The mechanism of DNA repair by uracil-DNA glycosylase: studies using nucleotide analogues.

2',4'-Dideoxy-4'-methyleneuridine incorporated into oligodeoxynucleotides forms regular B-DNA duplexes as shown by Tm and CD measurements. Such oligomers are not cleaved by the DNA repair enzyme, UDG, which cleaves the glycosylic bond in dU but not in dT nor in dC nucleosides in single stranded and double stranded DNA. Differential binding of oligomers containing carbadU, 4'-thiodU, and dU residues to wild type and mutant UDG proteins identify an essential role for the furanose 4'-oxygen in recognition and cleavage of dU residues in DNA.

Crystal structure of a thwarted mismatch glycosylase DNA repair complex.

The bacterial mismatch-specific uracil-DNA glycosylase (MUG) and eukaryotic thymine-DNA glycosylase (TDG) enzymes form a homologous family of DNA glycosylases that initiate base-excision repair of G:U/T mismatches. Despite low sequence homology, the MUG/TDG enzymes are structurally related to the uracil-DNA glycosylase enzymes, but have a very different mechanism for substrate recognition. We have now determined the crystal structure of the Escherichia coli MUG enzyme complexed with an oligonucleotide containing a non-hydrolysable deoxyuridine analogue mismatched with guanine, providing the first structure of an intact substrate-nucleotide productively bound to a hydrolytic DNA glycosylase. The structure of this complex explains the preference for G:U over G:T mispairs, and reveals an essentially non-specific pyrimidine-binding pocket that allows MUG/TDG enzymes to excise the alkylated base, 3, N(4)-ethenocytosine. Together with structures for the free enzyme and for an abasic-DNA product complex, the MUG-substrate analogue complex reveals the conformational changes accompanying the catalytic cycle of substrate binding, base excision and product release.

Structure of a DNA base-excision product resembling a cisplatin inter-strand adduct.

Base-excision of a self-complementary oligonucleotide with central G:T mismatches by the G:T/U-specific mismatch DNA glycosylase (MUG), generates an unusual DNA structure which is remarkably similar in conformation to an interstrand DNA adduct of the anti-tumor drug cis-diamminedichloroplatinum. The abasic sugars generated by excision of the mismatched thymines are extruded from the double-helix, and the 'widowed' deoxyguanosines rotate so that their N7 and O6 groups protrude into the minor groove of the duplex and restack in an interleaved intercalative geometry, generating a kink in the helix axis.

Crystal structure of a G:T/U mismatch-specific DNA glycosylase: mismatch recognition by complementary-strand interactions.

G:U mismatches resulting from deamination of cytosine are the most common promutagenic lesions occurring in DNA. Uracil is removed in a base-excision repair pathway by uracil DNA-glycosylase (UDG), which excises uracil from both single- and double-stranded DNA. Recently, a biochemically distinct family of DNA repair enzymes has been identified, which excises both uracil and thymine, but only from mispairs with guanine. Crystal structures of the mismatch-specific uracil DNA-glycosylase (MUG) from E. coli, and of a DNA complex, reveal a remarkable structural and functional homology to UDGs despite low sequence identity. Details of the MUG structure explain its thymine DNA-glycosylase activity and the specificity for G:U/T mispairs, which derives from direct recognition of guanine on the complementary strand.

Direct measurement of the substrate preference of uracil-DNA glycosylase.

Site-directed mutants of the herpes simplex virus type 1 uracil-DNA glycosylase lacking catalytic activity have been used to probe the substrate recognition of this highly conserved and ubiquitous class of DNA-repair enzyme utilizing surface plasmon resonance. The residues aspartic acid-88 and histidine-210, implicated in the catalytic mechanism of the enzyme (Savva, R., McAuley-Hecht, K., Brown, T., and Pearl, L. (1995) Nature 373, 487-493; Slupphaug, G., Mol, C. D., Kavli, B., Arvai, A. S., Krokan, H. E. and Tainer, J. A. (1996) Nature 384, 87-92) were separately mutated to asparagine to allow investigations of substrate recognition in the absence of catalysis. The mutants were shown to be correctly folded and to lack catalytic activity. Binding to single- and double-stranded oligonucleotides, with or without uracil, was monitored by real-time biomolecular interaction analysis using surface plasmon resonance. Both mutants exhibited comparable rates of binding and dissociation on the same uracil-containing substrates. Interaction with single-stranded uracil-DNA was found to be stronger than with double-stranded uracil-DNA, and the binding to Gua:Ura mismatches was significantly stronger than that to Ade:Ura base pairs suggesting that the stability of the base pair determines the efficiency of interaction. Also, there was negligible interaction between the mutants and single- or double-stranded DNA lacking uracil, or with DNA containing abasic sites. These results suggest that it is uracil in the DNA, rather than DNA itself, that is recognized by the uracil-DNA glycosylases.

Uracil-DNA glycosylase activities in hyperthermophilic micro-organisms.

Hyperthermophiles exist in conditions which present an increased threat to the informational integrity of their DNA, particularly by hydrolytic damage. As in mesophilic organisms, specific activities must exist to restore and protect this template function of DNA. In this study we have demonstrated the presence of thermally stable uracil-DNA glycosylase activities in seven hyperthermophiles; one bacterial: Thermotoga maritima, and six archaeal: Sulfolobus solfataricus, Sulfolobus shibatae, Sulfolobus acidocaldarius, Thermococcus litoralis, Pyrococcus furiosus and Pyrobaculum islandicum. Uracil-DNA glycosylase inhibitor protein of the Bacillus subtilis bacteriophage PBS1 shows activity against all of these, suggesting a highly conserved tertiary structure between hyperthermophilic and mesophilic uracil-DNA glycosylases.

Nucleotide mimicry in the crystal structure of the uracil-DNA glycosylase-uracil glycosylase inhibitor protein complex.

The Bacillus subtilis bacteriophages PBS-1 and PBS-2 protect their uracil-containing DNA by expressing an inhibitor protein (UGI) which inactivates the host uracil-DNA glycosylase (UDGase) base-excision repair enzyme. Also, PBS1/2 UGI efficiently inactivates UDGases from other biological sources, including the enzyme from herpes simplex virus type-1 (HSV-1). The crystal structure of the HSV-1 UDGase-PBS1 UGI complex at 2.7 angstrum reveals an alpha-beta-alpha sandwich structure for UGI which interacts with conserved regions of UDGase involved in DNA binding, and directly mimics protein-DNA interactions observed in the UDGase-oligonucleotide complex. The inhibitor completely blocks access to the active site of UDGase, but makes no direct contact with the uracil-binding pocket itself.

Cloning and expression of the uracil-DNA glycosylase inhibitor (UGI) from bacteriophage PBS-1 and crystallization of a uracil-DNA glycosylase-UGI complex.

The uracil-DNA glycosylase inhibitory protein (UGI) from the bacteriophage PBS-1 has been cloned and overexpressed. The nucleotide sequence is identical to that for the previously described PBS-2 inhibitor. The recombinant PBS-1 UGI inhibits the uracil-DNA glycosylase from herpes simplex virus type-1 (HSV-1 UDGase), and a complex between the HSV-1 UDGase and PBS-1 UGI has been crystallized. The crystals have unit cell dimensions a = 143.21 A, c = 40.78 A and are in a polar hexagonal space group. There is a single complex in the asymmetric unit with a solvent content of 62% by volume and the crystals diffract to 2.5A on a synchrotron radiation source.

The structural basis of specific base-excision repair by uracil-DNA glycosylase.

The 1.75-A crystal structure of the uracil-DNA glycosylase from herpes simplex virus type-1 reveals a new fold, distantly related to dinucleotide-binding proteins. Complexes with a trideoxynucleotide, and with uracil, define the DNA-binding site and allow a detailed understanding of the exquisitely specific recognition of uracil in DNA. The overall structure suggests binding models for elongated single- and double-stranded DNA substrates. Conserved residues close to the uracil-binding site suggest a catalytic mechanism for hydrolytic base excision.

Crystallization and preliminary X-ray analysis of the uracil-DNA glycosylase DNA repair enzyme from herpes simplex virus type 1.

A 28.5 kDa catalytic fragment of the uracil-DNA glycosylase DNA repair enzyme from Herpes simplex virus type 1 (HSV-1) has been crystallized using protein from a highly expressing Escherichia coli clone of the Herpes simplex virus type 1 UL2 gene. The protein crystallizes at 12 mg/ml from 11% (w/v) polyethylene glycol 8000 at pH values in the range 6.8 to 7.0, in the presence of (NH4)2SO4. Long trigonal rods (0.08 mm x 0.08 mm x > 0.5 mm) diffract beyond 3.0 A using a laboratory source. The enzyme crystallizes in P3(1) (or P3(2)) a = 65.3 A, c = 49.0 A with a single molecule in the asymmetric unit and an estimated solvent content of 41% by volume.