Novel mechanisms for the removal of strong replication-blocking HMCES- and thiazolidine-DNA adducts in humans.

Apurinic/apyrimidinic (AP) sites are DNA lesions created under normal growth conditions that result in cytotoxicity, replication-blocks, and mutations. AP sites are susceptible to ß-elimination and are liable to be converted to DNA strand breaks. HMCES (5-hydroxymethylcytosine binding, ES cell specific) protein interacts with AP sites in single stranded (ss) DNA exposed at DNA replication forks to generate a stable thiazolidine protein-DNA crosslink and protect cells against AP site toxicity. The crosslinked HMCES is resolved by proteasome-mediated degradation; however, it is unclear how HMCES-crosslinked ssDNA and the resulting proteasome-degraded HMCES adducts are processed and repaired. Here, we describe methods for the preparation of thiazolidine adduct-containing oligonucleotides and determination of their structure. We demonstrate that the HMCES-crosslink is a strong replication blocking adduct and that protease-digested HMCES adducts block DNA replication to a similar extent as AP sites. Moreover, we show that the human AP endonuclease APE1 incises DNA 5' to the protease-digested HMCES adduct. Interestingly, while HMCES-ssDNA crosslinks are stable, the crosslink is reversed upon the formation of dsDNA, possibly due to a catalytic reverse reaction. Our results shed new light on damage tolerance and repair pathways for HMCES-DNA crosslinks in human cells.

RFWD3 and translesion DNA polymerases contribute to PCNA modification-dependent DNA damage tolerance.

DNA damage tolerance pathways are regulated by proliferating cell nuclear antigen (PCNA) modifications at lysine 164. Translesion DNA synthesis by DNA polymerase η (Polη) is well studied, but less is known about Polη-independent mechanisms. Illudin S and its derivatives induce alkyl DNA adducts, which are repaired by transcription-coupled nucleotide excision repair (TC-NER). We demonstrate that in addition to TC-NER, PCNA modification at K164 plays an essential role in cellular resistance to these compounds by overcoming replication blockages, with no requirement for Polη. Polκ and RING finger and WD repeat domain 3 (RFWD3) contribute to tolerance, and are both dependent on PCNA modifications. Although RFWD3 is a FANC protein, we demonstrate that it plays a role in DNA damage tolerance independent of the FANC pathway. Finally, we demonstrate that RFWD3-mediated cellular survival after UV irradiation is dependent on PCNA modifications but is independent of Polη. Thus, RFWD3 contributes to PCNA modification-dependent DNA damage tolerance in addition to translesion DNA polymerases.

Acetaldehyde induces NER repairable mutagenic DNA lesions.

Nucleotide excision repair (NER) is a repair mechanism that removes DNA lesions induced by UV radiation, environmental mutagens and carcinogens. There exists sufficient evidence against acetaldehyde suggesting it to cause a variety of DNA lesions and be carcinogenic to humans. Previously, we found that acetaldehyde induces reversible intra-strand GG crosslinks in DNA similar to those induced by cis-diammineplatinum(II) that is subsequently repaired by NER. In this study, we analysed the repairability by NER mechanism and the mutagenesis of acetaldehyde. In an in vitro reaction setup with NER-proficient and NER-deficient xeroderma pigmentosum group A (XPA) cell extracts, NER reactions were observed in the presence of XPA recombinant proteins in acetaldehyde-treated plasmids. Using an in vivo assay with living XPA cells and XPA-correcting XPA cells, the repair reactions were also observed. Additionally, it was observed that DNA polymerase eta inserted dATP opposite guanine in acetaldehyde-treated oligonucleotides, suggesting that acetaldehyde-induced GG-to-TT transversions. These findings show that acetaldehyde induces NER repairable mutagenic DNA lesions.

Spatiotemporal regulation of PCNA ubiquitination in damage tolerance pathways.

DNA is constantly exposed to a wide variety of exogenous and endogenous agents, and most DNA lesions inhibit DNA synthesis. To cope with such problems during replication, cells have molecular mechanisms to resume DNA synthesis in the presence of DNA lesions, which are known as DNA damage tolerance (DDT) pathways. The concept of ubiquitination-mediated regulation of DDT pathways in eukaryotes was established via genetic studies in the yeast Saccharomyces cerevisiae, in which two branches of the DDT pathway are regulated via ubiquitination of proliferating cell nuclear antigen (PCNA): translesion DNA synthesis (TLS) and homology-dependent repair (HDR), which are stimulated by mono- and polyubiquitination of PCNA, respectively. Over the subsequent nearly two decades, significant progress has been made in understanding the mechanisms that regulate DDT pathways in other eukaryotes. Importantly, TLS is intrinsically error-prone because of the miscoding nature of most damaged nucleotides and inaccurate replication of undamaged templates by TLS polymerases (pols), whereas HDR is theoretically error-free because the DNA synthesis is thought to be predominantly performed by pol δ, an accurate replicative DNA pol, using the undamaged sister chromatid as its template. Thus, the regulation of the choice between the TLS and HDR pathways is critical to determine the appropriate biological outcomes caused by DNA damage. In this review, we summarize our current understanding of the species-specific regulatory mechanisms of PCNA ubiquitination and how cells choose between TLS and HDR. We then provide a hypothetical model for the spatiotemporal regulation of DDT pathways in human cells.

Stepwise multipolyubiquitination of p53 by the E6AP-E6 ubiquitin ligase complex.

The human papillomavirus (HPV) oncoprotein E6 specifically binds to E6AP (E6-associated protein), a HECT (homologous to the E6AP C terminus)-type ubiquitin ligase, and directs its ligase activity toward the tumor suppressor p53. To examine the biochemical reaction in vitro, we established an efficient reconstitution system for the polyubiquitination of p53 by the E6AP-E6 complex. We demonstrate that E6AP-E6 formed a stable ternary complex with p53, which underwent extensive polyubiquitination when the isolated ternary complex was incubated with E1, E2, and ubiquitin. Mass spectrometry and biochemical analysis of the reaction products identified lysine residues as p53 ubiquitination sites. A p53 mutant with arginine substitutions of its 18 lysine residues was not ubiquitinated. Analysis of additional p53 mutants retaining only one or two intact ubiquitination sites revealed that chain elongation at each of these sites was limited to 5-6-mers. We also determined the size distribution of ubiquitin chains released by en bloc cleavage from polyubiquitinated p53 to be 2-6-mers. Taken together, these results strongly suggest that p53 is multipolyubiquitinated with short chains by E6AP-E6. In addition, analysis of growing chains provided strong evidence for step-by-step chain elongation. Thus, we hypothesize that p53 is polyubiquitinated in a stepwise manner through the back-and-forth movement of the C-lobe, and the permissive distance for the movement of the C-lobe restricts the length of the chains in the E6AP-E6-p53 ternary complex. Finally, we show that multipolyubiquitination at different sites provides a signal for proteasomal degradation.

Preferential digestion of PCNA-ubiquitin and p53-ubiquitin linkages by USP7 to remove polyubiquitin chains from substrates.

Ubiquitin-specific protease 7 (USP7) regulates various cellular pathways through its deubiquitination activity. Despite the identification of a growing number of substrates of USP7, the molecular mechanism by which USP7 removes ubiquitin chains from polyubiquitinated substrates remains unexplored. The present study investigated the mechanism underlying the deubiquitination of Lys63-linked polyubiquitinated proliferating cell nuclear antigen (PCNA). Biochemical analyses demonstrated that USP7 efficiently removes polyubiquitin chains from polyubiquitinated PCNA by preferential cleavage of the PCNA-ubiquitin linkage. This property was largely attributed to the poor activity toward Lys63-linked ubiquitin chains. The preferential cleavage of substrate-ubiquitin linkages was also observed for Lys48-linked polyubiquitinated p53 because of the inefficient cleavage of the Lys48-linked ubiquitin chains. The present findings suggest a mechanism underlying the removal of polyubiquitin signals by USP7.

Regulation of HLTF-mediated PCNA polyubiquitination by RFC and PCNA monoubiquitination levels determines choice of damage tolerance pathway.

DNA-damage tolerance protects cells via at least two sub-pathways regulated by proliferating cell nuclear antigen (PCNA) ubiquitination in eukaryotes: translesion DNA synthesis (TLS) and template switching (TS), which are stimulated by mono- and polyubiquitination, respectively. However, how cells choose between the two pathways remains unclear. The regulation of ubiquitin ligases catalyzing polyubiquitination, such as helicase-like transcription factor (HLTF), could play a role in the choice of pathway. Here, we demonstrate that the ligase activity of HLTF is stimulated by double-stranded DNA via HIRAN domain-dependent recruitment to stalled primer ends. Replication factor C (RFC) and PCNA located at primer ends, however, suppress en bloc polyubiquitination in the complex, redirecting toward sequential chain elongation. When PCNA in the complex is monoubiquitinated by RAD6-RAD18, the resulting ubiquitin moiety is immediately polyubiquitinated by coexisting HLTF, indicating a coupling reaction between mono- and polyubiquitination. By contrast, when PCNA was monoubiquitinated in the absence of HLTF, it was not polyubiquitinated by subsequently recruited HLTF unless all three-subunits of PCNA were monoubiquitinated, indicating that the uncoupling reaction specifically occurs on three-subunit-monoubiquitinated PCNA. We discuss the physiological relevance of the different modes of the polyubiquitination to the choice of cells between TLS and TS under different conditions.

Regulation of DNA damage tolerance in mammalian cells by post-translational modifications of PCNA.

DNA damage tolerance pathways, which include translesion DNA synthesis (TLS) and template switching, are crucial for prevention of DNA replication arrest and maintenance of genomic stability. However, these pathways utilize error-prone DNA polymerases or template exchange between sister DNA strands, and consequently have the potential to induce mutations or chromosomal rearrangements. Post-translational modifications of proliferating cell nuclear antigen (PCNA) play important roles in controlling these pathways. For example, TLS is mediated by mono-ubiquitination of PCNA at lysine 164, for which RAD6-RAD18 is the primary E2-E3 complex. Elaborate protein-protein interactions between mono-ubiquitinated PCNA and Y-family DNA polymerases constitute the core of the TLS regulatory system, and enhancers of PCNA mono-ubiquitination and de-ubiquitinating enzymes finely regulate TLS and suppress TLS-mediated mutagenesis. The template switching pathway is promoted by K63-linked poly-ubiquitination of PCNA at lysine 164. Poly-ubiquitination is achieved by a coupled reaction mediated by two sets of E2-E3 complexes, RAD6-RAD18 and MMS2-UBC13-HTLF/SHPRH. In addition to these mono- and poly-ubiquitinations, simultaneous mono-ubiquitinations on multiple units of the PCNA homotrimeric ring promote an unidentified damage tolerance mechanism that remains to be fully characterized. Furthermore, SUMOylation of PCNA in mammalian cells can negatively regulate recombination. Other modifications, including ISGylation, acetylation, methylation, or phosphorylation, may also play roles in DNA damage tolerance and control of genomic stability.

USP7 Is a Suppressor of PCNA Ubiquitination and Oxidative-Stress-Induced Mutagenesis in Human Cells.

Mono-ubiquitinated PCNA activates error-prone DNA polymerases; therefore, strict regulation of PCNA mono-ubiquitination is crucial in avoiding undesired mutagenesis. In this study, we used an in vitro assay system to identify USP7 as a deubiquitinating enzyme of mono-ubiquitinated PCNA. Suppression of USP1, a previously identified PCNA deubiquitinase, or USP7 increased UV- and H2O2-induced PCNA mono-ubiquitination in a distinct and additive manner, suggesting that USP1 and USP7 make different contributions to PCNA deubiquitination in human cells. Cell-cycle-synchronization analyses revealed that USP7 suppression increased H2O2-induced PCNA ubiquitination throughout interphase, whereas USP1 suppression specifically increased ubiquitination in S-phase cells. UV-induced mutagenesis was elevated in USP1-suppressed cells, whereas H2O2-induced mutagenesis was elevated in USP7-suppressed cells. These results suggest that USP1 suppresses UV-induced mutations produced in a manner involving DNA replication, whereas USP7 suppresses H2O2-induced mutagenesis involving cell-cycle-independent processes such as DNA repair.

Different types of interaction between PCNA and PIP boxes contribute to distinct cellular functions of Y-family DNA polymerases.

Translesion DNA synthesis (TLS) by the Y-family DNA polymerases Polη, Polι and Polκ, mediated via interaction with proliferating cell nuclear antigen (PCNA), is a crucial pathway that protects human cells against DNA damage. We report that Polη has three PCNA-interacting protein (PIP) boxes (PIP1, 2, 3) that contribute differentially to two distinct functions, stimulation of DNA synthesis and promotion of PCNA ubiquitination. The latter function is strongly associated with formation of nuclear Polη foci, which co-localize with PCNA. We also show that Polκ has two functionally distinct PIP boxes, like Polη, whereas Polι has a single PIP box involved in stimulation of DNA synthesis. All three polymerases were additionally stimulated by mono-ubiquitinated PCNA in vitro. The three PIP boxes and a ubiquitin-binding zinc-finger of Polη exert redundant and additive effects in vivo via distinct molecular mechanisms. These findings provide an integrated picture of the orchestration of TLS polymerases.

The BAH domain of BAF180 is required for PCNA ubiquitination.

Monoubiquitination of proliferating cell nuclear antigen (PCNA) is a critical regulator of post replication repair (PRR). The depletion of BAF180, a unique subunit of the PBAF chromatin remodeling complex in human cells results in reduced PCNA ubiquitination leading to less efficient fork progression following DNA damage, but little is known about the mechanism. Here, we report that the expression of exogenous BAF180 in cells promotes PCNA ubiquitination during S-phase after UV irradiation and it persists for many hours. No correlation was observed between the protein level of ubiquitin-specific protease 1 (USP1) and ubiquitinated PCNA in BAF180 expressing cells. Analysis of cells expressing BAF180 deletion mutants showed that the bromo-adjacent homology (BAH) domains are responsible for this effect. Surprisingly, a deletion construct encoding only the BAH domain region is able to increase the level of ubiquitinated PCNA, even though it is unable to be assembled into the PBAF complex. These results suggest that the ATPase-dependent chromatin remodeling activity of PBAF is not necessary, but instead the BAH domains are sufficient to promote PCNA ubiquitination.

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.

Relevance of simultaneous mono-ubiquitinations of multiple units of PCNA homo-trimers in DNA damage tolerance.

DNA damage tolerance (DDT) pathways, including translesion synthesis (TLS) and additional unknown mechanisms, enable recovery from replication arrest at DNA lesions. DDT pathways are regulated by post-translational modifications of proliferating cell nuclear antigen (PCNA) at its K164 residue. In particular, mono-ubiquitination by the ubiquitin ligase RAD18 is crucial for Polη-mediated TLS. Although the importance of modifications of PCNA to DDT pathways is well known, the relevance of its homo-trimer form, in which three K164 residues are present in a single ring, remains to be elucidated. Here, we show that multiple units of a PCNA homo-trimer are simultaneously mono-ubiquitinated in vitro and in vivo. RAD18 catalyzed sequential mono-ubiquitinations of multiple units of a PCNA homo-trimer in a reconstituted system. Exogenous PCNA formed hetero-trimers with endogenous PCNA in WI38VA13 cell transformants. When K164R-mutated PCNA was expressed in these cells at levels that depleted endogenous PCNA homo-trimers, multiple modifications of PCNA complexes were reduced and the cells showed defects in DDT after UV irradiation. Notably, ectopic expression of mutant PCNA increased the UV sensitivities of Polη-proficient, Polη-deficient, and REV1-depleted cells, suggesting the disruption of a DDT pathway distinct from the Polη- and REV1-mediated pathways. These results suggest that simultaneous modifications of multiple units of a PCNA homo-trimer are required for a certain DDT pathway in human cells.

Guanine- 5-carboxylcytosine base pairs mimic mismatches during DNA replication.

The genetic information encoded in genomes must be faithfully replicated and transmitted to daughter cells. The recent discovery of consecutive DNA conversions by TET family proteins of 5-methylcytosine into 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine (5caC) suggests these modified cytosines act as DNA lesions, which could threaten genome integrity. Here, we have shown that although 5caC pairs with guanine during DNA replication in vitro, G·5caC pairs stimulated DNA polymerase exonuclease activity and were recognized by the mismatch repair (MMR) proteins. Knockdown of thymine DNA glycosylase increased 5caC in genome, affected cell proliferation via MMR, indicating MMR is a novel reader for 5caC. These results suggest the epigenetic modification products of 5caC behave as DNA lesions.

Sphingosine, a modulator of human translesion DNA polymerase activity.

Translesion (TLS) DNA polymerases are specialized, error-prone enzymes that synthesize DNA across bulky, replication-stalling DNA adducts. In so doing, they facilitate the progression of DNA synthesis and promote cell proliferation. To potentiate the effect of cancer chemotherapeutic regimens, we sought to identify inhibitors of TLS DNA polymerases. We screened five libraries of ∼ 3000 small molecules, including one comprising ∼ 600 nucleoside analogs, for their effect on primer extension activity of DNA polymerase η (Pol η). We serendipitously identified sphingosine, a lipid-signaling molecule that robustly stimulates the activity of Pol η by ∼ 100-fold at low micromolar concentrations but inhibits it at higher concentrations. This effect is specific to the Y-family DNA polymerases, Pols η, κ, and ι. The addition of a single phosphate group on sphingosine completely abrogates this effect. Likewise, the inclusion of other sphingolipids, including ceramide and sphingomyelin to extension reactions does not elicit this response. Sphingosine increases the rate of correct and incorrect nucleotide incorporation while having no effect on polymerase processivity. Endogenous Pol η activity is modulated similarly as the recombinant enzyme. Importantly, sphingosine-treated cells exhibit increased lesion bypass activity, and sphingosine tethered to membrane lipids mimics the effects of free sphingosine. Our studies have uncovered sphingosine as a modulator of TLS DNA polymerase activity; this property of sphingosine may be associated with its known role as a signaling molecule in regulating cell proliferation in response to cellular stress.

A cyclobutane thymine-N4-methylcytosine dimer is resistant to hydrolysis but strongly blocks DNA synthesis.

Exposure of DNA to ultraviolet light produces harmful crosslinks between adjacent pyrimidine bases, to form cyclobutane pyrimidine dimers (CPDs) and pyrimidine(6-4)pyrimidone photoproducts. The CPD is frequently formed, and its repair mechanisms have been exclusively studied by using a CPD formed at a TT site. On the other hand, biochemical analyses using CPDs formed within cytosine-containing sequence contexts are practically difficult, because saturated cytosine easily undergoes hydrolytic deamination. Here, we found that N-alkylation of the exocyclic amino group of 2'-deoxycytidine prevents hydrolysis in CPD formation, and an N-methylated cytosine-containing CPD was stable enough to be derivatized into its phosphoramidite building block and incorporated into oligonucleotides. Kinetic studies of the CPD-containing oligonucleotide indicated that its lifetime under physiological conditions is relatively long (∼ 7 days). In biochemical analyses using human DNA polymerase η, incorporation of TMP opposite the N-methylcytosine moiety of the CPD was clearly detected, in addition to dGMP incorporation, and the incorrect TMP incorporation blocked DNA synthesis. The thermodynamic parameters confirmed the formation of this unusual base pair.

En bloc transfer of polyubiquitin chains to PCNA in vitro is mediated by two different human E2-E3 pairs.

Post-replication DNA repair in eukaryotes is regulated by ubiquitination of proliferating cell nuclear antigen (PCNA). Monoubiquitination catalyzed by RAD6-RAD18 (an E2-E3 complex) stimulates translesion DNA synthesis, whereas polyubiquitination, promoted by additional factors such as MMS2-UBC13 (a UEV-E2 complex) and HLTF (an E3 ligase), leads to template switching in humans. Here, using an in vitro ubiquitination reaction system reconstituted with purified human proteins, we demonstrated that PCNA is polyubiquitinated predominantly via en bloc transfer of a pre-formed ubiquitin (Ub) chain rather than by extension of the Ub chain on monoubiquitinated PCNA. Our results support a model in which HLTF forms a thiol-linked Ub chain on UBC13 (UBC13∼Ubn) and then transfers the chain to RAD6∼Ub, forming RAD6∼Ubn+1. The resultant Ub chain is subsequently transferred to PCNA by RAD18. Thus, template switching may be promoted under certain circumstances in which both RAD18 and HLTF are coordinately recruited to sites of stalled replication.

NBS1 recruits RAD18 via a RAD6-like domain and regulates Pol η-dependent translesion DNA synthesis.

Translesion DNA synthesis, a process orchestrated by monoubiquitinated PCNA, is critical for DNA damage tolerance. While the ubiquitin-conjugating enzyme RAD6 and ubiquitin ligase RAD18 are known to monoubiquitinate PCNA, how they are regulated by DNA damage is not fully understood. We show that NBS1 (mutated in Nijmegen breakage syndrome) binds to RAD18 after UV irradiation and mediates the recruitment of RAD18 to sites of DNA damage. Disruption of NBS1 abolished RAD18-dependent PCNA ubiquitination and Polη focus formation, leading to elevated UV sensitivity and mutation. Unexpectedly, the RAD18-interacting domain of NBS1, which was mapped to its C terminus, shares structural and functional similarity with the RAD18-interacting domain of RAD6. These domains of NBS1 and RAD6 allow the two proteins to interact with RAD18 homodimers simultaneously and are crucial for Polη-dependent UV tolerance. Thus, in addition to chromosomal break repair, NBS1 plays a key role in translesion DNA synthesis.

Molecular chaperone Hsp90 regulates REV1-mediated mutagenesis.

REV1 is a Y-family polymerase that plays a central role in mutagenic translesion DNA synthesis (TLS), contributing to tumor initiation and progression. In a current model, a monoubiquitinated form of the replication accessory protein, proliferating cell nuclear antigen (PCNA), serves as a platform to recruit REV1 to damaged sites on the DNA template. Emerging evidence indicates that posttranslational mechanisms regulate REV1 in yeast; however, the regulation of REV1 in higher eukaryotes is poorly understood. Here we show that the molecular chaperone Hsp90 is a critical regulator of REV1 in human cells. Hsp90 specifically binds REV1 in vivo and in vitro. Treatment with a specific inhibitor of Hsp90 reduces REV1 protein levels in several cell types through proteasomal degradation. This is associated with suppression of UV-induced mutagenesis. Furthermore, Hsp90 inhibition disrupts the interaction between REV1 and monoubiquitinated PCNA and suppresses UV-induced focus formation. These results indicate that Hsp90 promotes folding of REV1 into a stable and/or functional form(s) to bind to monoubiquitinated PCNA. The present findings reveal a novel role of Hsp90 in the regulation of TLS-mediated mutagenesis.

Photosensitized [2 + 2] cycloaddition of N-acetylated cytosine affords stereoselective formation of cyclobutane pyrimidine dimer.

Photocycloaddition between two adjacent bases in DNA produces a cyclobutane pyrimidine dimer (CPD), which is one of the major UV-induced DNA lesions, with either the cis-syn or trans-syn structure. In this study, we investigated the photosensitized intramolecular cycloaddition of partially-protected thymidylyl-(3'→5')-N(4)-acetyl-2'-deoxy-5-methylcytidine, to clarify the effect of the base modification on the cycloaddition reaction. The reaction resulted in the stereoselective formation of the trans-syn CPD, followed by hydrolysis of the acetylamino group. The same result was obtained for the photocycloaddition of thymidylyl-(3'→5')-N(4)-acetyl-2'-deoxycytidine, whereas both the cis-syn and trans-syn CPDs were formed from thymidylyl-(3'→5')-thymidine. Kinetic analyses revealed that the activation energy of the acid-catalyzed hydrolysis is comparable to that reported for the thymine-cytosine CPD. These findings provided a new strategy for the synthesis of oligonucleotides containing the trans-syn CPD. Using the synthesized oligonucleotide, translesion synthesis by human DNA polymerase η was analyzed.