Characterization of a Thermostable 8-Oxoguanine DNA Glycosylase Specific for GO/N Mismatches from the Thermoacidophilic Archaeon Thermoplasma volcanium

A thermophilic 8-oxoguanine DNA glycosylase from Thermococcus barophilus Ch5 is a new member of AGOG DNA glycosylase family

8-Oxoguanine (8oxoG) in DNA is a major oxidized base that poses a severe threat to genome stability. To counteract the mutagenic effect generated by 8oxoG in DNA, cells have evolved 8oxoG DNA glycosylase (OGG) that can excise this oxidized base from DNA. Currently, OGG enzymes have been divided into three families: OGG1, OGG2 and AGOG (archaeal 8oxoG DNA glycosylase). Due to the limited reports, our understanding on AGOG enzymes remains incomplete. Herein, we present evidence that an AGOG from the hyperthermophilic euryarchaeon Ch5 (Tb-AGOG) excises 8oxoG from DNA at high temperature. The enzyme displays maximum efficiency at 75°C-95°C and at pH 9.0. As expected, Tb-AGOG is a bifunctional glycosylase that harbors glycosylase activity and AP (apurinic/apyrimidinic) lyase activity. Importantly, we reveal for the first time that residue D41 in Tb-AGOG is essential for 8oxoG excision and intermediate formation, but not essential for DNA binding or AP cleavage. Furthermore, residue E79 in Tb-AGOG is essential for 8oxoG excision and intermediate formation, and is partially involved in DNA binding and AP cleavage, which has not been described among the reported AGOG members to date. Overall, our work provides new insights into catalytic mechanism of AGOG enzymes.

Genetic Control of Oxidative Mutagenesis in Sulfolobus acidocaldarius

To identify DNA-oxidation defenses of hyperthermophilic archaea, we deleted genes encoding the putative 7,8-dihydro-8-oxoguanine (oxoG)-targeted N-glycosylase of S. acidocaldarius (ogg; Saci_01367), the Y-family DNA polymerase (dbh; Saci_0554), or both, and measured the effects on cellular survival, replication accuracy, and oxoG bypass in vivo Spontaneous G:C to T:A transversions were elevated in all Δogg and Δdbh constructs, and the Δogg Δdbh double mutant lost viability at a faster rate than isogenic WT and ogg strains. The distribution of G:C to T:A transversions within mutation-detector genes suggested that reactivity of G toward oxidation and the effect on translation contribute heavily to the pattern of mutations that are recovered. An impact of the Ogg protein on overall efficiency of bypassing oxoG in transforming DNA was evident only in the absence of Dbh, and Ogg status did not affect the accuracy of bypass. Dbh function, in contrast, dramatically influenced both the efficiency and accuracy of oxoG bypass. Thus, Ogg and Dbh were found to work independently to avoid mutagenesis by oxoG, and inactivating this simple but effective defense system by deleting both genes imposed a severe mutational burden on S. acidocaldarius cells.IMPORTANCE Hyperthermophilic archaea are expected to have effective (and perhaps atypical) mechanisms to limit the genetic consequences of DNA damage, but few gene products have been demonstrated to have genome-preserving functions in vivo This study confirmed by genetic criteria that the S. acidocaldarius Ogg protein avoids the characteristic mutagenesis of G oxidation. This enzyme and the bypass polymerase Dbh have similar impacts on genome stability but work independently, and may comprise most of the DNA-oxidation defense of S. acidocaldarius The critical dependence of accurate oxoG bypass on the accessory DNA polymerase Dbh further argues that some form of polymerase exchange is important for accurate genome replication in Sulfolobus, and perhaps in related hyperthermophilic archaea.

Biochemical reconstitution and genetic characterization of the major oxidative damage base excision DNA repair pathway in Thermococcus kodakarensis

Reactive oxygen species drive the oxidation of guanine to 8-oxoguanine (8oxoG), which threatens genome integrity. The repair of 8oxoG is carried out by base excision repair enzymes in Bacteria and Eukarya, however, little is known about archaeal 8oxoG repair. This study identifies a member of the Ogg-subfamily archaeal GO glycosylase (AGOG) in Thermococcus kodakarensis, an anaerobic, hyperthermophilic archaeon, and delineates its mechanism, kinetics, and substrate specificity. TkoAGOG is the major 8oxoG glycosylase in T. kodakarensis, but is non-essential. In addition to TkoAGOG, the major apurinic/apyrimidinic (AP) endonuclease (TkoEndoIV) required for archaeal base excision repair and cell viability was identified and characterized. Enzymes required for the archaeal oxidative damage base excision repair pathway were identified and the complete pathway was reconstituted. This study illustrates the conservation of oxidative damage repair across all Domains of life.