Reduced sulfhydryls maintain specific incision of BPDE-DNA adducts by recombinant thermoresistant Bacillus caldotenax UvrABC endonuclease

The intrinsic ATPase activity of Mycobacterium tuberculosis UvrC is crucial for its damage-specific DNA incision function

To ensure genome stability, bacteria have evolved a network of DNA repair mechanisms; among them, the UvrABC-dependent nucleotide excision repair (NER) pathway is essential for the incision of a variety of bulky adducts generated by exogenous chemicals, UV radiation and by-products of cellular metabolism. However, very little is known about the enzymatic properties of Mycobacterium tuberculosis UvrABC excinuclease complex. Furthermore, the biochemical properties of Escherichia coli UvrC (EcUvrC) are not well understood (compared to UvrA and UvrB), perhaps due to its limited availability and/or activity instability in vitro. In addition, homology modelling of M. tuberculosis UvrC (MtUvrC) revealed the presence of a putative ATP-binding pocket, although its function remains unknown. To elucidate the biochemical properties of UvrC, we constructed and purified wild-type MtUvrC and its eight variants harbouring mutations within the ATP-binding pocket. The data from DNA-binding studies suggest that MtUvrC exhibits high-affinity for duplex DNA containing a bubble or fluorescein-dT moiety, over fluorescein-adducted single-stranded DNA. Most notably, MtUvrC has an intrinsic UvrB-independent ATPase activity, which drives dual incision of the damaged DNA strand. In contrast, EcUvrC is devoid of ATPase activity; however, it retains the ability to bind ATP at levels comparable to that of MtUvrC. The ATPase-deficient variants map to residues lining the MtUvrC ATP-binding pocket. Further analysis of these variants revealed separation of function between ATPase and DNA-binding activities in MtUvrC. Altogether, these findings reveal functional diversity of the bacterial NER machinery and a paradigm for the evolution of a catalytic scaffold in UvrC.

Removal of misincorporated ribonucleotides from prokaryotic genomes: an unexpected role for nucleotide excision repair

Stringent steric exclusion mechanisms limit the misincorporation of ribonucleotides by high-fidelity DNA polymerases into genomic DNA. In contrast, low-fidelity Escherichia coli DNA polymerase V (pol V) has relatively poor sugar discrimination and frequently misincorporates ribonucleotides. Substitution of a steric gate tyrosine residue with alanine (umuC_Y11A) reduces sugar selectivity further and allows pol V to readily misincorporate ribonucleotides as easily as deoxynucleotides, whilst leaving its poor base-substitution fidelity essentially unchanged. However, the mutability of cells expressing the steric gate pol V mutant is very low due to efficient repair mechanisms that are triggered by the misincorporated rNMPs. Comparison of the mutation frequency between strains expressing wild-type and mutant pol V therefore allows us to identify pathways specifically directed at ribonucleotide excision repair (RER). We previously demonstrated that rNMPs incorporated by umuC_Y11A are efficiently removed from DNA in a repair pathway initiated by RNase HII. Using the same approach, we show here that mismatch repair and base excision repair play minimal back-up roles in RER in vivo. In contrast, in the absence of functional RNase HII, umuC_Y11A-dependent mutagenesis increases significantly in ΔuvrA, uvrB5 and ΔuvrC strains, suggesting that rNMPs misincorporated into DNA are actively repaired by nucleotide excision repair (NER) in vivo. Participation of NER in RER was confirmed by reconstituting ribonucleotide-dependent NER in vitro. We show that UvrABC nuclease-catalyzed incisions are readily made on DNA templates containing one, two, or five rNMPs and that the reactions are stimulated by the presence of mispaired bases. Similar to NER of DNA lesions, excision of rNMPs proceeds through dual incisions made at the 8^th phosphodiester bond 5′ and 4^th–5^th phosphodiester bonds 3′ of the ribonucleotide. Ribonucleotides misinserted into DNA can therefore be added to the broad list of helix-distorting modifications that are substrates for NER.

Most DNA polymerases differentiate between ribo- and deoxyribonucleotides quite effectively, thereby deterring insertion of nucleotides with the “wrong” sugar into chromosomes. Nevertheless, a significant number of ribonucleotides still get stably incorporated into genomic DNA. E.coli pol V is among the most inaccurate DNA polymerases in terms of both sugar selectivity and base substitution fidelity. The umuC_Y11A steric gate variant of pol V is even less discriminating when selecting sugar of the incoming nucleotide while keeping a similar capacity to form non-Watson-Crick base pairs. In the present study, we describe mechanisms employed by E. coli to excise rNMPs from DNA and to concomitantly reduce the extent of spontaneous mutagenesis induced by umuC_Y11A. The first line of defense comes from Ribonuclease HII, which initiates the ribonucleotide excision repair pathway. In the absence of RNase HII, alternate repair pathways help remove the misincorporated ribonucleotides. Here, we present the first direct evidence that nucleotide excision repair (NER) has the capacity to recognize both correctly and incorrectly paired rNMPs embedded in DNA. The combined actions of RNase HII and NER thereby reduce the mutagenic potential of ribonucleotides errantly incorporated into prokaryotic genomes.

Role of human CYP1A1 and NAT2 in 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-induced mutagenicity and DNA adducts

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is carcinogenic in multiple organs and numerous species. Bioactivation of PhIP is initiated by PhIP N(2)-hydroxylation catalysed by cytochrome P450s. Following N-hydroxylation, O-acetylation catalysed by N-acetyltransferase 2 (NAT2) is considered a further possible activation pathway. Genetic polymorphisms in NAT2 may modify cancer risk following exposure. Nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells stably transfected with human cytochrome P4501A1 (CYP1A1) and a single copy of either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles were used to test the effect of CYP1A1 and NAT2 polymorphism on PhIP genotoxicity. Cells transfected with NAT2*4 had significantly higher levels of N-hydroxy-PhIP O-acetyltransferase (p = 0.0150) activity than cells transfected with NAT2*5B. Following PhIP treatment, CHO cell lines transfected with CYP1A1, CYP1A1/NAT2*4 and CYP1A1/NAT2*5B each showed concentration-dependent cytotoxicity and hypoxanthine phosphoribosyl transferase (hprt) mutagenesis not observed in untransfected CHO cells. dG-C8-PhIP was the primary DNA adduct formed and levels were dose dependent in transfected CHO cells in the order: CYP1A1 < CYP1A1 and NAT2*5B < CYP1A1 and NAT2*4, although levels did not differ significantly (p > 0.05) following one-way analysis of variance. These results strongly support activation of PhIP by CYP1A1 with little effect of human NAT2 genetic polymorphism on mutagenesis and DNA damage.

Sequence context- and temperature-dependent nucleotide excision repair of a benzo[a]pyrene diol epoxide-guanine DNA adduct catalyzed by thermophilic UvrABC proteins

The influence of DNA base sequence context on the removal of a bulky benzo[a]pyrene diol epoxide-guanine adduct, (+)-trans-B[a]P-N2-dG (G*), by UvrABC nuclease from the thermophilic organism Bacillus caldotenax was investigated. The lesion was flanked by either T or C in otherwise identical complementary 43-mer duplexes (TG*T or CG*C, respectively). It was reported earlier that in the CG*C context, a dominant minor groove adduct structure was observed by NMR methods with all Watson-Crick base pairs intact, and the duplex exhibited a rigid bend. In contrast, in the TG*T context, a highly flexible bend was observed, base pairing at G*, and two 5'-base pairs flanking the adduct were impaired, and multiple solvent-accessible adduct conformations were observed. The TG*T-43-mer duplexes are incised with consistently greater efficiency by UvrABC proteins from B. caldotenax by a factor of 2.3 +/- 0.3. The rates of incisions increase with increasing temperature and are characterized by linear Arrhenius plots with activation energies of 27.0 +/- 1.5 and 23.4 +/- 1.0 kcal/mol for CG*C and TG*T duplexes, respectively. These values reflect the thermophilic characteristics of the UVrABC nuclease complex and the contributions of the different DNA substrates to the overall activation energies. These effects are consistent with base sequence context-dependent differences in structural disorder engendered by a loss of local base stacking interactions and Watson-Crick base pairing in the immediate vicinity of the lesions in the TG*T duplexes. The local weakening of base pairing interactions constitutes a recognition element of the UvrABC nucleotide excision repair apparatus.

Robust incision of Benoz[a]pyrene-7,8-dihyrodiol-9,10-epoxide-DNA adducts by a recombinant thermoresistant interspecies combination UvrABC endonuclease system

Prokaryotic DNA repair nucleases are useful reagents for detecting DNA lesions. UvrABC endonuclease, encoded by the UvrA, UvrB, and UvrC genes can incise DNA containing bulky nucleotide adducts and intrastrand cross-links. UvrA, UvrB, and UvrC were cloned from Bacillus caldotenax (Bca)and UvrC from Thermatoga maritima (Tma), and recombinant proteins were overexpressed in and purified from Escherichia coli. Incision activities of UvrABC composed of all Bca-derived subunits (UvrABC(Bca)) and an interspecies combination UvrABC composed of Bca-derived UvrA and UvrB and Tma-derived UvrC (UvrABC(Tma)) were compared on benoz[a]pyrene-7,8-dihyrodiol-9,10-epoxide (BPDE)-adducted substrates. Both UvrABC(Bca) and UvrABC(Tma) specifically incised both BPDE-adducted plasmid DNAs and site-specifically modified 50-bp oligonucleotides containing a single (+)-trans- or (+)-cis-BPDE adduct. Incision activity was maximal at 55-60 degrees C. However, UvrABC(Tma) was more robust than UvrABC(Bca) with 4-fold greater incision activity on BPDE-adducted oligonucleotides and 1.5-fold greater on [(3)H]BPDE-adducted plasmid DNAs. Remarkably, UvrABC(Bca) incised only at the eighth phosphodiester bond 5' to the BPDE-modified guanosine. In contrast, UvrABC(Tma) performed dual incision, cutting at both the fifth phosphodiester bond 3' and eighth phosphodiester bond 5' from BPDE-modified guanosine. BPDE adduct stereochemistry influenced incision activity, and cis adducts on oligonucleotide substrates were incised more efficiently than trans adducts by both UvrABC(Bca) and UvrABC(Tma). UvrAB-DNA complex formation was similar with (+)-trans- and (+)-cis-BPDE-adducted substrates, suggesting that UvrAB binds both adducts equally and that adduct configuration modifies UvrC recognition of the UvrAB-DNA complex. The dual incision capabilities and higher incision activity of UvrABC(Tma) make it a robust tool for DNA adduct studies.

Incision of trivalent chromium [Cr(III)]-induced DNA damage by Bacillus caldotenax UvrABC endonuclease

Some hexavalent chromium [Cr(VI)]-containing compounds are lung carcinogens. Once within cells, Cr(VI) is reduced to trivalent chromium [Cr(III)] which displays an affinity for both DNA bases and the phosphate backbone. A diverse array of genetic lesions is produced by Cr including Cr-DNA monoadducts, DNA interstrand crosslinks (ICLs), DNA-Cr-protein crosslinks (DPCs), abasic sites, DNA strand breaks and oxidized bases. Despite the large amount of information available on the genotoxicity of Cr, little is known regarding the molecular mechanisms involved in the removal of these lesions from damaged DNA. Recent work indicates that nucleotide excision repair (NER) is involved in the processing of Cr-DNA adducts in human and rodent cells. In order to better understand this process at the molecular level and begin to identify the Cr-DNA adducts processed by NER, the incision of CrCl(3) [Cr(III)]-damaged plasmid DNA was studied using a thermal-resistant UvrABC NER endonuclease from Bacillus caldotenax (Bca). Treatment of plasmid DNA with Cr(III) (as CrCl(3)) increased DNA binding as a function of dose. For example, at a Cr(III) concentration of 1 microM we observed approximately 2 Cr(III)-DNA adducts per plasmid. At this same concentration of Cr(III) we found that approximately 17% of the plasmid DNA contained ICLs ( approximately 0.2 ICLs/plasmid). When plasmid DNA treated with Cr(III) (1 microM) was incubated with Bca UvrABC we observed approximately 0.8 incisions/plasmid. The formation of endonuclease IV-sensitive abasic lesions or Fpg-sensitive oxidized DNA bases was not detected suggesting that the incision of Cr(III)-damaged plasmid DNA by UvrABC was not related to the generation of oxidized DNA damage. Taken together, our data suggest that a sub-fraction of Cr(III)-DNA adducts is recognized and processed by the prokaryotic NER machinery and that ICLs are not necessarily the sole lesions generated by Cr(III) that are substrates for NER.

Purification and characterization of Escherichia coli and human nucleotide excision repair enzyme systems

Nucleotide excision repair is a multicomponent, multistep enzymatic system that removes a wide spectrum of DNA damage by dual incisions in the damaged strand on both sides of the lesion. The basic steps are damage recognition, dual incisions, resynthesis to replace the excised DNA, and ligation. Each step has been studied in vitro using cell extracts or highly purified repair factors and radiolabeled DNA of known sequence with DNA damage at a defined site. This chapter describes procedures for preparation of DNA substrates designed for analysis of damage recognition, either the 5' or the 3' incision event, excision (resulting from concerted dual incisions), and repair synthesis. Excision in Escherichia coli is accomplished by the three-subunit Uvr(A)BC excision nuclease and in humans by six repair factors: XPA, RPA, XPChR23B, TFIIH, XPFERCC1, and XPG. This chapter outlines methods for expression and purification of these essential repair factors and provides protocols for performing each of the in vitro repair assays with either the E. coli or the human excision nuclease.