Lesion-Induced Mutation in the Hyperthermophilic Archaeon Sulfolobus acidocaldarius and Its Avoidance by the Y-Family DNA Polymerase Dbh

Translesion synthesis of apurinic/apyrimidic site analogues by Y-family DNA polymerase Dbh from Sulfolobus acidocaldarius

Apurinic/apyrimidic (AP) sites are severe DNA damages and strongly block DNA extension by major DNA polymerases. Y-family DNA polymerases possess a strong ability to bypass AP sites and continue the DNA synthesis reaction, which is called translesion synthesis (TLS) activity. To investigate the effect of the molecular structure of the AP site on the TLS efficiency of Dbh, a Y-family DNA polymerase from Sulfolobus acidocaldarius, a series of different AP site analogues (various spacers) are used to characterize the bypass efficiency. We find that not only the molecular structure and atomic composition but also the number and position of AP site analogues determine the TLS efficiency of Dbh. Increasing the spacer length decreases TLS activity. The TLS efficiency also decreases when more than one spacer exists on the DNA template. The position of the AP site analogues is also an important factor for TLS. When the spacer is opposite to the first incorporated dNTPs, the TLS efficiency is the lowest, suggesting that AP sites are largely harmful for the formation of hydrogen bonds. These results deepen our understanding of the TLS activity of Y-family DNA polymerases and provide a biochemical basis for elucidating the TLS mechanism in Sulfolobus acidocaldarius cells.

PolB1 Is Sufficient for DNA Replication and Repair Under Normal Growth Conditions in the Extremely Thermophilic Crenarchaeon Sulfolobus acidocaldarius

The thermophilic crenarchaeon Sulfolobus acidocaldarius has four DNA polymerases (DNAPs): PolB1, PolB2, PolB3, and Dbh (PolY). Previous in vitro studies suggested that PolB1 is the main replicative DNAP of Sulfolobales whereas PolB2 and Y-family polymerases Dpo4 (Saccharolobus solfataricus) or Dbh are involved in DNA repair and translesion DNA synthesis. On the other hand, there are various opinions about the role of PolB3, which remains to be clearly resolved. In order to examine the roles of the DNAPs of S. acidocaldarius through in vivo experiments, we constructed polB2, polB3, and dbh deletion strains and characterized their phenotypes. Efforts to construct a polB1 deletion strain were not successful; in contrast, it was possible to isolate triple gene-deletion strains lacking polB2, polB3, and dbh. The growth of these strains was nearly the same as that of the parent strains under normal growth conditions. The polB2, polB3, and dbh single-deletion strains were sensitive to some types of DNA-damaging treatments, but exhibited normal sensitivity to UV irradiation and several other damaging treatments. Overall, the genotype which exhibited the greatest sensitivity to the DNA-damaging treatments we tested was the ΔpolB2 ΔpolB3 combination, providing the first evidence of overlapping function for these two DNAPs in vivo. The results of our study strongly suggest that PolB1 is responsible for the DNA replication of both the leading and lagging strands and is sufficient to complete the repair of most DNA damage under normal growth conditions in S. acidocaldarius.

Heterotrimeric PCNA increases the activity and fidelity of Dbh, a Y-family translesion DNA polymerase prone to creating single-base deletion mutations

Dbh is a Y-family translesion DNA polymerase from Sulfolobus acidocaldarius, an archaeal species that grows in harsh environmental conditions. Biochemically, Dbh displays a distinctive mutational profile, creating single-base deletion mutations at extraordinarily high frequencies (up to 50 %) in specific repeat sequences. In cells, however, Dbh does not appear to contribute significantly to spontaneous frameshifts in these same sequence contexts. This suggests that either the error-prone DNA synthesis activity of Dbh is reduced in vivo and/or Dbh is restricted from replicating these sequences. Here, we test the hypothesis that the propensity for Dbh to make single base deletion mutations is reduced through interaction with the S. acidocaldarius heterotrimeric sliding clamp processivity factor, PCNA-123. We first confirm that Dbh physically interacts with PCNA-123, with the interaction requiring both the PCNA-1 subunit and the C-terminal 10 amino acids of Dbh, which contain a predicted PCNA-interaction peptide (PIP) motif. This interaction stimulates the polymerase activity of Dbh, even on short, linear primer-template DNA, by increasing the rate of nucleotide incorporation. This stimulation requires an intact PCNA-123 heterotrimer and a DNA duplex length of at least 18 basepairs, the minimal length predicted from structural data to bind to both the polymerase and the clamp. Finally, we find that PCNA-123 increases the fidelity of Dbh on a single-base deletion hotspot sequence 3-fold by promoting an increase in the rate of correct, but not incorrect, nucleotide addition and propose that PCNA-123 induces Dbh to adopt a more active conformation that is less prone to creating deletions during DNA synthesis.

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.

Archaeal DNA polymerases: new frontiers in DNA replication and repair

Archaeal DNA polymerases have long been studied due to their superior properties for DNA amplification in the polymerase chain reaction and DNA sequencing technologies. However, a full comprehension of their functions, recruitment and regulation as part of the replisome during genome replication and DNA repair lags behind well-established bacterial and eukaryotic model systems. The archaea are evolutionarily very broad, but many studies in the major model systems of both Crenarchaeota and Euryarchaeota are starting to yield significant increases in understanding of the functions of DNA polymerases in the respective phyla. Recent advances in biochemical approaches and in archaeal genetic models allowing knockout and epitope tagging have led to significant increases in our understanding, including DNA polymerase roles in Okazaki fragment maturation on the lagging strand, towards reconstitution of the replisome itself. Furthermore, poorly characterised DNA polymerase paralogues are finding roles in DNA repair and CRISPR immunity. This review attempts to provide a current update on the roles of archaeal DNA polymerases in both DNA replication and repair, addressing significant questions that remain for this field.