Identification of a new uracil-DNA glycosylase family by expression cloning using synthetic inhibitors.
BACKGROUND: The cellular environment exposes DNA to a wide variety of endogenous and exogenous reactive species that can damage DNA, thereby leading to genetic mutations. DNA glycosylases protect the integrity of the genome by catalyzing the first step in the base excision-repair of lesions in DNA. RESULTS: Here, we report a strategy to conduct genome-wide screening for expressed DNA glycosylases, based on their ability to bind to a library of four synthetic inhibitors that target the enzyme's active site. These inhibitors, used in conjunction with the in vitro expression cloning procedure, led to the identification of novel Xenopus and human proteins, xSMUG1 and hSMUG1, respectively, that efficiently excise uracil residues from DNA. Despite a lack of statistically significant overall sequence similarity to the two established classes of uracil-DNA glycosylases, the SMUG1 enzymes contain motifs that are hallmarks of a shared active-site structure and overall protein architecture. The unusual preference of SMUG1 for single-stranded rather than double-stranded DNA suggests a unique biological function in ridding the genome of uracil residues, which are potent endogenous mutagens. CONCLUSIONS: The 'proteomics' approach described here has led to the isolation of a new family of uracil-DNA glycosylases. The three classes of uracil-excising enzymes (SMUG1 being the most recently discovered) represent a striking example of structural and functional conservation in the almost complete absence of sequence conservation. (+info)
Age-associated increase in 8-oxo-deoxyguanosine glycosylase/AP lyase activity in rat mitochondria.
The mitochondrial theory of aging postulates that organisms age due to the accumulation of DNA damage and mutations in the multiple mitochondrial genomes, leading to mitochondrial dysfunction. Among the wide variety of DNA damage, 8-oxo-deoxyguanosine (8-oxo-dG) has received the most attention due to its mutagenicity and because of the possible correlation between its accumulation and pathological processes like cancer, degenerative diseases and aging. Although still controversial, many studies show that 8-oxo-dG accumulates with age in the mitochondrial (mt) DNA. However, little is known about the processing of this lesion and no study has yet examined whether mtDNA repair changes with age. Here, we report the first study on age-related changes in mtDNA repair, accomplished by assessing the cleavage activity of mitochondrial extracts towards an 8-oxo-dG-containing substrate. In this study, mitochondria obtained from rat heart and liver were used. We find that this enzymatic activity is higher in 12 and 23 month-old rats than in 6 month-old rats, in both liver and heart extracts. These mitochondrial extracts also cleave oligonucleotides containing a U:A mismatch, at the uracil position, reflecting the combined action of mitochondrial uracil DNA glycosylase (mtUDG) and mitochondrial apurinic/apyrimidinic (AP) endonucleases. The mtUDG activity did not change with age in liver mitochondria, but there was a small increase in activity from 6 to 23 months in rat heart extracts, after normalization to citrate synthase activity. Endonuclease G activity, measured by a plasmid relaxation assay, did not show any age-associated change in liver, but there was a significant decrease from 6 to 23 months in heart mitochondria. Our results suggest that the mitochondrial capacity to repair 8-oxo-dG, the main oxidative base damage suggested to accumulate with age in mtDNA, does not decrease, but rather increases with age. The specific increase in 8-oxo-dG endonuclease activity, rather than a general up-regulation of DNA repair in mitochondria, suggests an induction of the 8-oxo-dG-specific repair pathway with age. (+info)
Linear free-energy model description of the conformational stability of uracil-DNA glycosylase inhibitor A thermodynamic characterization of interaction with denaturant and cold denaturation.
The equilibrium unfolding of uracil DNA glycosylase inhibitor (Ugi), a small acidic protein of molecular mass 9474 Da, has been studied by a combination of thermal-induced and guanidine hydrochloride (GdnCl)-induced denaturation. The analysis of the denaturation data provides a measure of the changes in conformational free energy, enthalpy, entropy and heat capacity DeltaCp that accompany the equilibrium unfolding of Ugi over a wide range of temperature and GdnCl concentration. The unfolding of Ugi is a simple two-state, reversible process. The protein undergoes both low-temperature and high-temperature unfolding even in the absence of GdnCl but more so in the presence of denaturant. The data are consistent with the linear free-energy model and with a temperature independent DeltaCp over the large temperature range of unfolding. The small DeltaCp (6.52 kJ.mol-1.K-1) for the unfolding of Ugi, is perhaps a reflection of a relatively small, buried hydrophobic core in the folded form of this small monomeric protein. Despite a relatively low value of DeltaG(H2O), 7.40 kJ.mol-1 at pH 8.3, Ugi displays considerable stability with the temperature of maximum stability being 301.6 K. (+info)
Thermostable uracil-DNA glycosylase from Thermotoga maritima a member of a novel class of DNA repair enzymes.
Uracil-DNA glycosylase (UDG) is a ubiquitous enzyme found in eukaryotes and prokaryotes . This enzyme removes uracil bases that are present in DNA as a result of either deamination of cytosine or misincorporation of dUMP instead of dTMP  , and it is the primary activity in the DNA base excision repair pathway. Although UDG activities have been shown to be present in several thermophiles , no sequences have been found that are complementary to the Escherichia coli ung gene, which encodes UDG . Here, we describe a UDG from the thermophile Thermotoga maritima. The T. maritima UDG gene has a low level of homology to the E. coli G-T/U mismatch-specific DNA glycosylase gene (mug). The expressed protein is capable of removing uracil from DNA containing either a U-A or a U-G base pair and is heat-stable up to 75 degrees C. The enzyme is also active on single-stranded DNA containing uracil. Analogous genes appear to be present in several prokaryotic organisms, including thermophilic and mesophilic eubacteria as well as archaebacteria, the human-disease pathogens Treponema palladium and Rickettsia prowazekii, and the extremely radioresistant organism Deinococcus radiodurans. These findings suggest that the T. maritima UDG is a member of a new class of DNA repair enzymes. (+info)
Post-replicative base excision repair in replication foci.
Base excision repair (BER) is initiated by a DNA glycosylase and is completed by alternative routes, one of which requires proliferating cell nuclear antigen (PCNA) and other proteins also involved in DNA replication. We report that the major nuclear uracil-DNA glycosylase (UNG2) increases in S phase, during which it co-localizes with incorporated BrdUrd in replication foci. Uracil is rapidly removed from replicatively incorporated dUMP residues in isolated nuclei. Neutralizing antibodies to UNG2 inhibit this removal, indicating that UNG2 is the major uracil-DNA glycosylase responsible. PCNA and replication protein A (RPA) co-localize with UNG2 in replication foci, and a direct molecular interaction of UNG2 with PCNA (one binding site) and RPA (two binding sites) was demonstrated using two-hybrid assays, a peptide SPOT assay and enzyme-linked immunosorbent assays. These results demonstrate rapid post-replicative removal of incorporated uracil by UNG2 and indicate the formation of a BER complex that contains UNG2, RPA and PCNA close to the replication fork. (+info)
Differential effects of single-stranded DNA binding proteins (SSBs) on uracil DNA glycosylases (UDGs) from Escherichia coli and mycobacteria.
Deamination of cytosines results in accumulation of uracil residues in DNA, which unless repaired lead to GC-->AT transition mutations. Uracil DNA glyco-sylase excises uracil residues from DNA and initiates the base excision repair pathway to safeguard the genomic integrity. In this study, we have investigated the effect of single-stranded DNA binding proteins (SSBs) from Escherichia coli (Eco SSB) and Mycobacterium tuberculosis (Mtu SSB) on uracil excision from synthetic substrates by uracil DNA glycosylases (UDGs) from E. coli, Mycobacterium smegmatis and M.tuberculosis (referred to as Eco -, Msm - and Mtu UDGs respectively). Presence of SSBs with all the three UDGs resulted in decreased efficiency of uracil excision from a single-stranded 'unstructured' oligonucleo-tide, SS-U9. On the other hand, addition of Eco SSB to Eco UDG, or Mtu SSB to Mtu UDG reactions resulted in increased efficiency of uracil excision from a hairpin oligonucleotide containing dU at the second position in a tetraloop (Loop-U2). Interestingly, the efficiency of uracil excision by Msm UDG from the same substrate was decreased in the presence of either Eco- or Mtu SSBs. Furthermore, Mtu SSB also decreased uracil excision from Loop-U2 by Eco UDG. Our studies using surface plasmon resonance technique demonstrated interactions between the homologous combinations of SSBs and UDGs. Heterologous combinations either did not show detectable interaction (Eco SSB with Mtu UDG) or showed a relatively weaker interaction (Mtu SSB with Eco UDG). Taken together, our studies suggest differential interactions between the two groups (SSBs and UDGs) of the highly conserved proteins. Such studies may provide important clues to design selective inhibitors against this important class of DNA repair enzymes. (+info)
Repair of apurinic/apyrimidinic sites by UV damage endonuclease; a repair protein for UV and oxidative damage.
UV damage endonuclease (UVDE) initiates a novel form of excision repair by introducing a nick imme-diately 5" to UV-induced cyclobutane pyrimidine dimers or 6-4 photoproducts. Here, we report that apurinic/apyrimidinic (AP) sites are also nicked by Neurospora crassa and Schizosaccharomyces pombe UVDE. UVDE introduces a nick immediately 5" to the AP site leaving a 3"-OH and a 5"-phosphate AP. Apyrimidinic sites are more effectively nicked by UVDE than apurinic sites. UVDE also possesses 3"-repair activities for AP sites nicked by AP lyase and for 3"-phosphoglycolate produced by bleomycin. The Uvde gene introduced into Escherichia coli cells lacking two types of AP endonuclease, Exo III and Endo IV, gave the host cells resistance to methylmethane sulfonate and t-butyl hydroperoxide. We identified two AP endonuclease activities in S.pombe cell extracts. Besides cyclobutane pyrimidine dimers and 6-4 photoproducts, N. crassa UVDE also nicks Dewar photoproducts. Thus, UVDE is able to repair both of the major forms of DNA damage in living organisms: UV-induced DNA lesions and AP sites. (+info)
Cloning of polymorphisms (COP): enrichment of polymorphic sequences from complex genomes.
Here we describe a new procedure (cloning of polymorphisms, COP) for enrichment of single nucleotide polymorphisms (SNPs) that represent restriction fragment length polymorphisms (RFLPs). COP would be applicable to the isolation of SNPs from particular regions of the genome, e.g. CpG islands, chromosomal bands, YACs or PAC contigs. A combination of digestion with restriction enzymes, treatment with uracil-DNA glycosylase and mung bean nuclease, PCR amplification and purification with streptavidin magnetic beads was used to isolate polymorphic sequences from the genomes of two human samples. After only two cycles of enrichment, 80% of the isolated clones were found to contain RFLPs. A simple method for the PCR detection of these polymorphisms was also developed. (+info)