Normal hypermutation in antibody genes from congenic mice defective for DNA polymerase iota

Several low fidelity DNA polymerases participate in generating mutations in immunoglobulin genes. Polymerase eta is clearly involved in the process by causing substitutions of A:T base pairs, whereas polymerase iota has a controversial role. Although the frequency of mutations was decreased in the BL2 cell line deficient for polymerase iota, hypermutation was normal in the 129 strain of mice, which has a natural nonsense mutation in the Poli gene. It is possible that the mice compensated for the defect over time, or that polymerase eta substituted in the absence of polymerase iota. To examine polymerase iota in a genetically defined background, we backcrossed the 129 nonsense mutation to the C57BL/6 strain for six generations. Class switch recombination and hypermutation were studied in these mice and in congenic mice doubly deficient for both polymerases iota and eta. The absence of both polymerases did not affect production of IgG1, indicating that these enzymes are not involved in switch recombination. Poli(-/-F6) mice had the same types of nucleotide substitutions in variable genes as their C57BL/6 counterparts, and mice doubly deficient for polymerases iota and eta had the same mutational spectrum as Polh-/- mice. Thus, polymerase iota did not contribute to the mutational spectra, even in the absence of polymerase eta.     

UV-induced mutations in epidermal cells of mice defective in DNA polymerase η and/or ι

Xeroderma pigmentosum variant (XP-V) is a human rare inherited recessive disease, predisposed to sunlight-induced skin cancer, which is caused by deficiency in DNA polymerase η (Polη). Polη catalyzes accurate translesion synthesis (TLS) past pyrimidine dimers, the most prominent UV-induced lesions. DNA polymerase ι (Polι) is a paralog of Polη that has been suggested to participate in TLS past UV-induced lesions, but its function in vivo remains uncertain. We have previously reported that Polη-deficient and Polη/Polι double-deficient mice showed increased susceptibility to UV-induced carcinogenesis. Here, we investigated UV-induced mutation frequencies and spectra in the epidermal cells of Polη- and/or Polι-deficient mice. While Polη-deficient mice showed significantly higher UV-induced mutation frequencies than wild-type mice, Polι deficiency did not influence the frequencies in the presence of Polη. Interestingly, the frequencies in Polη/Polι double-deficient mice were statistically lower than those in Polη-deficient mice, although they were still higher than those of wild-type mice. Sequence analysis revealed that most of the UV-induced mutations in Polη-deficient and Polη/Polι double-deficient mice were base substitutions at dipyrimidine sites. An increase in UV-induced mutations at both G:C and A:T pairs associated with Polη deficiency suggests that Polη contributes to accurate TLS past both thymine- and cytosine-containing dimers in vivo. A significant decrease in G:C to A:T transition in Polη/Polι double-deficient mice when compared with Polη-deficient mice suggests that Polι is involved in error-prone TLS past cytosine-containing dimers when Polη is inactivated.     

The role of DNA polymerase iota in UV mutational spectra

UVB (280-320 nm) and UVC (200-280 nm) irradiation generate predominantly cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts in DNA. CPDs are thought to be responsible for most of the UV-induced mutations. Thymine-thymine CPDs, and probably also CPDs containing cytosine, are replicated in vivo in a largely accurate manner by a DNA polymerase eta (Pol eta) dependent process. Pol eta is a DNA damage-tolerant and error-prone DNA polymerase encoded by the POLH (XPV) gene in humans. Another member of the Y family of error-prone DNA polymerases is POLI encoding DNA polymerase iota (Pol iota). In order to clarify the specific role of Pol iota in UV mutagenesis, we have used an siRNA knockdown approach in combination with a supF shuttle vector which replicates in mammalian cells, similar as we have previously done for Pol eta. Synthetic RNA duplexes were used to efficiently inhibit Pol iota expression in 293 T cells. The supF shuttle vector was irradiated with 254 nm UVC and replicated in 293 T cells in presence of anti-Pol iota siRNA. Surprisingly, there was a consistent reduction of recovered plasmid from cells with Pol iota knockdown and this was independent of UV irradiation of the plasmid. The supF mutant frequency was unchanged in the siRNA knockdown cells relative to control cells confirming that Pol iota does not play an important role in UV mutagenesis. UV-induced supF mutants were sequenced from siRNA-treated cells and controls. Neither the type of mutations nor their distribution along the supF gene were significantly different between controls and siRNA knockdown cells and were predominantly C to T and CC to TT transitions at dipyrimidine sites. These results show that Pol iota has no significant role in UV lesion bypass and mutagenesis in vivo and provides some initial data suggesting that this polymerase may be involved in replication of extrachromosomal DNA.     

Role of DNA polymerases eta, iota and zeta in UV resistance and UV-induced mutagenesis in a human cell line

Genes coding for DNA polymerases eta, iota and zeta, or for both Pol eta and Pol iota have been inactivated by homologous recombination in the Burkitt's lymphoma BL2 cell line, thus providing for the first time the total suppression of these enzymes in a human context. The UV sensitivities and UV-induced mutagenesis on an irradiated shuttle vector have been analyzed for these deficient cell lines. The double Pol eta/iota deficient cell line was more UV sensitive than the Pol eta-deficient cell line and mutation hotspots specific to the Pol eta-deficient context appeared to require the presence of Pol iota, thus strengthening the view that Pol iota is involved in UV damage translesion synthesis and UV-induced mutagenesis. A role for Pol zeta in a damage repair process at late replicative stages is reported, which may explain the drastic UV-sensitivity phenotype observed when this polymerase is absent. A specific mutation pattern was observed for the UV-irradiated shuttle vector transfected in Pol zeta-deficient cell lines, which, in contrast to mutagenesis at the HPRT locus previously reported, strikingly resembled mutations observed in UV-induced skin cancers in humans. Finally, a Pol eta PIP-box mutant (without its PCNA binding domain) could completely restore the UV resistance in a Pol eta deficient cell line, in the absence of UV-induced foci, suggesting, as observed for Pol iota in a Pol eta-deficient background, that TLS may occur without the accumulation of microscopically visible repair factories.     

Reevaluation of the role of DNA polymerase theta in somatic hypermutation of immunoglobulin genes

DNA polymerase theta has been implicated in the process of somatic hypermutation in immunoglobulin variable genes based on several reports of alterations in the frequency and spectra of mutations from Polq(-/-) mice. However, these studies have contrasting results on mutation frequencies and the types of nucleotide substitutions, which question the role of polymerase theta in hypermutation. DNA polymerase eta has a dominant effect on mutation and may substitute in the absence of polymerase theta to affect the pattern. Therefore, we have examined mutation in mice deficient for both polymerases theta and eta. The mutation frequencies in rearranged variable genes from Peyer's patches were similar in wild type, Polq(-/-), Polh(-/-), and Polq(-/-)Polh(-/-) mice. The types of substitutions were also similar between wild type and Polq(-/-) clones, and between Polh(-/-) and Polq(-/-)Polh(-/-) clones. Furthermore, there was no difference in heavy chain class switching in splenic B cells from the four groups of mice. These results indicate that polymerase theta does not play a significant role in the generation of somatic mutation in immunoglobulin genes.     

Proofreading of DNA polymerase eta-dependent replication errors

Human DNA polymerase eta, the product of the skin cancer susceptibility gene XPV, bypasses UV photoproducts in template DNA that block synthesis by other DNA polymerases. Pol eta lacks an intrinsic proofreading exonuclease and copies DNA with low fidelity, such that pol eta errors could contribute to mutagenesis unless they are corrected. Here we provide evidence that pol eta can compete with other human polymerases during replication of duplex DNA, and in so doing it lowers replication fidelity. However, we show that pol eta has low processivity and extends mismatched primer termini less efficiently than matched termini. These properties could provide an opportunity for extrinsic exonuclease(s) to proofread pol eta-induced replication errors. When we tested this hypothesis during replication in human cell extracts, pol eta-induced replication infidelity was found to be modulated by changing the dNTP concentration and to be enhanced by adding dGMP to a replication reaction. Both effects are classical hallmarks of exonucleolytic proofreading. Thus, pol eta is ideally suited for its role in reducing UV-induced mutagenesis and skin cancer risk, in that its relaxed base selectivity may facilitate efficient bypass of UV photoproducts, while subsequent proofreading by extrinsic exonuclease(s) may reduce its mutagenic potential.     

Two distinct translesion synthesis pathways across a lipid peroxidation-derived DNA adduct in mammalian cells

Translesion DNA synthesis (TLS) of damaged DNA templates is catalyzed by specialized DNA polymerases. To probe the cellular TLS mechanism, a host-vector system consisting of mouse fibroblasts and a replicating plasmid bearing a single DNA adduct was developed. This system was used to explore the TLS mechanism of a heptanone-etheno-dC (H-epsilondC) adduct, an endogenous lesion produced by lipid peroxidation. In wild-type cells, H-epsilondC almost exclusively directed incorporation of dT and dA. Whereas knockout of the Y family TLS polymerase genes, Polh, Polk, or Poli, did not qualitatively affect these TLS events, inactivation of the Rev3 gene coding for a subunit of polymerase zeta or of the Rev1 gene abolished TLS associated with dA, but not dT, insertion. The analysis of results of the cellular studies and in vitro TLS studies using purified polymerases has revealed that the insertion of dA and dT was catalyzed by different polymerases in cells. While insertion of dT can be catalyzed by polymerase eta, kappa, and iota, insertion of dA is catalyzed by an unidentified polymerase that cannot catalyze extension from the resulting dA terminus. Therefore, the extension from this terminus requires the activity of polymerase zeta-REV1. These results provide new insight into how cells use different TLS pathways to overcome a synthesis block.     

DNA聚合酶Polι的研究进展

Y家族DNA聚合酶是一种跨损伤复制酶,即能以损伤的DNA为模板进行复制。Y家族DNA聚合酶广泛分布生物界,人类细胞中Y家族DNA聚合酶至少包括Rev1、Polκ、Polι、Polη四种,Polι在以DNA为模板进行复制时错配率很高而不同于其他跨损伤DNA聚合酶,Polι是目前发现的所有DNA聚合酶中保真性最低的DNA聚合酶。很高的错配率导致很高的突变率,最后基因的突变导致癌症的发生,因此Polι在各个国家被广泛的研究,并且对Polι的各个不同的特性进行了研究,取得了一系列成果,现对Polι的研究进展予以综述,并展望了未来的研究趋势。     

Mouse Rev1 protein interacts with multiple DNA polymerases involved in translesion DNA synthesis

Pol kappa and Rev1 are members of the Y family of DNA polymerases involved in tolerance to DNA damage by replicative bypass [translesion DNA synthesis (TLS)]. We demonstrate that mouse Rev1 protein physically associates with Pol kappa. We show too that Rev1 interacts independently with Rev7 (a subunit of a TLS polymerase, Pol zeta) and with two other Y-family polymerases, Pol iota and Pol eta. Mouse Pol kappa, Rev7, Pol iota and Pol eta each bind to the same approximately 100 amino acid C-terminal region of Rev1. Furthermore, Rev7 competes directly with Pol kappa for binding to the Rev1 C-terminus. Notwithstanding the physical interaction between Rev1 and Pol kappa, the DNA polymerase activity of each measured by primer extension in vitro is unaffected by the complex, either when extending normal primer-termini, when bypassing a single thymine glycol lesion, or when extending certain mismatched primer termini. Our observations suggest that Rev1 plays a role(s) in mediating protein-protein interactions among DNA polymerases required for TLS. The precise function(s) of these interactions during TLS remains to be determined.     

DNA polymerase iota and related rad30-like enzymes

Until recently, the molecular mechanisms of translesion DNA synthesis (TLS), a process whereby a damaged base is used as a template for continued replication, was poorly understood. This area of scientific research has, however, been revolutionized by the finding that proteins long implicated in TLS are, in fact, DNA polymerases. Members of this so-called UmuC/DinB/Rev1/Rad30 superfamily of polymerases have been identified in prokaryotes, eukaryotes and archaea. Biochemical studies with the highly purified polymerases reveal that some, but not all, can traverse blocking lesions in template DNA. All of them share a common feature, however, in that they exhibit low fidelity when replicating undamaged DNA. Of particular interest to us is the Rad30 subfamily of polymerases found exclusively in eukaryotes. Humans possess two Rad30 paralogs, Rad30A and Rad30B. The RAD30A gene encodes DNA polymerase eta and defects in the protein lead to the xeroderma pigmentosum variant (XP-V) phenotype in humans. Very recently RAD30B has also been shown to encode a novel DNA polymerase, designated as Pol iota. Based upon in vitro studies, it appears that Pol iota has the lowest fidelity of any eukaryotic polymerase studied to date and we speculate as to the possible cellular functions of such a remarkably error-prone DNA polymerase.     

The role of DNA polymerase eta in UV mutational spectra

UV irradiation generates predominantly cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts in DNA. CPDs are thought to be responsible for most of the UV-induced mutations. Thymine-thymine CPDs, and probably also CPDs containing cytosine, are replicated in vivo in a largely accurate manner by a DNA polymerase eta (Pol eta) dependent process. Pol eta is encoded by the POLH (XPV) gene in humans. In order to clarify the specific role of Pol eta in UV mutagenesis, we have used an siRNA knockdown approach in combination with a supF shuttle vector which replicates in mammalian cells. This strategy provides an advantage over studying mutagenesis in cell lines derived from normal individuals and XP-V patients, since the genetic background of the cells is identical. Synthetic RNA duplexes were used to inhibit Pol eta expression in 293T cells. The reduction of Pol eta mRNA and protein was greater than 90%. The supF shuttle vector was irradiated with UVC and replicated in 293T cells in presence of anti-Pol eta siRNA. The supF mutant frequency was increased by up to 3.6-fold in the siRNA knockdown cells relative to control cells confirming that Pol eta plays an important role in mutation avoidance and that the pol eta knockdown was efficient. UV-induced supF mutants were sequenced from siRNA-treated cells and controls. Surprisingly, neither the type of mutations nor their distribution along the supF gene were substantially different between controls and siRNA knockdown cells and were predominantly C to T and CC to TT transitions at dipyrimidine sites. The data are compatible with two models. (i) Incorrect replication of cytosine-containing photoproducts by a polymerase other than Pol eta produces similar mutations as when Pol eta is present but at a higher frequency. (ii) Due to lack of Pol eta or low levels of remaining Pol eta, lesion replication is delayed allowing more time for cytosine deamination within CPDs to occur. We provide proof of principle that siRNA technology can be used to dissect the in vivo roles of lesion bypass DNA polymerases in DNA damage-induced mutagenesis.     

Evidence of finely tuned expression of DNA polymerase beta in vivo using transgenic mice

DNA polymerase (Pol) is an error-prone repair DNA polymerase that has been shown to create genetic instability and tumorigenesis when overexpressed by only 2-fold in cells, suggesting that a rigorous regulation of its expression may be essential in vivo. To address this question, we have generated mice which express a transgene (Tg) bearing the Pol cDNA under the control of the ubiquitous promoter of the mouse H-2K gene from the major histocompatibility complex. These mice express the Tg only in thymus, an organ which normally contains the most abundant endogenous Pol mRNA and protein, supporting the idea of a tight regulation of Pol in vivo. Furthermore, we found no tumor incidence, suggesting that the single Pol overexpression event is not sufficient to initiate tumorigenesis in vivo.     

Mouse DNA polymerase kappa has a functional role in the repair of DNA strand breaks

The Y-family of DNA polymerases support of translesion DNA synthesis (TLS) associated with stalled DNA replication by DNA damage. Recently, a number of studies suggest that some specialized TLS polymerases also support other aspects of DNA metabolism beyond TLS in vivo. Here we show that mouse polymerase kappa (Polκ) could accumulate at laser-induced sites of damage in vivo resembling polymerases eta and iota. The recruitment was mediated through Polκ C-terminus which contains the PCNA-interacting peptide, ubiquitin zinc finger motif 2 and nuclear localization signal. Interestingly, this recruitment was significantly reduced in MSH2-deficient LoVo cells and Rad18-depleted cells. We further observed that Polκ-deficient mouse embryo fibroblasts were abnormally sensitive to H₂O₂ treatment and displayed defects in both single-strand break repair and double-strand break repair. We speculate that Polκ may have an important role in strand break repair following oxidative stress in vivo.     

A false note of DNA polymerase iota in the choir of genome caretakers in mammals

DNA polymerase iota (Pol iota) of mammals is a member of the Y family of DNA polymerases. Among many other genome caretakers, these enzymes are responsible for maintaining genome stability. The members of the Y-family DNA polymerases take part in translesion DNA synthesis, bypassing some DNA lesions, and are characterized by low fidelity of DNA synthesis. A unique ability of Pol iota to predominantly incorporate G opposite T allowed us to identify the product of this enzyme among those synthesized by other DNA polymerases. This product can be called a "false note" of Pol iota. We measured the enzyme activity of Pol iota in crude extracts of cells from different organs of five inbred strains of mice (N3H/Sn, 101/H, C57BL/6, BALB/c, 129/J) that differed in a number of parameters. The "false note" of Pol iota was clearly sounding only in the extracts of testis and brain cells from four analyzed strains: N3H/Sn, 101/H, C57BL/6, BALB/c. In mice of 129/J strain that had a nonsense mutation in the second exon of the pol iota gene, the Pol iota activity was reliably detectable only in the extracts of brain. The data show that the active enzyme can be formed in some cell types even if they carry a nonsense mutation in the pol iota gene. This supports tissue-specific regulation of pol iota gene expression through alternative splicing. A semiquantitative determination of pol iota activity in mice strains different in their radiosensitivity suggests a reciprocal correlation between the enzyme activity of pol iota in testis and the resistance of mice to radiation.     

Sequence context-dependent replication of DNA templates containing UV-induced lesions by human DNA polymerase iota

Humans possess four Y-family polymerases: pols eta, iota, kappa and the Rev1 protein. The pivotal role that pol eta plays in protecting us from UV-induced skin cancers is unquestioned given that mutations in the POLH gene (encoding pol eta), lead to the sunlight-sensitive and cancer-prone xeroderma pigmentosum variant phenotype. The roles that pols iota, kappa and Rev1 play in the tolerance of UV-induced DNA damage is, however, much less clear. For example, in vitro studies in which the ability of pol iota to bypass UV-induced cyclobutane pyrimidine dimers (CPDs) or 6-4 pyrimidine-pyrimidone (6-4PP) lesions has been assayed, are somewhat varied with results ranging from limited misinsertion opposite CPDs to complete lesion bypass. We have tested the hypothesis that such discrepancies might have arisen from different assay conditions and local sequence contexts surrounding each UV-photoproduct and find that pol iota can facilitate significant levels of unassisted highly error-prone bypass of a T-T CPD, particularly when the lesion is located in a 3'-A[T-T]A-5' template sequence context and the reaction buffer contains no KCl. When encountering a T-T 6-4PP dimer under the same assay conditions, pol iota efficiently and accurately inserts the correct base, A, opposite the 3'T of the 6-4PP by factors of approximately 10(2) over the incorporation of incorrect nucleotides, while incorporation opposite the 5'T is highly mutagenic. Pol kappa has been proposed to function in the bypass of UV-induced lesions by helping extend primers terminated opposite CPDs. However, we find no evidence that the combined actions of pol iota and pol kappa result in a significant increase in bypass of T-T CPDs when compared to pol iota alone. Our data suggest that under certain conditions and sequence contexts, pol iota can bypass T-T CPDs unassisted and can efficiently incorporate one or more bases opposite a T-T 6-4PP. Such biochemical activities may, therefore, be of biological significance especially in XP-V cells lacking the primary T-T CPD bypassing enzyme, pol eta.     

Specificity of DNA lesion bypass by the yeast DNA polymerase eta

DNA polymerase eta (Pol(eta), xeroderma pigmentosum variant, or Rad30) plays an important role in an error-free response to unrepaired UV damage during replication. It faithfully synthesizes DNA opposite a thymine-thymine cis-syn-cyclobutane dimer. We have purified the yeast Pol(eta) and studied its lesion bypass activity in vitro with various types of DNA damage. The yeast Pol(eta) lacked a nuclease or a proofreading activity. It efficiently bypassed 8-oxoguanine, incorporating C, A, and G opposite the lesion with a relative efficiency of approximately 100:56:14, respectively. The yeast Pol(eta) efficiently incorporated a C opposite an acetylaminofluorene-modified G, and efficiently inserted a G or less frequently an A opposite an apurinic/apyrimidinic (AP) site but was unable to extend the DNA synthesis further in both cases. However, some continued DNA synthesis was observed in the presence of the yeast Pol(zeta) following the Pol(eta) action opposite an AP site, achieving true lesion bypass. In contrast, the yeast Pol(alpha) was able to bypass efficiently a template AP site, predominantly incorporating an A residue opposite the lesion. These results suggest that other than UV damage, Pol(eta) may also play a role in bypassing additional DNA lesions, some of which can be error-prone.     

Alternative splicing at exon 2 results in the loss of the catalytic activity of mouse DNA polymerase iota in vitro

Y-family DNA polymerase iota (Pol ι) possesses both DNA polymerase and dRP lyase activities and was suggested to be involved in DNA translesion synthesis and base excision repair in mammals. The 129 strain of mice and its derivatives have a natural nonsense codon mutation in the second exon of the Pol ι gene resulting in truncation of the Pol ι protein. These mice were widely used as a Pol ι-null model for in vivo studies of the Pol ι function. However whether 129-derived strains of mice are fully deficient in the Pol ι functions was a subject of discussion since Pol ι mRNA undergoes alternative splicing at exon 2. Here we report purification of mouse Pol ι lacking the region encoded by exon 2, which includes several conserved residues involved in catalysis. We show that the deletion abrogates both the DNA polymerase and dRP lyase activities of Pol ι in the presence of either Mg²⁺ or Mn²⁺ ions. Thus, 129-derived strains of mice express catalytically inactive alternatively spliced Pol ι variant, whose cellular functions, if any exist, remain to be established.     

Enhanced Activity of DNA Polymerase Iota in Mouse Brain Cells Is Associated with Aggressiveness

Recent studies performed with crude extracts of mouse tissues showed that the activity of DNA-polymerase iota (Pol iota) can be detected only in brain and testis extracts. To assess whether the activity of Pol iota is associated with animal behavior, we determined Pol iota activity in brain extracts of mice of two lines sharply differing in aggressiveness (RSB and RLB). We found that Pol iota activity in the mice with aggressive behavior was three times higher than in the less aggressive mice. The possible relationship between the activity of Pol iota and animal behavior is discussed.     

A single domain in human DNA polymerase iota mediates interaction with PCNA: implications for translesion DNA synthesis

DNA polymerases (Pols) of the Y family rescue stalled replication forks by promoting replication through DNA lesions. Humans have four Y family Pols, eta, iota, kappa, and Rev1, of which Pols eta, iota, and kappa have been shown to physically interact with proliferating cell nuclear antigen (PCNA) and be functionally stimulated by it. However, in sharp contrast to the large increase in processivity that PCNA binding imparts to the replicative Pol, Poldelta, the processivity of Y family Pols is not enhanced upon PCNA binding. Instead, PCNA binding improves the efficiency of nucleotide incorporation via a reduction in the apparent K(m) for the nucleotide. Here we show that Poliota interacts with PCNA via only one of its conserved PCNA binding motifs, regardless of whether PCNA is bound to DNA or not. The mode of PCNA binding by Poliota is quite unlike that in Poldelta, where multisite interactions with PCNA provide for a very tight binding of the replicating Pol with PCNA. We discuss the implications of these observations for the accuracy of DNA synthesis during translesion synthesis and for the process of Pol exchange at the lesion site.     

Preferential incorporation of G opposite template T by the low-fidelity human DNA polymerase iota

DNA polymerase activity is essential for replication, recombination, repair, and mutagenesis. All DNA polymerases studied so far from any biological source synthesize DNA by the Watson-Crick base-pairing rule, incorporating A, G, C, and T opposite the templates T, C, G, and A, respectively. Non-Watson-Crick base pairs would lead to mutations. In this report, we describe the ninth human DNA polymerase, Pol(iota), encoded by the RAD30B gene. We show that human Pol(iota) violates the Watson-Crick base-pairing rule opposite template T. During base selection, human Pol(iota) preferred T-G base pairing, leading to G incorporation opposite template T. The resulting T-G base pair was less efficiently extended by human Pol(iota) compared to the Watson-Crick base pairs. Consequently, DNA synthesis frequently aborted opposite template T, a property we designated the T stop. This T stop restricted human Pol(iota) to a very short stretch of DNA synthesis. Furthermore, kinetic analyses show that human Pol(iota) copies template C with extraordinarily low fidelity, misincorporating T, A, and C with unprecedented frequencies of 1/9, 1/10, and 1/11, respectively. Human Pol(iota) incorporated one nucleotide opposite a template abasic site more efficiently than opposite a template T, suggesting a role for human Pol(iota) in DNA lesion bypass. The unique features of preferential G incorporation opposite template T and T stop suggest that DNA Pol(iota) may additionally play a specialized function in human biology.