional interaction between archaeal UDG and PCNA.

Specific interactions of three proliferating cell nuclear antigens with replication-related proteins in Aeropyrum pernix

Proliferating cell nuclear antigen (PCNA) is a well-known multifunctional protein involved in eukaryotic and archaeal DNA transactions. The homotrimeric PCNA ring encircles double-stranded DNA within its central hole and tethers many proteins on DNA. Plural genes encoding PCNA-like proteins have been found in the genome sequence of crenarchaeal organisms. We describe here the biochemical properties of the three PCNAs, PCNA1, PCNA2 and PCNA3, from the hyperthermophilic archaeon, Aeropyrum pernix. PCNA2 can form a trimeric structure by itself, and it also forms heterotrimeric structures with PCNA1 and PCNA3. However, neither PCNA1 nor PCNA3 can form homotrimers. The DNA synthesis activity of DNA polymerase I and II, the endonuclease activity of FEN1, and the nick-sealing activity of DNA ligase were stimulated by the complex of PCNA2 and 3 or PCNA1, 2 and 3. These results suggest that the heterotrimeric PCNA at least including PCNA2 and 3 function as the clamp in the replisome. However, PCNA2 is the most abundant in the cells throughout the growth stages among the three PCNAs, and therefore, PCNA2 may perform multitasks by changing complex composition.

Biochemical evidence of a physical interaction between Sulfolobus solfataricus B-family and Y-family DNA polymerases

The hyper-thermophilic archaeon Sulfolobus solfataricus possesses two functional DNA polymerases belonging to the B-family (Sso DNA pol B1) and to the Y-family (Sso DNA pol Y1). Sso DNA pol B1 recognizes the presence of uracil and hypoxanthine in the template strand and stalls synthesis 3-4 bases upstream of this lesion ("read-ahead" function). On the other hand, Sso DNA pol Y1 is able to synthesize across these and other lesions on the template strand. Herein we report evidence that Sso DNA pol B1 physically interacts with DNA pol Y1 by surface plasmon resonance measurements and immuno-precipitation experiments. The region of DNA pol B1 responsible for this interaction has been mapped in the central portion of the polypeptide chain (from the amino acid residue 482 to 617), which includes an extended protease hyper-sensitive linker between the N- and C-terminal modules (amino acid residues Asn482-Ala497) and the alpha-helices forming the "fingers" sub-domain (alpha-helices R, R' and S). These results have important implications for understanding the polymerase-switching mechanism on the damaged template strand during genome replication in S. solfataricus.

Characterization of an archaeal family 4 uracil DNA glycosylase and its interaction with PCNA and chromatin proteins

We describe the characterization of a family 4 UDG1 (uracil DNA glycosylase) from the crenarchaeote Sulfolobus solfataricus. UDG1 is found to have a marked preference for substrates containing a G:U base pair over either A:U or single-stranded uracil-containing DNA substrates. UDG1 is found to interact with the sliding clamp PCNA (proliferating cell nuclear antigen), and does so by a conserved motif in the C-terminus of the protein. S. solfataricus has a heterotrimeric PCNA, and only one of the subunits, PCNA3, interacts with UDG1. We have been unable to detect any stimulation of UDG activity by PCNA, in contrast with the observed effects of PCNA on a number of DNA metabolic enzymes. However, analysis of the effects of Sulfolobus chromatin proteins on UDG1 leads us to propose a mechanistic basis for coupling UDG1 to the replication fork.

Human cytomegalovirus uracil DNA glycosylase associates with ppUL44 and accelerates the accumulation of viral DNA

Background

Human cytomegalovirus UL114 encodes a uracil-DNA glycosylase homolog that is highly conserved in all characterized herpesviruses that infect mammals. Previous studies demonstrated that the deletion of this nonessential gene delays significantly the onset of viral DNA synthesis and results in a prolonged replication cycle. The gene product, pUL114, also appears to be important in late phase DNA synthesis presumably by introducing single stranded breaks.

Results

A series of experiments was performed to formally assign the observed phenotype to pUL114 and to characterize the function of the protein in viral replication. A cell line expressing pUL114 complemented the observed phenotype of a UL114 deletion virus in trans, confirming that the observed defects were the result of a deficiency in this gene product. Stocks of recombinant viruses without elevated levels of uracil were produced in the complementing cells; however they retained the phenotype of poor growth in normal fibroblasts suggesting that poor replication was unrelated to uracil content of input genomes. Recombinant viruses expressing epitope tagged versions of this gene demonstrated that pUL114 was expressed at early times and that it localized to viral replication compartments. This protein also coprecipitated with the DNA polymerase processivity factor, ppUL44 suggesting that these proteins associate in infected cells. This apparent interaction did not appear to require other viral proteins since ppUL44 could recruit pUL114 to the nucleus in uninfected cells. An analysis of DNA replication kinetics revealed that the initial rate of DNA synthesis and the accumulation of progeny viral genomes were significantly reduced compared to the parent virus.

Conclusion

These data suggest that pUL114 associates with ppUL44 and that it functions as part of the viral DNA replication complex to increase the efficiency of both early and late phase viral DNA synthesis.

Cloning, expression, and characterization of uracil-DNA glycosylase of Chlamydia pneumoniae in Escherichia coli

A uracil-DNA glycosylase gene was cloned from Chlamydia pneumoniae AR39 and expressed in E. coli strains BL21 (DE3) and BL21 (DE3) pLysS. After purification by Ni-NTA His x Bind Resin and DEAE Sepharose Fast Flow column chromatography, recombinant CpUDG with a specific activity of 1,000,000 U/mg was obtained. The enzymatic activity of the purified CpUDG protein was further characterized using oligodeoxyribonucleotides carrying uracil bases as substrates. The base opposite to uracil in double strand DNAs affected uracil removal efficiencies in the order: U/- > U/T > U/C > U/G > U/A. Free uracil and abasic sites (AP site) could inhibit the reaction. The optimal temperature and pH for uracil removal by CpUDG were 37 degrees C and pH 8.0, respectively. Site-directed mutagenesis studies indicated that amino acids D77, H200, and A205 were important for the catalytic activity of CpUDG. Together, these data suggest that CpUDG is a member of the UDG family-I protein. This is the first report on cloning, expression, and characterization of Chlamydia uracil-DNA glycosylase.

In vitro and in vivo dimerization of human endonuclease III stimulates its activity

Human endonuclease III (hNTH1), a DNA glycosylase with associated abasic lyase activity, repairs various mutagenic and toxic-oxidized DNA lesions, including thymine glycol. We demonstrate for the first time that the full-length hNTH1 positively cooperates in product formation as a function of enzyme concentration. The protein concentrations that caused cooperativity in turnover also exhibited dimerization, independent of DNA binding. Earlier we had found that the hNTH1 consists of two domains: a well conserved catalytic domain, and an inhibitory N-terminal tail. The N-terminal truncated proteins neither undergo dimerization, nor do they show cooperativity in turnover, indicating that the homodimerization of hNTH1 is specific and requires the N-terminal tail. Further kinetic analysis at transition states reveals that this homodimerization stimulates an 11-fold increase in the rate of release of the final product, an AP-site with a 3'-nick, and that it does not affect other intermediate reaction rates, including those of DNA N-glycosylase or AP lyase activities that are modulated by previously reported interacting proteins, YB-1, APE1, and XPG. Thus, the site of modulating action of the dimer on the hNTH1 reaction steps is unique. Moreover, the high intranuclear (2.3 microM) and cytosolic (0.65 microM) concentrations of hNTH1 determined here support the possibility of in vivo dimerization; indeed, in vivo protein cross-linking showed the presence of the dimer in the nucleus of HeLa cells. Therefore, it is likely that the dimerization of hNTH1 involving the N-terminal tail masks the inhibitory effect of this tail and plays a critical role in its catalytic turnover in the cell.

The diverse spectrum of sliding clamp interacting proteins

DNA polymerase sliding clamps are a family of ring-shaped proteins that play essential roles in DNA metabolism. The proteins from the three domains of life, Bacteria, Archaea and Eukarya, as well as those from bacteriophages and viruses, were shown to interact with a large number of cellular factors and to influence their activity. In the last several years a large number of such proteins have been identified and studied. Here the various proteins that have been shown to interact with the sliding clamps of Bacteria, Archaea and Eukarya are summarized.

Enzymology of base excision repair in the hyperthermophilic archaeon Pyrobaculum aerophilum

DNA of all living organisms is constantly modified by exogenous and endogenous reagents. The mutagenic threat of modifications such as methylation, oxidation, and hydrolytic deamination of DNA bases is counteracted by base excision repair (BER). This process is initiated by the action of one of several DNA glycosylases, which removes the aberrant base and thus initiates a cascade of events that involves scission of the DNA backbone, removal of the baseless sugar-phosphate residue, filling in of the resulting single nucleotide gap, and ligation of the remaining nick. We were interested to find out how the BER process functions in hyperthermophiles, organisms growing at temperatures around 100 degrees C, where the rates of these spontaneous reactions are greatly accelerated. In our previous studies, we could show that the crenarchaeon Pyrobaculum aerophilum has at least three uracil-DNA glycosylases, Pa-UDGa, Pa-UDGb, and Pa-MIG, that can initiate the BER process by catalyzing the removal of uracil residues arising through the spontaneous deamination of cytosines. We now report that the genome of P. aerophilum encodes also the remaining functions necessary for BER and show that a system consisting of four P. aerophilum encoded enzymes, Pa-UDGb, AP endonuclease IV, DNA polymerase B2, and DNA ligase, can efficiently repair a G.U mispair in an oligonucleotide substrate to a G.C pair. Interestingly, the efficiency of the in vitro repair reaction was stimulated by Pa-PCNA1, the processivity clamp of DNA polymerases.

An archaeal XPF repair endonuclease dependent on a heterotrimeric PCNA

Archaea share many similarities with eukarya in their information processing pathways and have proven to be a useful model for studies of DNA replication and transcription, but DNA repair pathways are not well understood in archaea. Nucleotide Excision Repair (NER) deals with many bulky DNA lesions and involves over 30 proteins in eukarya. Archaeal NER has not been characterized biochemically, but homologues of the human repair nucleases XPF and XPG have been identified by homology searches. Crenarchaeal XPF proteins have a simplified domain structure, consisting of the C-terminal nuclease domain conserved in XPF and Mus81 but lacking the N-terminal 'helicase' domain that is found in eukaryal and euryarchaeal sequences. Unexpectedly, Sulfolobus XPF is only active in the presence of the sliding clamp PCNA, which is a heterotrimer in this organism. Interactions with two of the three subunits of PCNA are mediated via a C-terminal interaction motif. The PCNA-XPF complex acts as a structure-specific nuclease on a similar range of DNA flap, bubble and junction substrates as the human protein, suggesting a fundamental conservation through billions of years of evolution.

A novel uracil-DNA glycosylase with broad substrate specificity and an unusual active site

Uracil-DNA glycosylases (UDGs) catalyse the removal of uracil by flipping it out of the double helix into their binding pockets, where the glycosidic bond is hydrolysed by a water molecule activated by a polar amino acid. Interestingly, the four known UDG families differ in their active site make-up. The activating residues in UNG and SMUG enzymes are aspartates, thermostable UDGs resemble UNG-type enzymes, but carry glutamate rather than aspartate residues in their active sites, and the less active MUG/TDG enzymes contain an active site asparagine. We now describe the first member of a fifth UDG family, Pa-UDGb from the hyperthermophilic crenarchaeon Pyrobaculum aerophilum, the active site of which lacks the polar residue that was hitherto thought to be essential for catalysis. Moreover, Pa-UDGb is the first member of the UDG family that efficiently catalyses the removal of an aberrant purine, hypoxanthine, from DNA. We postulate that this enzyme has evolved to counteract the mutagenic threat of cytosine and adenine deamination, which becomes particularly acute in organisms living at elevated temperatures.