Primary sequence and biological functions of a Saccharomyces cerevisiae O6-methylguanine/O4-methylthymine DNA repair methyltransferase gene

Profiling The Compendium Of Changes In Saccharomyces cerevisiae Due To Mutations That Alter Availability Of The Main Methyl Donor S-Adenosylmethionine

The SAM1 and SAM2 genes encode for S-AdenosylMethionine (AdoMet) synthetase enzymes, with AdoMet serving as the main methyl donor. We have previously shown that independent deletion of these genes alters chromosome stability and AdoMet concentrations in opposite ways in S. cerevisiae. To characterize other changes occurring in these mutants, we grew wildtype, sam1∆/sam1∆, and sam2∆/sam2∆ strains in 15 different Phenotypic Microarray plates with different components, equal to 1440 wells, and measured for growth variations. RNA-Sequencing was also carried out on these strains and differential gene expression determined for each mutant. In this study, we explore how the phenotypic growth differences are linked to the altered gene expression, and thereby predict the mechanisms by which loss of the SAM genes and subsequent AdoMet level changes, impact S. cerevisiae pathways and processes. We present six stories, discussing changes in sensitivity or resistance to azoles, cisplatin, oxidative stress, arginine biosynthesis perturbations, DNA synthesis inhibitors, and tamoxifen, to demonstrate the power of this novel methodology to broadly profile changes due to gene mutations. The large number of conditions that result in altered growth, as well as the large number of differentially expressed genes with wide-ranging functionality, speaks to the broad array of impacts that altering methyl donor abundance can impart, even when the conditions tested were not specifically selected as targeting known methyl involving pathways. Our findings demonstrate that some cellular changes are directly related to AdoMet-dependent methyltransferases and AdoMet availability, some are directly linked to the methyl cycle and its role is production of several important cellular components, and others reveal impacts of SAM gene mutations on previously unconnected pathways.

DNA Repair in Haploid Context

DNA repair is a well-covered topic as alteration of genetic integrity underlies many pathological conditions and important transgenerational consequences. Surprisingly, the ploidy status is rarely considered although the presence of homologous chromosomes dramatically impacts the repair capacities of cells. This is especially important for the haploid gametes as they must transfer genetic information to the offspring. An understanding of the different mechanisms monitoring genetic integrity in this context is, therefore, essential as differences in repair pathways exist that differentiate the gamete’s role in transgenerational inheritance. Hence, the oocyte must have the most reliable repair capacity while sperm, produced in large numbers and from many differentiation steps, are expected to carry de novo variations. This review describes the main DNA repair pathways with a special emphasis on ploidy. Differences between Saccharomyces cerevisiae and Schizosaccharomyces pombe are especially useful to this aim as they can maintain a diploid and haploid life cycle respectively.

A Candida parapsilosis Overexpression Collection Reveals Genes Required for Pathogenesis

Relative to the vast data regarding the virulence mechanisms of Candida albicans, there is limited knowledge on the emerging opportunistic human pathogen Candida parapsilosis. The aim of this study was to generate and characterize an overexpression mutant collection to identify and explore virulence factors in C. parapsilosis. With the obtained mutants, we investigated stress tolerance, morphology switch, biofilm formation, phagocytosis, and in vivo virulence in Galleria mellonella larvae and mouse models. In order to evaluate the results, we compared the data from the C. parapsilosis overexpression collection analysis to the results derived from previous deletion mutant library characterizations. Of the 37 overexpression C. parapsilosis mutants, we identified eight with altered phenotypes compared to the controls. This work is the first report to identify CPAR2_107240, CPAR2_108840, CPAR2_302400, CPAR2_406400, and CPAR2_602820 as contributors to C. parapsilosis virulence by regulating functions associated with host-pathogen interactions and biofilm formation. Our findings also confirmed the role of CPAR2_109520, CPAR2_200040, and CPAR2_500180 in pathogenesis. This study was the first attempt to use an overexpression strategy to systematically assess gene function in C. parapsilosis, and our results demonstrate that this approach is effective for such investigations.

Genome-wide surveillance of transcription errors in response to genotoxic stress

Mutagenic compounds are a potent source of human disease. By inducing genetic instability, they can accelerate the evolution of human cancers or lead to the development of genetically inherited diseases. Here, we show that in addition to genetic mutations, mutagens are also a powerful source of transcription errors. These errors arise in dividing and nondividing cells alike, affect every class of transcripts inside cells, and, in certain cases, greatly exceed the number of mutations that arise in the genome. In addition, we reveal the kinetics of transcription errors in response to mutagen exposure and find that DNA repair is required to mitigate transcriptional mutagenesis after exposure. Together, these observations have far-reaching consequences for our understanding of mutagenesis in human aging and disease, and suggest that the impact of DNA damage on human physiology has been greatly underestimated.

The Major Replicative Histone Chaperone CAF-1 Suppresses the Activity of the DNA Mismatch Repair System in the Cytotoxic Response to a DNA-methylating Agent

The DNA mismatch repair (MMR) system corrects DNA mismatches in the genome. It is also required for the cytotoxic response of O⁶-methylguanine-DNA methyltransferase (MGMT)-deficient mammalian cells and yeast mgt1Δ rad52Δ cells to treatment with S_n1-type methylating agents, which produce cytotoxic O⁶-methylguanine (O⁶-mG) DNA lesions. Specifically, an activity of the MMR system causes degradation of irreparable O⁶-mG-T mispair-containing DNA, triggering cell death; this process forms the basis of treatments of MGMT-deficient cancers with S_n1-type methylating drugs. Recent research supports the view that degradation of irreparable O⁶-mG-T mispair-containing DNA by the MMR system and CAF-1-dependent packaging of the newly replicated DNA into nucleosomes are two concomitant processes that interact with each other. Here, we studied whether CAF-1 modulates the activity of the MMR system in the cytotoxic response to S_n1-type methylating agents. We found that CAF-1 suppresses the activity of the MMR system in the cytotoxic response of yeast mgt1Δ rad52Δ cells to the prototypic S_n1-type methylating agent N-methyl-N'-nitro-N-nitrosoguanidine. We also report evidence that in human MGMT-deficient cell-free extracts, CAF-1-dependent packaging of irreparable O⁶-mG-T mispair-containing DNA into nucleosomes suppresses its degradation by the MMR system. Taken together, these findings suggest that CAF-1-dependent incorporation of irreparable O⁶-mG-T mispair-containing DNA into nucleosomes suppresses its degradation by the MMR system, thereby defending the cell against killing by the S_n1-type methylating agent.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

DNA repair mechanisms and the bypass of DNA damage in Saccharomyces cerevisiae

DNA repair mechanisms are critical for maintaining the integrity of genomic DNA, and their loss is associated with cancer predisposition syndromes. Studies in Saccharomyces cerevisiae have played a central role in elucidating the highly conserved mechanisms that promote eukaryotic genome stability. This review will focus on repair mechanisms that involve excision of a single strand from duplex DNA with the intact, complementary strand serving as a template to fill the resulting gap. These mechanisms are of two general types: those that remove damage from DNA and those that repair errors made during DNA synthesis. The major DNA-damage repair pathways are base excision repair and nucleotide excision repair, which, in the most simple terms, are distinguished by the extent of single-strand DNA removed together with the lesion. Mistakes made by DNA polymerases are corrected by the mismatch repair pathway, which also corrects mismatches generated when single strands of non-identical duplexes are exchanged during homologous recombination. In addition to the true repair pathways, the postreplication repair pathway allows lesions or structural aberrations that block replicative DNA polymerases to be tolerated. There are two bypass mechanisms: an error-free mechanism that involves a switch to an undamaged template for synthesis past the lesion and an error-prone mechanism that utilizes specialized translesion synthesis DNA polymerases to directly synthesize DNA across the lesion. A high level of functional redundancy exists among the pathways that deal with lesions, which minimizes the detrimental effects of endogenous and exogenous DNA damage.

Molecular characterization of an adaptive response to alkylating agents in the opportunistic pathogen Aspergillus fumigatus

An adaptive response to alkylating agents based upon the conformational change of a methylphosphotriester (MPT) DNA repair protein to a transcriptional activator has been demonstrated in a number of bacterial species, but this mechanism appears largely absent from eukaryotes. Here, we demonstrate that the human pathogen Aspergillus fumigatus elicits an adaptive response to sub-lethal doses of the mono-functional alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). We have identified genes that encode MPT and O(6)-alkylguanine DNA alkyltransferase (AGT) DNA repair proteins; deletions of either of these genes abolish the adaptive response and sensitize the organism to MNNG. In vitro DNA repair assays confirm the ability of MPT and AGT to repair methylphosphotriester and O(6)-methylguanine lesions respectively. In eukaryotes, the MPT protein is confined to a select group of fungal species, some of which are major mammalian and plant pathogens. The evolutionary origin of the adaptive response is bacterial and rooted within the Firmicutes phylum. Inter-kingdom horizontal gene transfer between Firmicutes and Ascomycete ancestors introduced the adaptive response into the Fungal kingdom. Our data constitute the first detailed characterization of the molecular mechanism of the adaptive response in a lower eukaryote and has applications for development of novel fungal therapeutics targeting this DNA repair system.

Endocrine disrupting chemicals modulate expression of O⁶-methylguanine DNA methyltransferase (O⁶-MGMT) gene in the hermaphroditic fish, Kryptolebias marmoratus

O⁶-methylguanine-DNA methyltransferase (O⁶-MGMT; EC 2.1.1.63) is a key repair enzyme that helps to protect the cell against alkylation on DNA by removing a methyl group from the O⁶-position of guanine. Here, we cloned and sequenced the full-length O⁶-MGMT cDNA from the hermaphroditic fish, Kryptolebias marmoratus. Complete Km-O⁶-MGMT cDNA was 1324 bp in length, and the open reading frame of 567 bp encoded a polypeptide of 188 amino acid residues. Phylogenetic analysis revealed that Km-O⁶-MGMT was clustered with those of other fish species. Embryo, juveniles, and aged secondary fish had low levels of Km-O⁶-MGMT mRNA than adults, indicating more susceptibility to DNA damage by alkylating agent exposure during these developmental stages. Km-O⁶-MGMT mRNA levels differed according to tissue type and was highest in the liver. Exposure to an alkylating agent, N-methyl-N-nitrosourea (MNU) exposure increased the mRNA expression of tumor suppressor gene such as p53 and oncogenes such as R-ras1, R-ras3, N-ras, c-fos as well as Km-O⁶-MGMT mRNA in a time-dependent manner. On the contrary, several (anti)estrogenic compounds (17β-estradiol 100 ng/L, tamoxifen 10 μg/L, bisphenol A 600 μg/L, and 4-tert-octylphenol 300 μg/L) suppressed mRNA expression of Km-O⁶-MGMT in most tissues, especially the liver. In juvenile fish, 17β-estradiol, bisphenol A, and 4-tert-octylphenol also decreased the expression of Km-O⁶-MGMT mRNA in a time-dependent manner. Overall, our finding shows that Km-O⁶-MGMT mRNA levels can be modulated by environmental estrogenic compounds as well as alkylating agents. This finding will be helpful to improve our knowledge of the effects of estrogenic compounds that contain the genotoxic ability to inhibit the DNA repair process in aquatic animals.

Requirement of the Saccharomyces cerevisiae APN1 gene for the repair of mitochondrial DNA alkylation damage

The Saccharomyces cerevisiae APN1 gene that participates in base excision repair has been localized both in the nucleus and the mitochondria. APN1 deficient cells (apn1 Delta) show increased mutation frequencies in mitochondrial DNA (mtDNA) suggesting that APN1 is also important for mtDNA stability. To understand APN1-dependent mtDNA repair processes we studied the formation and repair of mtDNA lesions in cells exposed to methyl methanesulfonate (MMS). We show that MMS induces mtDNA damage in a dose-dependent fashion and that deletion of the APN1 gene enhances the susceptibility of mtDNA to MMS. Repair kinetic experiments demonstrate that in wild-type cells (WT) it takes 4 hr to repair the damage induced by 0.1% MMS, whereas in the apn1 Delta strain there is a lag in mtDNA repair that results in significant differences in the repair capacity between the two yeast strains. Analysis of lesions in nuclear DNA (nDNA) after treatment with 0.1% MMS shows a significant difference in the amount of nDNA lesions between WT and apn1 Delta cells. Interestingly, comparisons between nDNA and mtDNA damage show that nDNA is more sensitive to the effects of MMS treatment. However, both strains are able to repair the nDNA lesions, contrary to mtDNA repair, which is compromised in the apn1 Delta mutant strain. Therefore, although nDNA is more sensitive than mtDNA to the effects of MMS, deletion of APN1 has a stronger phenotype in mtDNA repair than in nDNA. These results highlight the prominent role of APN1 in the repair of environmentally induced mtDNA damage.

Interplay of DNA repair pathways controls methylation damage toxicity in Saccharomyces cerevisiae

Methylating agents of S(N)1 type are widely used in cancer chemotherapy, but their mode of action is poorly understood. In particular, it is unclear how the primary cytotoxic lesion, O(6)-methylguanine ((Me)G), causes cell death. One hypothesis stipulates that binding of mismatch repair (MMR) proteins to (Me)G/T mispairs arising during DNA replication triggers cell-cycle arrest and cell death. An alternative hypothesis posits that (Me)G cytotoxicity is linked to futile processing of (Me)G-containing base pairs by the MMR system. In this study, we provide compelling genetic evidence in support of the latter hypothesis. Treatment of 4644 deletion mutants of Saccharomyces cerevisiae with the prototypic S(N)1-type methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) identified MMR as the only pathway that sensitizes cells to MNNG. In contrast, homologous recombination (HR), postreplicative repair, DNA helicases, and chromatin maintenance factors protect yeast cells against the cytotoxicity of this chemical. Notably, DNA damage signaling proteins played a protective rather than sensitizing role in the MNNG response. Taken together, this evidence demonstrates that (Me)G-containing lesions in yeast must be processed to be cytotoxic.

MGMT: a personal perspective

This review describes the history of studies on alkylation damage of mammalian genomes and its carcinogenic consequences that led to the discovery of a unique DNA repair protein, named MGMT. MGMT repairs O(6)-alkylguanine, a critical mutagenic lesion induced by alkylating agents. The follow-up studies in mammalian cells following the discovery of the ubiquitous repair protein in E. coli are summarized.

Homologous recombination rescues mismatch-repair-dependent cytotoxicity of S(N)1-type methylating agents in S. cerevisiae

Resistance of mammalian cells to S(N)1-type methylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) generally arises through increased expression of methylguanine methyltransferase (MGMT), which reverts the cytotoxic O(6)-methylguanine ((Me)G) to guanine, or through inactivation of the mismatch repair (MMR) system, which triggers cell death through aberrant processing of (Me)G/T mispairs generated during DNA replication when MGMT capacity is exceeded. Given that MMR and (Me)G-detoxifying proteins are functionally conserved through evolution, and that MMR-deficient Escherichia coli dam(-) strains are also resistant to MNNG, the finding that MMR status did not affect the sensitivity of Saccharomyces cerevisiae to MNNG was unexpected. Because (Me)G residues in DNA trigger homologous recombination (HR), we wondered whether the efficient HR in S. cerevisiae might alleviate the cytotoxic effects of (Me)G processing. We now show that HR inactivation sensitizes S. cerevisiae to MNNG and that, as in human cells, defects in the MMR genes MLH1 and MSH2 rescue this sensitivity. Inactivation of the EXO1 gene, which encodes the only exonuclease implicated in MMR to date, failed to rescue the hypersensitivity, which implies that scExo1 is not involved in the processing of (Me)G residues by the S. cerevisiae MMR system.

Alkylation damage in DNA and RNA--repair mechanisms and medical significance

Alkylation lesions in DNA and RNA result from endogenous compounds, environmental agents and alkylating drugs. Simple methylating agents, e.g. methylnitrosourea, tobacco-specific nitrosamines and drugs like temozolomide or streptozotocin, form adducts at N- and O-atoms in DNA bases. These lesions are mainly repaired by direct base repair, base excision repair, and to some extent by nucleotide excision repair (NER). The identified carcinogenicity of O(6)-methylguanine (O(6)-meG) is largely caused by its miscoding properties. Mutations from this lesion are prevented by O(6)-alkylG-DNA alkyltransferase (MGMT or AGT) that repairs the base in one step. However, the genotoxicity and cytotoxicity of O(6)-meG is mainly due to recognition of O(6)-meG/T (or C) mispairs by the mismatch repair system (MMR) and induction of futile repair cycles, eventually resulting in cytotoxic double-strand breaks. Therefore, inactivation of the MMR system in an AGT-defective background causes resistance to the killing effects of O(6)-alkylating agents, but not to the mutagenic effect. Bifunctional alkylating agents, such as chlorambucil or carmustine (BCNU), are commonly used anti-cancer drugs. DNA lesions caused by these agents are complex and require complex repair mechanisms. Thus, primary chloroethyl adducts at O(6)-G are repaired by AGT, while the secondary highly cytotoxic interstrand cross-links (ICLs) require nucleotide excision repair factors (e.g. XPF-ERCC1) for incision and homologous recombination to complete repair. Recently, Escherichia coli protein AlkB and human homologues were shown to be oxidative demethylases that repair cytotoxic 1-methyladenine (1-meA) and 3-methylcytosine (3-meC) residues. Numerous AlkB homologues are found in viruses, bacteria and eukaryotes, including eight human homologues (hABH1-8). These have distinct locations in subcellular compartments and their functions are only starting to become understood. Surprisingly, AlkB and hABH3 also repair RNA. An evaluation of the biological effects of environmental mutagens, as well as understanding the mechanism of action and resistance to alkylating drugs require a detailed understanding of DNA repair processes.

Yeast as a model system for anticancer drug discovery

Saccharomyces cerevisiae has been used extensively as a model for higher eukaryotes in the study of basic cellular processes. The high degree of conservation in terms of sequence similarity and function has made this organism useful in elucidating biological pathways, both yeast and human. Among these are pathways responsible for DNA damage repair and cell cycle control. This review presents an overview of opportunities for using yeast as a model system for anticancer drug discovery. It covers screens directed against specific cancer-related targets as well as contexts created by cancer-related alterations. The methodologies covered include pharmacological and genetic screens, as well as genome-wide approaches to drug target identification.

How heterologously expressed Escherichia coli genes contribute to understanding DNA repair processes in Saccharomyces cerevisiae

DNA-damaging agents constantly challenge cellular DNA; and efficient DNA repair is therefore essential to maintain genome stability and cell viability. Several DNA repair mechanisms have evolved and these have been shown to be highly conserved from bacteria to man. DNA repair studies were originally initiated in very simple organisms such as Escherichia coli and Saccharomyces cerevisiae, bacteria being the best understood organism to date. As a consequence, bacterial DNA repair genes encoding proteins with well characterized functions have been transferred into higher organisms in order to increase repair capacity, or to complement repair defects, in heterologous cells. While indicating the contribution of these repair functions to protection against the genotoxic effects of DNA-damaging agents, heterologous expression studies also highlighted the role of the DNA lesions that are substrates for such processes. In addition, bacterial DNA repair-like functions could be identified in higher organisms using this approach. We heterologously expressed three well characterized E. coli repair genes in S. cerevisiae cells of different genetic backgrounds: (1) the ada gene encoding O(6)-methylguanine DNA-methyltransferase, a protein involved in the repair of alkylation damage to DNA, (2) the recA gene encoding the main recombinase in E. coli and (3) the nth gene, the product of which (endonuclease III) is responsible for the repair of oxidative base damage. Here, we summarize our results and indicate the possible implications they have for a better understanding of particular DNA repair processes in S. cerevisiae.

A single amino acid substitution in MSH5 results in DNA alkylation tolerance

DNA alkylation tolerance is a major concern in cancer chemotherapy. It has been suggested that mutations in DNA mismatch repair genes may result in alkylation tolerance. This alkylation tolerant phenotype is often manifested in cells lacking an O(6)-methylguanine DNA methyltransferase (MTase) activity. However, deletion of each mismatch repair gene in the MTase mutant of a model eukaryotic yeast does not result in alkylation tolerance. We previously isolated an alkylation tolerant mutant and mapped the mutation to MSH5. Here we present evidence that a single point mutation that results in a Y823H amino acid substitution, but not deletion, of the MSH5 gene is responsible for tolerance to killing by DNA alkylating agents. We also find that other preexisting amino acid variations may also enhance alkylation tolerance in the above mutation background. Since MSH5 encodes a protein homologous to DNA mismatch recognition proteins, mismatch repair genes are frequently mutated in cancers cells and, like mismatch repair genes, MSH5 is highly conserved from yeast to human, this observation suggests novel mechanisms of chemotherapeutic drug resistance that may occur in certain human cancer patients.

Importance of DNA repair in carcinogenesis: evidence from transgenic and gene targeting studies

We have generated transgenic mice by introducing copies of the E. coli O6-methylguanine-DNA methyltransferase gene, ada. Liver extracts from homozygotes demonstrate about three times the control enzyme activity and increase up to about eight-fold can be induced by treatment with zinc, since the metal-responsive metallothionein promoter is attached to the ada gene. Furthermore, studies of liver carcinogenesis in our transgenic mice demonstrated significantly reduced rates of development of hepatocellular tumors after treatment with dimethylnitrosamine or diethylnitrosamine. It is well known that xeroderma pigmentosum (XP) patients are deficient in DNA repair. The availability of XPA (XP group A complementing) knockout mice has enabled us to investigate the functional role of the XPA nucleotide excision repair gene in carcinogenesis in vivo, first using the mouse skin as a model system. XPA-/- mice demonstrated skin ulcers 5-7 days after 7,12-dimethylbenz[a]anthracene (DMBA) treatment and papilloma development within 4 weeks prior to promotion, skin tumor incidence being also much higher than in heterozygous and wild-type mice. Experiments targeting the lung, liver and tongue have also been conducted to answer the question of whether the internal organs of these mice are also susceptible to chemical carcinogens. For lung carcinogenesis, mice were instilled intratracheally with a small dose of benzo[a]pyrene. The pulmonary tumor incidence in XPA-/- mice was significantly higher than in XPA+/- and XPA+/+ mice. XPA-/- mice were also found to be have enhanced sensitivity to aflatoxin B1 regarding liver tumor induction. In addition, administration of 4-nitroquinoline-1-oxide in drinking water for 50 weeks resulted in tongue tumors only in XPA-/- mice. These studies, thus, provided convincing evidence that XPA mice are also sensitive to carcinogenesis in organs other than the skin.

Fidelity of nucleotide insertion at 8-oxo-7,8-dihydroguanine by mammalian DNA polymerase delta. Steady-state and pre-steady-state kinetic analysis

Nucleotide insertion opposite 8-oxo-7,8-dihydroguanine (8-oxoG) by fetal calf thymus DNA polymerase delta (pol delta) was examined by steady-state and pre-steady-state rapid quench kinetic analyses. In steady-state reactions with the accessory protein proliferating cell nuclear antigen (PCNA), pol delta preferred to incorporate dCTP opposite 8-oxoG with an efficiency of incorporation an order of magnitude lower than incorporation into unmodified DNA (mainly due to an increased K(m)). Pre-steady-state kinetic analysis of incorporation opposite 8-oxoG showed biphasic kinetics for incorporation of either dCTP or dATP, with rates similar to dCTP incorporation opposite G, large phosphorothioate effects (>100), and oligonucleotide dissociation apparently rate-limiting in the steady-state. Although pol delta preferred to incorporate dCTP (14% misincorporation of dATP) the extension past the A:8-oxoG mispair predominated. The presence of PCNA was found to be a more essential factor for nucleotide incorporation opposite 8-oxoG adducts than unmodified DNA, increased pre-steady-state rates of nucleotide incorporation by >2 orders of magnitude, and was essential for nucleotide extension beyond 8-oxoG. pol delta replication fidelity at 8-oxoG depends upon contributions from K(m), K(d)(dNTP), and rates of phosphodiester bond formation, and PCNA is an important accessory protein for incorporation and extension at 8-oxoG adducts.

REV3 is required for spontaneous but not methylation damage-induced mutagenesis of Saccharomyces cerevisiae cells lacking O6-methylguanine DNA methyltransferase

O6-methylguanine (O6-MeG) DNA methyltransferase (MTase) removes the methyl group from a DNA lesion and directly restores DNA structure. It has been shown previously that bacterial and yeast cells lacking such MTase activity are not only sensitive to killing and mutagenesis by DNA methylating agents, but also exhibit an increased spontaneous mutation rate. In order to understand molecular mechanisms of endogenous DNA alkylation damage and its effects on mutagenesis, we determined the spontaneous mutational spectra of the SUP4-o gene in various Saccharomyces cerevisiae strains. To our surprise, the mgt1 mutant deficient in DNA repair MTase activity exhibited a significant increase in G:C-->C:G transversions instead of the expected G:C-->A:T transition. Its mutational distribution strongly resembles that of the rad52 mutant defective in DNA recombinational repair. The rad52 mutational spectrum has been shown to be dependent on a mutagenesis pathway mediated by REV3. We demonstrate here that the mgt1 mutational spectrum is also REV3-dependent and that the rev3 deletion offsets the increase of the spontaneous mutation rate seen in the mgt1 strains. These results indicate that the eukaryotic mutagenesis pathway is directly involved in cellular processing of endogenous DNA alkylation damage possibly by the translesion bypass of lesions at the cost of G:C-->C:G transversion mutations. However, the rev3 deletion does not affect methylation damage-induced killing and mutagenesis of the mgt1 mutant, suggesting that endogenous alkyl lesions may be different from O6-MeG.

Methionine reduces spontaneous and alkylation-induced mutagenesis in Saccharomyces cerevisiae cells deficient in O6-methylguanine-DNA methyltransferase

The exposure of DNA to reactive intracellular metabolites is thought to be a major cause of spontaneous mutagenesis. DNA alkylation is implicated in the above process by the fact that bacterial and yeast cells lacking DNA alkylation-specific repair genes exhibit elevated spontaneous mutation rates. The origin of the intracellular alkylating molecules is not clear; however, S-adenosylmethionine (SAM) has been proposed as one source because it has a reactive methyl group known to methylate proteins and DNA. We supplemented yeast cultures with excess methionine and examined the effects of increased endogenous SAM concentration on spontaneous and alkylation-induced mutagenesis in the absence of various DNA repair pathways. Our results show that either the excess methionine, or the increased SAM produced as a result of this treatment, is able to protect yeast cells from mutagenesis, and that this effect is alkylation-damage-specific. The protective effect was observed only in the mgt1 mutant deficient in the O(6)-methylguanine-DNA repair methyltransferase, but not in the wild type or other DNA repair-deficient strains, indicating that the protection is specific for O-methyl lesions. Thus, our results may lend support to the recently reported chemopreventive effect of SAM in rodents and further suggest that the observed tumor prevention by SAM may be, in part, due to its suppression of spontaneous mutagenesis in mammals. Given that a strong correlation has been established between O(6)-methylguanine and carcinogenicity, this study may offer a novel approach to preventing carcinogenesis.

Identification and characterisation of the Drosophila melanogaster O6-alkylguanine-DNA alkyltransferase cDNA

The protein O 6-alkylguanine-DNA alkyltransferase(alkyltransferase) is involved in the repair of O 6-alkylguanine and O 4-alkylthymine in DNA and plays an important role in most organisms in attenuating the cytotoxic and mutagenic effects of certain classes of alkylating agents. A genomic clone encompassing the Drosophila melanogaster alkyltransferase gene ( DmAGT ) was identified on the basis of sequence homology with corresponding genes in Saccharomyces cerevisiae and man. The DmAGT gene is located at position 84A on the third chromosome. The nucleotide sequence of DmAGT cDNA revealed an open reading frame encoding 194 amino acids. The MNNG-hypersensitive phenotype of alkyltransferase-deficient bacteria was rescued by expression of the DmAGT cDNA. Furthermore, alkyltransferase activity was identified in crude extracts of Escherichia coli harbouring DmAGT cDNA and this activity was inhibited by preincubation of the extract with an oligonucleotide containing a single O6-methylguanine lesion. Similar to E.coli Ogt and yeast alkyltransferase but in contrast to the human alkyltransferase, the Drosophila alkyltransferase is resistant to inactivation by O 6-benzylguanine. In an E.coli lac Z reversion assay, expression of DmAGT efficiently suppressed MNNG-induced G:C-->A:T as well as A:T-->G:C transition mutations in vivo. These results demonstrate the presence of an alkyltransferase specific for the repair of O 6-methylguanine and O 4-methylthymine in Drosophila.

The Pol beta-14 dominant negative rat DNA polymerase beta mutator mutant commits errors during the gap-filling step of base excision repair in Saccharomyces cerevisiae

We demonstrated recently that dominant negative mutants of rat DNA polymerase beta (Pol beta) interfere with repair of alkylation damage in Saccharomyces cerevisiae. To identify the alkylation repair pathway that is disrupted by the Pol beta dominant negative mutants, we studied the epistatic relationship of the dominant negative Pol beta mutants to genes known to be involved in repair of DNA alkylation damage in S. cerevisiae. We demonstrate that the rat Pol beta mutants interfere with the base excision repair pathway in S. cerevisiae. In addition, expression of one of the Pol beta dominant negative mutants, Pol beta-14, increases the spontaneous mutation rate of S. cerevisiae whereas expression of another Pol beta dominant negative mutant, Pol beta-TR, does not. Expression of the Pol beta-14 mutant in cells lacking APN1 activity does not result in an increase in the spontaneous mutation rate. These results suggest that gaps are required for mutagenesis to occur in the presence of Pol beta-14 but that it is not merely the presence of a gap that results in mutagenesis. Our results suggest that mutagenesis can occur during the gap-filling step of base excision repair in vivo.

The potential role of glycine-160 of human O6-alkylguanine-DNA alkyltransferase in reaction with O6-benzylguanine as determined by site-directed mutagenesis and molecular modelling comparisons

O6-Alkylguanine DNA-alkyltransferase (ATase) repairs toxic, mutagenic and carcinogenic O6-alkylguanine (O6-alkG) lesions in DNA by a highly conserved reaction involving the stoichiometric transfer of the alkyl group to the active centre cysteine residue of the ATase protein. In the Escherichia coli Ada ATase, which is effectively refactory to inhibition by O6-benzylguanine (O6-BzG), the residue corresponding to glycine-160 (G160) for the mammalian proteins of this class is replaced by a tryptophan (W). Therefore, to investigate the potential role of the G160 of the human ATase (hAT) protein in determining sensitivity to O6-BzG, site-directed mutagenesis was used to produce a mutant protein (hATG160W) substituted at position 160 with a W residue. The hATG160W mutant was found to be stably expressed and was 3- and 5-fold more sensitive than hAT to inactivation by O6-BzG, in the absence and presence of additional calf-thymus DNA respectively. A similar, DNA dependent increased sensitivity of the hATG160W mutant relative to wild-type was also found for O6-methylguanine mediated inactivation. The potential role of the W160 residue in stabilising the binding of the O6-alkG to the protein is discussed in terms of a homology model of the structure of hAT. The region occupied by G/W-160 forms the site of a putative hinge that could be important in the conformational change that is likely to occur on DNA binding. Three sequence motifs have been identified in this region which may influence O6-BzG access to the active site; YSGG or YSGGG in mammals (YAGG in E. coli Ogt, YAGS in Dat from Bacillus subtilis), YRWG in E. coli Ada and Salmonella typhimurium (but YKWS in Saccharomyces cerevisiae) or YRGGF in AdaB from B. Subtilis. Finally,conformational and stereoelectronic analysis of the putative transition states for the alkyl transfer from a series of inactivators of hAT, including O6-BzG was undertaken to rationalise the unexpected weak inhibition shown by the alpha-pi-unsaturated electrophiles.

Construction and overexpression of a synthetic gene for human DNA methylguanine methyltransferase: renaturation and rapid purification of the protein

A synthetic gene was constructed that encodes human DNA methylguanine methyltransferase (hMGMT). The synthetic gene was designed with a number of unique restriction sites to facilitate cassette mutagenesis and to reflect the preferences found among genes in Escherichia coli. Both the full-length gene and a gene for a functional variant (hMGMT delta C) that lacks the C-terminal 28 codons were constructed, and the genes were overexpressed using a T7 RNA polymerase promoter. The proteins are made in the form of insoluble aggregates but the truncated form of the protein (hMGMT delta C) has been successfully denatured, renatured, and purified to near homogeneity by ion exchange. Methyltransferase activity assays of hMGMT delta C demonstrate that the reconstituted protein has substantial DNA repair activity, though somewhat less than full-length hMGMT that had been expressed and purified in a soluble form. Mass spectrometry of a mixture of proteolytic fragments confirmed the protein sequence and indicated no detectable oxidation of the active site cysteine. The protein was determined to be monomeric by gel filtration chromatography, and circular dichroism spectra for renatured hMGMT delta C and fully soluble hMGMT are consistent with the renatured protein preparation being fully folded. Refolded hMGMT delta C had a curious propensity to form large aggregates in a time-dependent manner when injected into a dynamic light scattering instrument; this aggregation behavior was not observed for hMGMT purified in a soluble form. Differences in susceptibility to aggregation may account for differences in methyltransfer activity. Yields of purified protein were approximately 5 mg/liter of culture.

RAD9, RAD17, and RAD24 are required for S phase regulation in Saccharomyces cerevisiae in response to DNA damage

We have previously shown that a checkpoint dependent on MEC1 and RAD53 slows the rate of S phase progression in Saccharomyces cerevisiae in response to alkylation damage. Whereas wild-type cells exhibit a slow S phase in response to damage, mec1-1 and rad53 mutants replicate rapidly in the presence or absence of DNA damage. In this report, we show that other genes (RAD9, RAD17, RAD24) involved in the DNA damage checkpoint pathway also play a role in regulating S phase in response to DNA damage. Furthermore, RAD9, RAD17, and RAD24 fall into two groups with respect to both sensitivity to alkylation and regulation of S phase. We also demonstrate that the more dramatic defect in S phase regulation in the mec1-1 and rad53 mutants is epistatic to a less severe defect seen in rad9 delta, rad 17 delta, and rad24 delta. Furthermore, the triple rad9 delta rad17 delta rad24 delta mutant also has a less severe defect than mec1-1 or rad53 mutants. Finally, we demonstrate the specificity of this phenotype by showing that the DNA repair and/or checkpoint mutants mgt1 delta, mag1 delta, apn1 delta, rev3 delta, rad18 delta, rad16 delta, dun1-delta 100, sad4-1, tel1 delta, rad26 delta, rad51 delta, rad52-1, rad54 delta, rad14 delta, rad1 delta, pol30-46, pol30-52, mad3 delta, pds1 delta/esp2 delta, pms1 delta, mlh1 delta, and msh2 delta are all proficient at S phase regulation, even though some of these mutations confer sensitivity to alkylation.

DNA repair functions in heterologous cells

Our genetic information is constantly challenged by exposure to endogenous and exogenous DNA-damaging agents, by DNA polymerase errors, and thereby inherent instability of the DNA molecule itself. The integrity of our genetic information is maintained by numerous DNA repair pathways, and the importance of these pathways is underscored by their remarkable structural and functional conservation across the evolutionary spectrum. Because of the highly conserved nature of DNA repair, the enzymes involved in this crucial function are often able to function in heterologous cells; as an example, the E. coli Ada DNA repair methyltransferase functions efficiently in yeast, in cultured rodent and human cells, in transgenic mice, and in ex vivo-modified mouse bone marrow cells. The heterologous expression of DNA repair functions has not only been used as a powerful cloning strategy, but also for the exploration of the biological and biochemical features of numerous enzymes involved in DNA repair pathways. In this review we highlight examples where the expression of DNA repair enzymes in heterologous cells was used to address fundamental questions about DNA repair processes in many different organisms.

DNA DAMAGE AND REPAIR IN PLANTS

The biological impact of any DNA damaging agent is a combined function of the chemical nature of the induced lesions and the efficiency and accuracy of their repair. Although much has been learned from microbes and mammals about both the repair of DNA damage and the biological effects of the persistence of these lesions, much remains to be learned about the mechanism and tissue-specificity of repair in plants. This review focuses on recent work on the induction and repair of DNA damage in higher plants, with special emphasis on UV-induced DNA damage products.

Amino acid residues affecting the activity and stability of human O6-alkylguanine-DNA alkyltransferase

Amino acid residues in the human O6-alkylguanine-DNA alkyltransferase (AGT) were mutated and seventeen of the mutant proteins expressed in the ada- ogt-E. coli strain GWR 109 which is very sensitive to killing by methylating agents because of the absence of endogenous alkyltransferases. Thirteen of the mutations tested (delta-10, delta 1-19, R128A, N137A, H146A, R147A, delta N157, Y158A, E172Q, delta 92-97, Y114E, C145A and E172stop) reduced activity to below detectable levels when crude cell extracts were tested for the ability to remove O6-[3H]methylguanine from 3H-methylated DNA. However, only 4 of these mutations (delta 92-97, Y114E, C145A and E172stop) led to a complete loss of activity when tested for the ability to protect the cells from killing by MNNG. This suggests that the other nine mutations do not lead to the complete inactivation of AGT but produce protein with a reduced activity or in reduced amounts. These results show that none of the residues altered in these mutations (delta 1-10, delta 1-19, R128A, N137A, delta N157, H146A, R147A, Y158A and E172Q) are absolutely essential for AGT activity in protection against killing by MNNG. The stability of the mutant AGT proteins was determined by measuring the half-life of the protein synthesis was blocked. These results indicated that five of mutants that lacked AGT activity when tested in the crude extracts (Y114E, R128A, C145A, delta N157 and Y158A) were stable in the cell showing that the alteration of these residues does greatly reduce AGT activity. The other eight mutants lacking activity in crude extracts (delta 1-10, delta 92-97, E172Q, E172stop, delta 1-19, N137A, H146A and R147A) produced a large decrease in the stability of the AGT protein. This may account for the inability to detect AGT activity in vitro despite the ability to protect from MNNG toxicity in vivo. It is of particular interest that mutation of residues His146, Arg147, Asn137 and Glu172 resulted in unstable AGT proteins active in vivo but not in vitro. The crystal structure of the related Ada-C alkyltransferase suggests the involvement of these residues with the Cys145 acceptor site in a hydrogen bond network that may stabilize the protein and aid in the reaction mechanism. The data presented here support the existence of such an interaction existing in the human AGT and stress its importance in maintaining the configuration of the protein.

DNA-repair methyltransferase as a molecular device for preventing mutation and cancer

Alkylation of DNA at the 0(6) position of guanine is regarded as one o f the most critical events leading to induction of mutations and cancers in organisms. Once 0(6)-methylguanine is formed, it can pair with thymine during DNA replication, the result being a conversion of the guanine.cytosine to an adenine.thymine pair in DNA, and such mutations are often found in tumors induced by alkylating agents. To counteract such effects, organisms possess a mechanism to repair 0(6)-methylguanine in DNA. An enzyme, 0(6)-methylguanine-DNA methyltransferase, is present in various organism, from bacteria to human cells, and appears to be responsible for preventing the occurrence of such mutations. The enzyme transfers methyl groups from 0(6)-methylguanine and other methylated moieties of the DNA to its own molecule, thereby repairing DNA lesions in a single-step reaction. To elucidate the role of methyltransferase in preventing cancers, animal models with altered levels of enzyme activity were generated. Transgenic mice carrying the foreign methyltransferase gene with functional promoters had higher levels of methyltransferase activity and showed a decreased susceptibility to N-nitroso compounds in regard to liver carcinogenesis. Mouse lines deficient in the methyltransferase gene, which were established by gene targeting, exhibited an extraordinarily high sensitivity to an alkylating carcinogen.

Molecular cloning and functional analysis of a human cDNA encoding an Escherichia coli AlkB homolog, a protein involved in DNA alkylation damage repair

The Escherichia coli AlkB protein is involved in protecting cells against mutation and cell death induced specifically by SN2-type alkylating agents such as methyl methanesulfonate (MMS). A human cDNA encoding a polypeptide homologous to E.coli AlkB was discovered by searching a database of expressed sequence tags (ESTs) derived from high throughput cDNA sequencing. The full-length human AlkB homolog (hABH) cDNA clone contains a 924 bp open reading frame encoding a 34 kDa protein which is 52% similar and 23% identical to E.coli AlkB. The hABH gene, which maps to chromosome 14q24, was ubiquitously expressed in 16 human tissues examined. When hABH was expressed in E.coli alkB mutant cells partial rescue of the cells from MMS-induced cell death occurred. Under the conditions used expression of hABH in skin fibroblasts was not regulated by treatment with MMS. Our findings show that the AlkB protein is structurally and functionally conserved from bacteria to human, but its regulation may have diverged during evolution.

Suppression of Escherichia coli alkB mutants by Saccharomyces cerevisiae genes

The alkB gene is one of a group of alkylation-inducible genes in Escherichia coli, and its product protects cells from SN2-type alkylating agents such as methyl methanesulfonate (MMS). However, the precise biochemical function of the AlkB protein remains unknown. Here, we describe the cloning, sequencing, and characterization of three Saccharomyces cerevisiae genes (YFW1, YFW12, and YFW16) that functionally complement E. coli alkB mutant cells. DNA sequence analysis showed that none of the three gene products have any amino acid sequence homology with the AlkB protein. The YFW1 and YFW12 proteins are highly serine and threonine rich, and YFW1 contains a stretch of 28 hydrophobic residues, indicating that it may be a membrane protein. The YFW16 gene turned out to be allelic with the S. cerevisiae STE11 gene. STE11 is a protein kinase known to be involved in pheromone signal transduction in S. cerevisiae; however, the kinase activity is not required for MMS resistance because mutant STE11 proteins lacking kinase activity could still complement E. coli alkB mutants. Despite the fact that YFW1, YFW12, and YFW16/STE11 each confer substantial MMS resistance upon E. coli alkB cells, S. cerevisiae null mutants for each gene were not MMS sensitive. Whether these three genes provide alkylation resistance in E. coli via an alkB-like mechanism remains to be determined, but protection appears to be specific for AlkB-deficient E. coli because none of the genes protect other alkylation-sensitive E. coli strains from killing by MMS.

Expression of Escherichia coli recA and ada genes in Saccharomyces cerevisiae using a vector with geneticin resistance

Construction of E. coli-yeast shuttle plasmids containing the neo selection gene is described. The protein-coding regions of the E. coli ada or recA genes under the control of the ADH1 promoter and terminator were ligated into the SphI unique site of pNF2 to produce pMSada and pMSrecA, respectively. The plasmids were used for transformation of the haploid and diploid pso4-1 strains of S. cerevisiae and their corresponding wild types. Transformants were obtained by selection for geneticin (G418) resistance. Crude protein samples were extracted from the individual transformants. Both the RecA and Ada proteins were present in all strains containing the recA and ada genes on plasmids, respectively. Thus the geneticin selection system was successfully used for the preparation of model yeast strains.

SNG1--a new gene involved in nitrosoguanidine resistance in Saccharomyces cerevisiae

We have molecularly characterized the SNG1 gene that confers hyper-resistance to the mutagen N-methyl-N'nitro-N-nitrosoguanidine (MNNG) in Saccharomyces cerevisiae when overexpressed on a multi-copy plasmid. This hyper-resistance to MNNG is not due to depletion of glutathione pools since multi-copy SNG1 containing yeast transformants contain at least wild type levels of glutathione; DNA repair seems unaffected in these transformants as the multi-copy SNG1-mediated MNNG hyper-resistance is also seen in DNA repair mutants belonging to each of the three epistasis groups of yeast repair mutants. It could be shown that SNG1 is not under control of the YAP1 encoded transcription activator that controls expression of at least two genes involved in MNNG metabolism in yeast. sng1 null mutants are viable but exhibit only slight sensitivity to MNNG, indicating that SNG1 does not encode a protein involved in a major detoxification step of this mutagen. Sequencing of the HYR-mediating passenger DNA revealed that SNG1 encodes a 547 a polypeptide containing seven transmembrane-spanning regions that may be membrane-bound. Comparison of the DNA sequence with established gene databanks revealed that SNG1 is a novel yeast gene.

Expression of the human MGMT O6-methylguanine DNA methyltransferase gene in a yeast alkylation-sensitive mutant: its effects on both exogenous and endogenous DNA alkylation damage

Common Mer- cell lines deficient in O6-methylguanine DNA methyltransferase (MTase) activity probably result from the down-regulation of, rather than mutations in, the MGMT gene. However, the down-regulation of other unrelated genes was also observed in some of these cell lines, making it difficult to determine the precise functions of the MGMT MTase gene. To study the biological function of human MGMT MTase, we seek to utilize a newly created yeast mgt1 mutant deficient in the DNA repair MTase activity. The human MGMT cDNA was cloned into yeast expression vectors so that the MGMT gene is under the control of either an inducible GAL1 promoter or a constitutive ADH1 promoter. Upon galactose induction, the PGAL1-MGMT transformant had about 40-fold MTase activity compared to the wild-type strain. MGMT overexpression protected the yeast mgt1 mutant against alkylation-induced killing and mutation. Limited expression of the MGMT gene in the mgt1 mutant still provides significant alkylation resistance, albeit at a reduced level. The yeast mgt1 mutants increase spontaneous mutation rate, whereas constitutive expression of the MGMT gene lowered the spontaneous mutation rate in the mgt1 mutant to the wild-type level. We suggest that MGMT MTase may play the same role in human cells as the MGT1 MTase in yeast cells. Thus our results demonstrate that the human MGMT gene functionally complements the yeast MTase-deficient mutant in the protection against exogenous and endogenous DNA alkylation damage, which provides a useful tool for the study of in vivo mammalian MTase functions.

Construction and characterization of mutants of Salmonella typhimurium deficient in DNA repair of O6-methylguanine

Escherichia coli has two O6-methylguanine DNA methyltransferases that repair alkylation damage in DNA and are encoded by the ada and ogt genes. The ada gene of E. coli also regulates the adaptive response to alkylation damage. The closely related species Salmonella typhimurium possesses methyltransferase activities but does not exhibit an adaptive response conferring detectable resistance to mutagenic methylating agents. We have previously cloned the ada-like gene of S. typhimurium (adaST) and constructed an adaST-deletion derivative of S. typhimurium TA1535. Unexpectedly, the sensitivity of the resulting strain to the mutagenic action of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was similar to that of the parent strain. In this study, we have cloned and sequenced the ogt-like gene of S. typhimurium (ogtST) and characterized ogtST-deletion derivatives of TA1535. The ogtST mutant was more sensitive than the parent strain to the mutagenicity of MNNG and other simple alkylating agents with longer alkyl groups (ethyl, propyl, and butyl). The adaST-ogtST double mutant had a level of hypersensitivity to these agents similar to that of the ogtST single mutant. The ogtST and the adaST-ogtST mutants also displayed a two to three times higher spontaneous mutation frequency than the parent strain and the adaST mutant. These results indicate that the OgtST protein, but not the AdaST protein, plays a major role in protecting S. typhimurium from the mutagenic action of endogenous as well as exogenous alkylating agents.

Requirement of the Pro-Cys-His-Arg sequence for O6-methylguanine-DNA methyltransferase activity revealed by saturation mutagenesis with negative and positive screening

O6-Methylguanine-DNA methyltransferase catalyzes transfer of a methyl group from O6-methylguanine and O4-methylthymine of DNA to a cysteine residue of the enzyme protein, thereby repairing the mutagenic and carcinogenic lesions in a single-step reaction. There are highly conserved amino acid sequences around the methyl-accepting cysteine site in eleven molecular species of methyltransferases. To elucidate the significance of the conserved sequence, amino acid substitutions were introduced by site-directed mutagenesis of the cloned DNA for Escherichia coli Ogt methyltransferase, and the activity and stability of mutant forms of the enzyme were examined. When cysteine-139, to which methyl transfer occurs, was replaced by other amino acids, all of the mutants showed the methyltransferase-negative phenotype. Methyltransferase-positive revertants, isolated from one of the negative mutants, had restored codons for cysteine. Thus the cysteine residue is essential for acceptance of the methyl group and is not replaceable by other amino acids. Using this negative and positive selection procedure, the analysis was extended to other residues near the acceptor site. At the histidine-140 and arginine-141 sites, all the positive revertants isolated carried codons for amino acids identical to those of the wild-type protein. At proline-138, five substitutions (serine, glutamine, threonine, histidine, and alanine) exhibited the positive phenotype but levels of methyltransferase activity in extracts of cells harboring these mutant forms were very low. This suggests that the proline residue at this site is important for maintaining the proper conformation of the protein. With valine-142 substitutions there were seven types of positive revertants, among which mutants carrying isoleucine, cysteine, leucine, and alanine showed relatively high levels of methyltransferase activity. These results indicate that the sequence Pro-Cys-His-Arg is a sine qua non for methyltransferase to exert its function.

Overexpression of the SNQ3/YAP1 gene confers hyper-resistance to nitrosoguanidine in Saccharomyces cerevisiae via a glutathione-independent mechanism

The MNNG hyper-resistance of yeast transformants containing multiple copies of the SNQ3/YAP1 yeast gene is not caused by lowered MNNG activation due to depleted pools of glutathione. On the contrary, the SNQ3/YAP1-encoded protein stimulates production of GSH, apparently by promoter activation due to the AP-1 recognition element. Expression of at least one further gene, encoding a protein with a strong detoxifying activity, must also be stimulated to explain the MNNG hyper-resistance phenotype.

Characterisation and correction of a mammalian cell mutant defective in late step of base excision repair

An Indian muntjac cell line, SVM, is unusually sensitive to cell killing induced by a range of alkylating agents. Cells transfected with the Escherichia coli ada gene or human genomic DNA have allowed the response of SVM to alkylating agents to be dissociated into two distinct components. Thus, in SVM, which expresses very low levels of alkyltransferase (AT), O6-alkylguanine appears to be the major cytotoxic, clastogenic, and recombinogenic lesion following exposure to agents such as methylnitrosourea (MNU). However, SVM is also very sensitive to agents such as dimethylsulfate (DMS), which produce only very low levels of O6-methylguanine damage. Sensitivity to DMS resides in an inability to complete base excision repair, with the appearance of persistent single-strand DNA breaks (SSBs), and does not appear to involve defects in glycosylase, apurinic/apyrimidinic endonuclease, or DNA ligase activities. Another, possibly related, phenotypic trait in SVM is its limited ability to ligate transfected linear plasmid DNA. Transfectants of SVM, harboring human DNA sequences, show a significant correction of DMS-induced cytotoxicity and clastogenicity and a reduction in the levels of DMS-induced DNA SSBs. The DMS-resistant transfectants have an increased ability to ligate linear plasmid DNA, and also express AT, making these lines resistant to alkylating agents such as MNU. These results suggest that cells possess a mechanism that regulates AT expression, plasmid break-joining ability, and certain aspects of base excision repair. Transfectants of SVM containing human DNA provide a means to isolate genes involved in a coordinate response to alkylation damage.

The suicidal DNA repair methyltransferases of microbes

Virtually every organism so far tested has been found to possess an extremely efficient DNA repair mechanism to ensure that certain alkylated oxygens do not accumulate in the genome. The repair is executed by DNA methyltransferases (MTases) which repair DNA O6-methylguanine (O6MeG), O4-methylthymine (O4MeT) and methylphosphotriesters (MePT). The mechanism is rather extravagant because an entire protein molecule is expended for the repair of just one, or sometimes two, O-alkyl DNA adduct(s). Cells profit from such an expensive transaction by earning protection against death and mutation by alkylating agents. This review considers the structure, function and biological roles of a number of well-characterized microbial DNA repair MTases.