Jeffrey O. Blaisdell

Abortive base-excision repair of radiation-induced clustered DNA lesions in Escherichia coli

It has been postulated that ionizing radiation produces a unique form of cellular DNA damage called “clustered damages” or “multiply damaged sites”. Here, we show that clustered DNA damages are indeed formed in Escherichia coli by ionizing radiation and are converted to lethal double-strand breaks during attempted base-excision repair. In wild-type cells possessing the oxidative DNA glycosylases that cleave DNA at repairable single damages, double-strand breaks are formed at radiation-induced clusters during postirradiation incubation and also in a dose-dependent fashion. E. coli mutants lacking these enzymes do not form double-strand breaks postirradiation and are substantially more radioresistant than wild-type cells. Furthermore, overproduction of one of the oxidative DNA glycosylases in mutant cells confers a radiosensitive phenotype and an increase in the number of double-strand breaks. Thus, the effect of the oxidative DNA glycosylases in potentiating DNA damage must be considered when estimating radiation risk.

Analysis of the Mechanisms of Action of the Saccharomyces cerevisiae Dominant Lethal cdc42 G12V and Dominant Negative cdc42 D118A Mutations

The Saccharomyces cerevisiae Cdc42p GTPase is localized to the plasma membrane and involved in signal transduction mechanisms controlling cell polarity. The mechanisms of action of the dominant negative cdc42 D118Amutant and the lethal, gain of functioncdc42 G12V mutant were examined. Cdc42D118A,C188Sp and its guanine-nucleotide exchange factor Cdc24p displayed a temperature-dependent interaction in the two-hybrid system, which correlated with the temperature dependence of the cdc42 D118A phenotype and supported a Cdc24p sequestration model for the mechanism ofcdc42 D118A action. Five cdc42mutations were isolated that led to decreased interactions with Cdc24p. The isolation of one mutation (V44A) correlated with the observations that the T35A effector domain mutation could interfere with Cdc42D118A,C188Sp-Cdc24p interactions and could suppress the cdc42 D118A mutation, suggesting that Cdc24p may interact with Cdc42p through its effector domain. Thecdc42 G12V mutant phenotypes were suppressed by the intragenic T35A and K183–187Q mutations and in skm1Δ and cla4Δ cells but not ste20Δ cells, suggesting that the mechanism of cdc42 G12Vaction is through the Skm1p and Cla4p protein kinases at the plasma membrane. Two intragenic suppressors ofcdc42 G12V were also identified that displayed a dominant negative phenotype at 16 °C, which was not suppressed by overexpression of Cdc24p, suggesting an alternate mechanism of action for these dominant negative mutations.

Rapid determination of the active fraction of DNA repair glycosylases: a novel fluorescence assay for trapped intermediates

Current methods to measure the fraction of active glycosylase molecules in a given enzyme preparation are slow and cumbersome. Here we report a novel assay for rapidly determining the active fraction based on molecular accessibility of a fluorescent DNA minor groove binder, 4′,6-diamidino-2-phenylindole (DAPI). Several 5,6-dihydrouracil-containing (DHU) DNA substrates were designed with sequence-dependent DAPI-binding sites to which base excision repair glycosylases were covalently trapped by reduction. Trapped complexes impeded the association of DAPI in a manner dependent on the enzyme used and the location of the DAPI-binding site in relation to the lesion. Of the sequences tested, one was shown to give an accurate measure of the fraction of active molecules for each enzyme tested from both the Fpg/Nei family and HhH-GPD Nth superfamily of DNA glycosylases. The validity of the approach was demonstrated by direct comparison with current gel-based methods. Additionally, the results are supported by in silico modeling based on available crystal structures.

Oxidative DNA Glycosylases: Recipes from Cloning to Characterization

As new organisms are being sequenced on a daily basis, new DNA glycosylases that recognize DNA damage can be easily identified in an effort to understand both their phylogenetics and substrate specificities. As a practical matter, existing bacterial and human homologs need to be readily available as laboratory reagents in order to compare the activities of the novel enzymes to existing enzymes. This chapter attempts to provide a primer for cloning, expression, and assay procedures for bacterial and human DNA glycosylases that recognize oxidative DNA damages. These methodologies can be translated readily to novel DNA glycosylases or to DNA glycosylases that recognize other types of DNA damages.

Multiprobe RNase Protection Assay Analysis of mRNA Levels for the Escherichia coli Oxidative DNA Glycosylase Genes under Conditions of Oxidative Stress

Escherichia coli formamidopyrimidine DNA glycosylase (Fpg), MutY DNA glycosylase, endonuclease VIII, and endonuclease III are oxidative base excision repair DNA glycosylases that remove oxidized bases from DNA, or an incorrect base paired with an oxidized base in the case of MutY. Since genes encoding other base excision repair proteins have been shown to be part of adaptive responses in E. coli, we wanted to determine whether the oxidative DNA glycosylase genes are induced in response to conditions that cause the type of damage their encoded proteins remove. The genes fpg, mutY, nei, and nth encode Fpg, MutY, endonuclease VIII, and endonuclease III, respectively. Multiprobe RNase protection assays were used to examine the transcript levels of these genes under conditions that induce the SoxRS, OxyR, and SOS regulons after a shift from anaerobic to aerobic growth and at different stages along the growth curve. Transcript levels for all four genes decreased as cells progressed from log-phase growth to stationary phase and increased after cells were shifted from anaerobic to aerobic growth. None of the genes were induced by hydrogen peroxide, paraquat, X rays, or conditions that induce the SOS response.

Engineering functional changes in Escherichia coli endonuclease III based on phylogenetic and structural analyses

Escherichia coli endonuclease III (EcoNth) plays an important cellular role by removing premutagenic pyrimidine damages produced by reactive oxygen species. EcoNth is a bifunctional enzyme that has DNA glycosylase and apurinic/apyrimidinic lyase activities. Using a phylogeny of natural sequences, we selected to study EcoNth serine 39, aspartate 44, and arginine 184, which are presumed to be in the vicinity of the damaged base in the glycosylase-substrate complex. These three amino acids are highly conserved among Nth orthologs, although not among homologous glycosylases, such as MutY, that have different base specificities and no lyase activity. To examine the role of these amino acids in catalysis, we constructed three mutants of EcoNth, in which Ser39 was replaced with leucine (S39L), Asp44 was replaced with valine (D44V), and Arg184 was replaced with alanine (R184A), which are the corresponding residues in EcoMutY. We showed that EcoNth S39L does not have significant glycosylase activity for oxidized pyrimidines, although it maintained AP lyase activity. In contrast, EcoNth D44V retained glycosylase activity against oxidized pyrimidines, but the apparent rate constant for the lyase activity of EcoNth D44V was significantly lower than that of EcoNth, indicating that Asp44 in EcoNth is required for β-elimination. Finally, EcoNth R184A maintained lyase activity but exhibited glycosylase specificity different from that of EcoNth. The functional consequences of each of these three substitutions can be rationalized in the context of high resolution protein structures. Thus phylogeny-based scanning mutagenesis has allowed us to identify novel roles for amino acids in the substrate binding pocket of EcoNth in base recognition and/or catalysis.

Processing and Consequences of Oxidative DNA Base Lesions

DNA base lesions produced by free radicals are common products of normal oxidative metabolism. These lesions are removed by base excision repair processing, the first step of which is recognition of the base lesion by a DNA-glycosylase. In general, the oxidative DNA base lesions are recognized by either a pyrimidine-specific or a purine-specific DNA-glycosylase. In this chapter, we describe the biochemical and biological properties of the newest of these activities the pyrimidine-specific oxidative DNA glycosylase, endonuclease VIII (nei) of Escherichia coli. We also describe an in vitro reconstitution of the base excision repair pathway using model DNAs that contain closely opposed lesions similar to multiply damaged sites produced by ionizing radiation. Here we show that when closely opposed lesions are more than three nucleotides apart, processing by base excision repair can lead to a potentially lethal double strand break. Finally, we describe the interaction between two ring saturation products of pyrimidines, one derived from cytosine, uracil glycol, the other from thymine, thymine glycol, with a model DNA polymerase, DNA polymerase I of E. coli. Both incorporation of the modified nucleoside triphosphate and translesion synthesis past the lesion in the DNA template, are considered