Bromouracil mutagenesis in Escherichia coli evidence for involvement of mismatch repair

DNA Mismatch Repair

DNA mismatch repair (MMR) corrects replication errors in newly synthesized DNA. It also has an antirecombination action on heteroduplexes that contain similar but not identical sequences. This review focuses on the genetics and development of MMR and not on the latest biochemical mechanisms. The main focus is on MMR in Escherichia coli, but examples from Streptococcuspneumoniae and Bacillussubtilis have also been included. In most organisms, only MutS (detects mismatches) and MutL (an endonuclease) and a single exonucleaseare present. How this system discriminates between newlysynthesized and parental DNA strands is not clear. In E. coli and its relatives, however, Dam methylation is an integral part of MMR and is the basis for strand discrimination. A dedicated site-specific endonuclease, MutH, is present, andMutL has no endonuclease activity; four exonucleases can participate in MMR. Although it might seem that the accumulated wealth of genetic and biochemical data has given us a detailed picture of the mechanism of MMR in E. coli, the existence of three competing models to explain the initiation phase indicates the complexity of the system. The mechanism of the antirecombination action of MMR is largely unknown, but only MutS and MutL appear to be necessary. A primary site of action appears to be on RecA, although subsequent steps of the recombination process can also be inhibited. In this review, the genetics of Very Short Patch (VSP) repair of T/G mismatches arising from deamination of 5-methylcytosineresidues is also discussed.

Integration of Principles of Systems Biology and Radiation Biology: Toward Development of in silico Models to Optimize IUdR-Mediated Radiosensitization of DNA Mismatch Repair Deficient (Damage Tolerant) Human Cancers

Over the last 7 years, we have focused our experimental and computational research efforts on improving our understanding of the biochemical, molecular, and cellular processing of iododeoxyuridine (IUdR) and ionizing radiation (IR) induced DNA base damage by DNA mismatch repair (MMR). These coordinated research efforts, sponsored by the National Cancer Institute Integrative Cancer Biology Program (ICBP), brought together system scientists with expertise in engineering, mathematics, and complex systems theory and translational cancer researchers with expertise in radiation biology. Our overall goal was to begin to develop computational models of IUdR- and/or IR-induced base damage processing by MMR that may provide new clinical strategies to optimize IUdR-mediated radiosensitization in MMR deficient (MMR⁻) “damage tolerant” human cancers. Using multiple scales of experimental testing, ranging from purified protein systems to in vitro (cellular) and to in vivo (human tumor xenografts in athymic mice) models, we have begun to integrate and interpolate these experimental data with hybrid stochastic biochemical models of MMR damage processing and probabilistic cell cycle regulation models through a systems biology approach. In this article, we highlight the results and current status of our integration of radiation biology approaches and computational modeling to enhance IUdR-mediated radiosensitization in MMR⁻ damage tolerant cancers.

The DNA N-glycosylase MED1 exhibits preference for halogenated pyrimidines and is involved in the cytotoxicity of 5-iododeoxyuridine

The base excision repair protein MED1 (also known as MBD4), an interactor with the mismatch repair protein MLH1, has a central role in the maintenance of genomic stability with dual functions in DNA damage response and repair. MED1 acts as a thymine and uracil DNA N-glycosylase on T:G and U:G mismatches that occur at cytosine-phosphate-guanine (CpG) methylation sites due to spontaneous deamination of 5-methylcytosine and cytosine, respectively. To elucidate the mechanisms that underlie sequence discrimination by MED1, we did single-turnover kinetics with the isolated, recombinant glycosylase domain of MED1. Quantification of MED1 substrate hierarchy confirmed MED1 preference for mismatches within a CpG context and showed preference for hemimethylated base mismatches. Furthermore, the k(st) values obtained with the uracil analogues 5-fluorouracil and 5-iodouracil were over 20- to 30-fold higher than those obtained with uracil, indicating substantially higher affinity for halogenated bases. A 5-iodouracil precursor is the halogenated nucleotide 5-iododeoxyuridine (5IdU), a cytotoxic and radiosensitizing agent. Cultures of mouse embryo fibroblasts (MEF) with different Med1 genotype derived from mice with targeted inactivation of the gene were evaluated for sensitivity to 5IdU. The results revealed that Med1-null MEFs are more sensitive to 5IdU than wild-type MEFs in both 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and colony formation assays. Furthermore, high-performance liquid chromatography analyses revealed that Med1-null cells exhibit increased levels of 5IdU in their DNA due to increased incorporation or reduced removal. These findings establish MED1 as a bona fide repair activity for the removal of halogenated bases and indicate that MED1 may play a significant role in 5IdU cytotoxicity.

Saturation of DNA mismatch repair and error catastrophe by a base analogue in Escherichia coli

Deoxyribosyl-dihydropyrimido[4,5-c][1,2]oxazin-7-one (dP) is a potent mutagenic deoxycytidine-derived base analogue capable of pairing with both A and G, thereby causing G. C --> A. T and A. T --> G. C transition mutations. We have found that the Escherichia coli DNA mismatch-repair system can protect cells against this mutagenic action. At a low dose, dP is much more mutagenic in mismatch-repair-defective mutH, mutL, and mutS strains than in a wild-type strain. At higher doses, the difference between the wild-type and the mutator strains becomes small, indicative of saturation of mismatch repair. Introduction of a plasmid containing the E. coli mutL(+) gene significantly reduces dP-induced mutagenesis. Together, the results indicate that the mismatch-repair system can remove dP-induced replication errors, but that its capacity to remove dP-containing mismatches can readily be saturated. When cells are cultured at high dP concentration, mutant frequencies reach exceptionally high levels and viable cell counts are reduced. The observations are consistent with a hypothesis in which dP-induced cell killing and growth impairment result from excess mutations (error catastrophe), as previously observed spontaneously in proofreading-deficient mutD (dnaQ) strains.

Illusory defects and mismatches: why must DNA repair always be (slightly) error prone?

There is growing evidence that recombination is mu;tagenic and that some forms of DNA repair synthesis are error prone. DNA synthesis in mismatch repair might also be error prone. DNA-repair systems detect structural defects in DNA with high efficiency but they occasionally also strike at normal sections of DNA. Considering the diversity of local DNA structure, some DNA sections with complementary sequences are bound to act as preferential false targets for a repair system (i.e. as "illusory defects"). However, if the repair system never changes the sequence upon repair, it will be solicited again and again by the illusory defect, a potentially harmful situation. It is therefore advantageous for a repair system to be, to some extent, error prone. Strong illusory defects may arise at the decanucleotide level and could be the cause of local increases in mutation levels. They might be used to initiate somatic hypermutation pathways.

The role of DNA mismatch repair in cisplatin mutagenicity

Cisplatin (DDP) is used with varying success for the treatment of a wide spectrum of human cancers. The most abundant lesions produced in DNA are intrastrand crosslinks, which are believed to account for not only the cytotoxic action but also the mutagenicity of the drug. The molecular basis for the mutagenicity of DDP adducts is believed to be related to bypass replication across the adducts by DNA polymerase. This results in misincorporation of non-complimentary bases by polymerase beta which, if left unpaired, will generate point or frameshift mutations. An important replication-associated correction function is provided by the post-replicative DNA mismatch repair (MMR) system. Loss of MMR activity is well documented to result in increased mutation rates and instability of genomic DNA. Inactivation of the MMR system also augments the intrinsic mutagenicity of DDP and enhances the risk of developing cells resistant to other drugs commonly used in combination with DDP. A future challenge will be to assess the clinical significance of the presence of MMR-deficient cells in tumors, and investigate new approaches to circumvent such multidrug resistance.

Enzymatic repair of 5-formyluracil. II. Mismatch formation between 5-formyluracil and guanine during dna replication and its recognition by two proteins involved in base excision repair (AlkA) and mismatch repair (MutS)

5-Formyluracil (fU), a major methyl oxidation product of thymine, forms correct (fU:A) and incorrect (fU:G) base pairs during DNA replication. In the accompanying paper (Masaoka, A., Terato, H., Kobayashi, M., Honsho, A., Ohyama, Y., and Ide, H. (1999) J. Biol. Chem. 274, 25136-25143), it has been shown that fU correctly paired with A is recognized by AlkA protein (Escherichia coli 3-methyladenine DNA glycosylase II). In the present work, mispairing frequency of fU with G and cellular repair protein that specifically recognized fU:G mispairs were studied using defined oligonucleotide substrates. Mispairing frequency of fU was determined by incorporation of 2'-deoxyribonucleoside 5'-triphosphate of fU opposite template G using DNA polymerase I Klenow fragment deficient in 3'-5' exonuclease. Mispairing frequency of fU was dependent on the nearest neighbor base pair in the primer terminus and 2-12 times higher than that of thymine at pH 7.8 and 2.6-6.7 times higher at pH 9.0 with an exception of the nearest neighbor T(template):A(primer). AlkA catalyzed the excision of fU placed opposite G, as well as A, and the excision efficiencies of fU for fU:G and fU:A pairs were comparable. In addition, MutS protein involved in methyl-directed mismatch repair also recognized fU:G mispairs and bound them with an efficiency comparable to T:G mispairs, but it did not recognize fU:A pairs. Prior complex formation between MutS and a heteroduplex containing an fU:G mispair inhibited the activity of AlkA to fU. These results suggest that fU present in DNA can be restored by two independent repair pathways, i.e. the base excision repair pathway initiated by AlkA and the methyl-directed mismatch repair pathway initiated by MutS. Biological relevance of the present results is discussed in light of DNA replication and repair in cells.

Dominant negative mutator mutations in the mutL gene of Escherichia coli

The mutL gene product is part of the dam-directed mismatch repair system of Escherichia coli but has no known enzymatic function. It forms a complex on heteroduplex DNA with the mismatch recognition MutS protein and with MutH, which has latent endonuclease activity. An N-terminal hexahistidine-tagged MutL was constructed which was active in vivo. As a first stop to determine the functional domains of MutL, we have isolated 72 hydroxylamine-induced plasmid-borne mutations which impart a dominant-negative phenotype to the wild-type strain for increased spontaneous mutagenesis. None of the mutations complement a mutL deletion mutant, indicating that the mutant proteins by themselves are inactive. All the dominant mutations but one could be complemented by the wild-type mutL at about the same gene dosage. DNA sequencing indicated that the mutations affected 22 amino acid residues located between positions 16 and 549 of the 615 amino acid protein. In the N-terminal half of the protein, 12 out of 15 amino acid replacements occur at positions conserved in various eukaryotic MutL homologs. All but one of the sequence changes affecting the C-terminal end of the protein are nonsense mutations.

Nucleoside and nucleobase analog mutagens

Compounds with structures close to those of normal nucleosides or nucleobases may be incorporated into cells and then become constituents of their DNA. Proliferation of such cells could yield mutants. In this article, the current status of studies on such nucleoside and nucleobase analogs is described. Base mispairing mechanisms for these analogs are discussed in light of recent biochemical and biophysical findings.

The extreme mutator effect of Escherichia coli mutD5 results from saturation of mismatch repair by excessive DNA replication errors

Escherichia coli mutator mutD5 is the most potent mutator known. The mutD5 mutation resides in the dnaQ gene encoding the proofreading exonuclease of DNA polymerase III holoenzyme. It has recently been shown that the extreme mutability of this strain results, in addition to a proofreading defect, from a defect in mutH, L, S-encoded postreplicational DNA mismatch repair. The following measurements of the mismatch-repair capacity of mutD5 cells demonstrate that this mismatch-repair defect is not structural, but transient. mutD5 cells in early log phase are as deficient in mismatch repair as mutL cells, but they become as proficient as wild-type cells in late log phase. Second, arrest of chromosomal replication in a mutD5-dnaA(Ts) strain at a nonpermissive temperature restores mismatch repair, even from the early log phase of growth. Third, transformation of mutD5 strains with multicopy plasmids expressing the mutH or mutL gene restores mismatch repair, even in rapidly growing cells. These observations suggest that the mismatch-repair deficiency of mutD strains results from a saturation of the mutHLS-mismatch-repair system by an excess of primary DNA replication errors due to the proofreading defect.

On the glucose effect in acridine-induced frameshift mutagenesis in Escherichia coli

Log-phase cells of E. coli growing in defined minimal media were washed, exposed to acridines in the same minimal salts solution, and plated to select for Nad+ revertants. At low mutagen concentration, treatment in the presence of the carbon source to which the cells were adapted resulted in a decrease in revertant yield of several orders of magnitude compared with the yield in the absence of a carbon source. At high mutagen concentration, however, a carbon source present during treatment caused a 2- to 150-fold increase in revertant yield (depending on the mutagen, the carbon source, and on the genetic background of the strain). In a strain lacking adenylate cyclase, acridine mutagenesis was not abolished under the experimental conditions used in this study, and the addition of cAMP during mutagenic treatment had no effect. In mismatch repair-deficient strains, the presence of glucose during treatment with low mutagen concentration did not cause a decrease in revertant yield as drastic as in the wild type. From the results reported here, we conclude that the glucose effect in acridine mutagenesis is due to an enhancement of mismatch repair.

Mutagenic nucleoside analog N4-aminocytidine: metabolism, incorporation into DNA, and mutagenesis in Escherichia coli

N4-Aminocytidine, a nucleoside analog, is strongly mutagenic to various organisms including Escherichia coli. Using E. coli WP2 (trp), we measured the incorporation of [5-3H]N4-aminocytidine into DNA and at the same time measured the frequency of reversion of the wild type, thereby attempting to correlate the incorporation with mutation induction. First, we observed that N4-aminocytidine uptake by the E. coli cells was as efficient as cytidine uptake. High-pressure liquid chromatographic analysis of nucleoside mixtures obtained by enzymatic digestion of isolated cellular DNA showed that the DNA contained [3H]N4-aminodeoxycytidine, corresponding to 0.01 to 0.07% of the total nucleoside; the content was dependent on the dose of N4-aminocytidine. There was a linear relationship between the N4-aminocytosine content in DNA and the mutation frequency observed. These results constitute strong evidence for the view that the N4-aminocytidine-induced mutation in E. coli is caused by the incorporation of this agent into DNA as N4-aminodeoxycytidine. We also found that the major portion of radioactivity in DNA of cells that had been treated with [5-3H]N4-aminocytidine was in the deoxycytidine fraction. We propose a metabolic pathway for N4-aminocytidine in cells of E. coli. This pathway involves the formation of both N4-aminodeoxycytidine 5'-triphosphate and deoxycytidine 5'-triphosphate; the deoxycytidine 5'-triphosphate formation is initiated by conversion of N4-aminocytidine into uridine. In support of this proposed scheme, a cytidine deaminase preparation obtained from E. coli catalyzed the decomposition of N4-aminocytidine into uridine and hydrazine.

Ability of base analogs to induce the SOS response: effect of a dam mutation and mismatch repair system

2-Aminopurine, 2-amino-N6-hydroxyadenine, 2-amino-N6-methoxyadenine and 2-amino-N6-methyl-N6-hydroxyadenine (but not N4-hydroxycytidine), strong mutagens of base analog type, may induce the SOS response in E. coli cells. This ability is greatly enhanced in dam3 mutants and abolished in dam3mutS, dam3mutH, and dam3mutL strains, thereby suggesting that the mismatch repair system is involved in the mechanism of induction.

Involvement of DNA lesions and SOS functions in 5-bromouracil-induced mutagenesis

Mutagenesis resulting from incorporation of 5-bromouracil (BU) in the DNA of E. coli K12 proceeds largely (approximately 80%) via misrepair of the lesions resulting from incorporation of the analogue. The premutational lesions are due principally to dehalogenation of incorporated BU residues, leading to formation of uracil residues, and removal of these by uracil-DNA glycosylase with formation of apyrimidinic sites. In the xthA mutant, defective in AP endonuclease, there is a several-fold increase in the frequency of BU-induced mutations, underlining the importance of AP sites in BU-induced mutagenesis. Premutational lesions undergo mutation frequency decline (MFD), which is subject to delay in the xthA mutant, pointing to some role of AP endonuclease in MFD, and further supporting involvement of AP sites in BU-induced mutagenesis. Efficient BU mutagenesis is dependent on the functions of the genes recA and umuC and non-mutated lexA protein.

Frameshift mutagenesis of lambda prophage by 9-aminoacridine, proflavin and ICR-191

The changes in DNA base sequence induced in the lambda cI gene in an E. coli lysogen have been determined following mutagenesis by three acridine derivatives: 9-aminoacridine and proflavin, which bind reversibly to DNA; and ICR-191, which attaches covalently to DNA through a half-mustard group. For all three derivatives, most mutations are +1 and -1 frameshifts in runs of adjacent G:C pairs. The specificity of mutagenesis at various sites is similar for all three compounds. Prophage in mutL host cells, deficient in mismatch repair, are much more susceptible to mutagenesis by 9-aminoacridine. The induced mutations are also frameshifts, and the site specificity is the same as in lysogens of wild type cells. Thus, additions or deletions of single bases can be corrected by the mismatch repair system, but mismatch repair does not play an important role in determining the sequence specificity of the mutational events.

Induction of SOS functions in Escherichia coli by lesions resulting from incorporation of 5-bromouracil into DNA

Lesions induced by 5-bromouracil (BU), after its incorporation into DNA, led to effective induction of prophage lambda and W reactivation (or BU reactivation). Prophage induction due to incorporated BU occurred only with the wild-type prophage, and not for the lambda c1857 mutant with a thermosensitive repressor. Antipain, a protease inhibitor, inhibited wild-type prophage induction 70-90%. This indicates that BU-induced lesions may induce the SOS repair system. The finding that such lesions provoke BU reactivation permits the inference that BU-induced mutagenesis also proceeds via involvement of the error-prone repair system, and not directly as a result of base-pairing errors. Genetic evidence suggests that induction of the SOS repair system as a result of incorporation of BU into DNA is linked to the subsequent appearance of uracil residues and apyrimidinic sites, resulting from dehalogenation of incorporated BU. Apyrimidinic sites appear to be more effective than uracil residues in induction of the SOS system.

Involvement of the mismatch repair system in base analogue-induced mutagenesis

The influence of the mismatch repair system, and the role of mutS, mutR, and mutL genes, in mutagenesis induced by n2ome6Ade, n2oh6Ade and n2Pur have been investigated. From the frequency of reversion of Arg-Thr- and His- markers in AB2497, and its mut- derivatives, it was concluded that mismatches introduced by n2-ome6Ade and n2oh6Ade are better substrates for mismatch repair enzymes than that introduced by n2Pur. All these mut-gene products are more active in removing spontaneous or base analogue-induced mismatches which, when unexcised, lead to transversion of base repairs, than those which lead to transitions. Active engagements of mutL, mutR, or mutS gene products depend on the kind of mutation, the site of mutagenesis, and the inducing agent. Dam- cells are over-mutated by both n2ome6Ade and n2oh6Ade, but are hyper-sensitive to n2oh6Ade only. It is proposed that hyper-sensitivity of dam- cells is due not only to an increase in overlaping gap formation on both strands of DNA, but to a greater lability of the impaired cells. Results are presented which strongly suggest that n2ome6Ade in mut+ cells and n2oh6Ade in mut- only, can induce GC leads to TA transversions.

Effect of proofreading and dam-instructed mismatch repair systems on N4-hydroxycytidine-induced mutagenesis

The role of the proofreading (3' leads to 5' exonuclease) function of T4 DNA polymerase and the mismatch repair system of E. coli on N4-hydroxycytidine (oh4Cyd) induced mutagenesis was investigated. oh4Cyd-induced mutation is strongly suppressed when the proofreading activity increases as a result of the presence of tsCB87--antimutator polymerase or elevated temperature (43 degrees C vs 30 degrees C). Mutagenic activity of oh4Cyd, however, is little, if at all, affected by the presence of the tsLB56 mutator allele of T4 DNA polymerase with suppressed proofreading activity. This leads to the conclusion that oh4Cyd nucleotides are not frequently removed by proofreading activity of wild-type T4 DNA polymerase. The number of mutations induced by oh4Cyd increases 3- to 5-fold due to damage of the genes mutS, mutL, uvrE, but not mutR. Dam- cells are more sensitive to, and hypermutable by, oh4Cyd in comparison with dam+ cells. This is compatible with the notion that oh4C residues are recognised and excised by mismatch repair enzymes. The results indicate that neither the proofreading function of T4 DNA polymerase, nor the mismatch repair enzymes, are responsible for the high specificity of oh4Cyd which causes AT leads to GC transition.

Mutants of Escherichia coli K12 which affect excision of transposon Tn10

We have described three illegitimate recombination events associated with, but not promoted by, transposon Tn10: precise excision, nearly precise excision, and precise excision of a nearly precise excision remnant. All three are structurally analogous: excision occurs between two short direct repeat sequences, removing all intervening material plus one copy of the direct repeat. In each case, the direct repeats border a larger inverted repeat. We report here the isolation of host mutants of Escherichia coli K12 which exhibit increased frequencies of precise excision of Tn10. Nineteen of the 39 mutants have been mapped to five distinct loci on the E. coli genetic map and have been designated texA through texE (for Tn10 excision). Mapping and genetic characterization indicate that each tex gene corresponds to a previously identified gene involved in cellular DNA metabolism: recB and/or recC, uvrD, mutH, mutS, and dam. The role of these various DNA repair and recombination genes in an illegitimate recombination process such as Tn10 excision will be discussed. In addition to an increase in precise excision frequency, all 39 tex mutants display an increased frequency for nearly precise excision. However, none of the mutants are increased for the third excision event, precise excision of a nearly precise excision remnant, supporting the idea that precise and nearly precise excision occur by closely related pathways which are distinct from those pathways which promote the third type of excision event.

Molecular cloning of the uvrD gene of Escherichia coli that controls ultraviolet sensitivity and spontaneous mutation frequency

The uvrD gene of Escherichia coli that controls UV sensitivity and spontaneous mutation frequency has been cloned with phage lambda as vector. The increased sensitivity to ultraviolet light (UV) of uvrD3, uvrE502, recL152, and pdeB41 mutants, high mutability of uvrD3 and pdeB41 mutants, and conditional lethality of strain TS41 that carried pdeB41, polA1, and supl26 mutations were all suppressed by lysogenization of the mutant cells with lambda uvrD+. These results were consistent with the idea that the uvrD, uvrE, recL, and pdeB mutations are alleles of the uvrD gene. In addition to the uvrD gene, lambda uvrD+ carried the corA gene that controls transport of Mg++, Mn++, and Co++ through the cell membrane. Hybrid plasmids carrying both uvrD and corA genes were also constructed by using pKY2289 as a cloning vehicle. Orientational isomers that carried the same 12.0 kb fragment in the opposite direction were equally efficient in complementing the UvrD- as well as CorA- defects of the transformed host cells, suggesting that the DNA insert contains all the genetic signals needed to express the two gene products. Insertion of the gamma delta sequence into recombinant plasmids was performed to generate appropriate restriction endonuclease target sites in the cloned DNA fragments.

Influence of DNA-repair deficiencies on MMS- and EMS-induced mutagenesis in Escherichia coli K-12

The mutagenic effects of the alkylating agents, MMS and EMS, were studied by measuring the reversion of an arg (ochre) mutation in various DNA-repair-deficient strains. When compared with the wild-type strain, EMS-induced mutagenesis was reduced in the recA13 strain but not in the lexA3 strain. MMS-induced mutagenesis was reduced to background levels in the recA13 strain and reduced to intermediate levels in the lexA3 strain. The umuC36 strain showed intermediate levels of mutagenesis with both mutagens which suggests that a substantial portion of both MMS- and EMS-induced mutagenesis depends upon this component of the error-prone "SOS" repair pathway. The uvrD101, recL152 and recF143 mutations produced increased levels of MMS-induced reversion but had no effect upon the levels of EMS-induced mutagenesis, suggesting that the pathways affected by these genes may play a role in the error-free repair of MMS but not EMS damage. In contrast, a large increase in the level of mutagenesis was noted in a delta uvrB101 mutant with EMS but not MMS. This hypermutability with EMS was also seen in uvrA6, uvrB5 and uvrC34 mutant strains and suggests a role for excision in the error-free repair of ethylation but not methylation damage to DNA.

Mutagenesis of lambda phage: weigle mutagenesis is induced by coincident lesions in the double helical DNA of the host cell genome

We have studied the increase in mutation in mutagenized lambda phage when the host cells are also irradiated with ultraviolet light, "Weigle mutagenesis." The increase in mutation is induced mainly on coincidences between a radiation-produced lesion in one strand of the host cell DNA and a second lesion in the complementary strand. This conclusion is based on experiments in which incorporation of the base analog bromouracil sensitized the host cells to ultraviolet light. For the same number of bromouracil incorporated per cell, uniform substitution gave a higher level of Weigle mutagenesis than did substitution in only one strand of the DNA double helix. The data also show some induction of Weigle mutagenesis by processes linear in ultraviolet fluence; possibility include: lesions involving both complementary strands such as crosslinks, lesions in one strand opposite pre-existing discontinuities in the complementary strand, and very small contributions to induction from lesions in one strand only of the DNA.

Non-random distribution of aberrations and identification with C- and G-bandings of the position of breakage points on Muntjac chromosomes induced by mitomycin c, bromodeoxyuridine and hydroxylamine

The analysis of chromosomes from muntjac after treatment of its lymphocyte cultures with 3 chemical mutagens having different base-pair affinities and modes of action, namely mitomycin C (MC), 5-bromodeoxyuridine (BUdR) and hydroxylamine hydrochloride (HA), with G- and C-band staining displayed non-random distribution of chemically specific damage points on them. The randomness of the involvement of each site on the chromosomes were examined by assuming an expected value calculated on the basis of its relative mitotic length. The observation revealed that a large fraction of MC-induced aberrations was preferentially located in the C-band positive constitutive heterochromatin, especially in the long "neck-like" centromeric region of the X-chromosome. On the chromosomal arms, the light G-bands were involved in aberrations either in proportion to or higher than that expected. When the cells were treated with BUdR, the dark G-bands on all the chromosomes of the complement were the preferred sites, displaying statistically significant higher numbers of aberrations. A single "hot-spot" for induced damage on 1 mid-q was also recorded. HA induced a very high frequency of damage in the secondary constriction regions of the chromosome pairs 1, X and Y2, and the frequency was slightly lower than this in the centromeres of 1, 2 and X chromosomes. The observation of specific distribution of damage points induced by the 3 chemicals lead to the suggestion that, though the effect of a chemical on chromosome segments depends on several factors, each being partially responsible for the end result, it is perhaps primarily depended by the chemical's base-pair affinity and mode of action.

Bromouracil-induced mutagenesis in a mismatch-repair-deficient strain of Haemophilus influenzae

Cells of wild-type Haemophilus influenzae and of a mismatch-repair-deficient mutant (hex-) were grown in a chemically defined medium containing either thymidine or 5-bromodeoxyuridine (BUdR). Spontaneous mutation frequencies to resistance against 3 antibiotics observed for the thymidine cultures were 10-30 times higher for the hex- mutant. The mutation frequencies observed for the BUdR hex- culture were increased by another 10 times while those for the wild-type suspension did not differ from the frequencies seen in the thymidine medium.

N4-hydroxycytidine: a mutagen specific for AT to GC transitions

N4-Hydroxycytidine is a mutagen of the base-analog type and one of the products formed by treatment of cytidine with hydroxylamine. In this communication evidence is presented showing that, in contrast with other known base analogs, N4-hydroxycytidine results mainly, if not exclusively, in AT leads to GC transitional alterations in Escherichia coli K12.

Isolation and characterization of Dam+ revertants and suppressor mutations that modify secondary phenotypes of dam-3 strains of Escherichia coli K-12

Bacteria mutant in the dam (DNA adenine methylation) gene and in either recA or recB or recC genes are inviable (Virm- phenotype). From crosses between dam-3 bacteria and recA1 or recB21 recC22 strains, Vrm+ recombinants were recovered. Among these recombinants, Dam+ revertants were present which did not show the phenotypes normally associated with dam-3 bacteria. Three classes of indirectly suppressed strains (dam-3 genotype) were also recovered which showed alterations in the secondary phenotypes normally associated with dam-3 bacteria. These strains contained a second unlinked mutation in either mutL or mutS or sin. In addition, mutation in either sbcA or sbcB supresses the Vrm- phenotype of dam-3 recB21 recC22 strains.

Escherichia coli mutator mutants deficient in methylation-instructed DNA mismatch correction

Our approach to the isolation of DNA mismatch-correction-deficient mutants was based upon the isolation of 2-aminopurine-resistant second-site revertants of Escherichia coli dam- mutants. We isolated such second-site revertants which, when separated from the dam- mutation, have a mutator character of their own. These new mutators all mapped at three known mutator loci, mutH, mutL, and mutS, which exhibit the same mutagenic spectrum as the dam- mutator: increased levels of base substitution and frameshift mutations. The mutator potencies of double and triple mut- mutants suggest that these mutators are involved in the same general mismatch-repair pathway. All these mutations result in a hyper-recombination phenotype, but in four-factor crosses among lambda phages, a specific loss of intragenic recombination (Pam3 X Pam80) was found in mutL and mutS mutants, as would be predicted from the postulated role of mismatch correction in gene conversion and high negative interference phenomena.

Influence of uvrD3, uvrE502, and recL152 mutations on the phenotypes of Escherichia coli K-12 dam mutants

The recF143 allele did not alter the phenotypes of dam mutants of Escherichia coli. The uvrD3, uvrE502, and recL152 mutations did alter some of the phenotypes of dam bacteria. It was concluded that the uvrD, uvrE, and recL gene products are involved in the same deoxyribonucleic acid repair pathway as the dam gene product.

Relationship between repair processes and mutation induction in bacteria

A summary is given of the main repair and replication-associated processes that can influence the induction of mutations by various mutagens in bacteria. These include both constitutive and induced, error-free and error-prone systems. The mutation yield from a treatment with a mutagen can be markedly affected by which of these systems is operating in a given bacterial species or strain. The effect of these systems on mutation induction by ultraviolet light, monofunctional alkylating agents, base analogues, and frameshift mutagens is discussed in some detail. The bearing of these studies on the practical problems of estimating hazards is briefly considered.

Influence of recC and uvrD on ultraviolet-induced mutation to valine resistance in E. coli K12

The production of mutants in E. coli exposed to ultraviolet light is initiated by photochemical reactions, and completed by metabolic processes controlled by recA and other genes. Ultraviolet-induced mutagenesis to valine resistance was measured in cells carrying recC, uvrD, or both recC and uvrD. The spontaneous and UV-induced mutagenesis was slightly greater in those carrying uvrD, as compared to recC or wild-type. At low doses, UV mutagenesis in the recC uvrD double mutant was greater than in either recC or wild-type, and was comparable to that in the uvrD strain, although this double mutant was very UV-sensitive and showed poor survival at doses above 2 J/m2.

A recA-dependent mutator of Escherichia coli K12: method of isolation and initial characterization

A number of mutator strains of E. coli were isolated using histochemical techniques which allow the identification of a single mutator colony on agar plates with as many as 2000 colonies. Several mutators isolated in this way were found by P1-mediated transduction to map to the proA--proB region of the E. coli chromosome. The map position of these mutators is very close to that of the conditional mutator, mutD. However, in contrast to mutD, one of these newly isolated mutators was suppressed in a thermosensitive recA strain at 43 degrees C, but not at 30 degrees C. This mutator mutation has been named mut-8. Besides being dependent upon recA, mut-8 is also dependent upon growth in enriched medium for the expression of its mutator activity. The mutator activity of mut-8 was found to be recessive to the wild-type allele.

Spontaneous mutagenesis in Escherichia coli strains lacking 6-methyladenine residues in their DNA: an altered mutational spectrum in dam- mutants

The mutational spectrum at the lacI locus in a dam-4 strain of Escherichia coli was examined. The observed 20-fold increase in spontaneous mutagenesis in a dam- strain was found to be due to base substitutions, primarily transitions, which had increased 140-fold. Using the trpE997 mutation it was found that the dam mutations also resulted in an increase in frameshift mutagenesis. The mutational spectrum of dam- strains was similar to that found with strains carrying the mutH, mutL, mutS and uvrE mutations thought to result in a defect in the repair of mismatched bases. These results are taken to be consistent with, and to support the hypothesis that, dam- strains are deficient in a post-replicative error-avoidance pathway which allows the directed elimination of mismatch lesions by a mechanism in which parental strands are recognized by their level of DNA methylation.

Genetic evidence for the nature, and excision repair, of DNA lesions resulting from incorporation of 5-bromouracil

Escherichia coli mutants defective in DNA uracil N-glycosidase (ung-) or endonuclease VI active against apurinic/apyrimidinic sites in DNA (xthA-) exhibit enhanced sensitivity towards 5-bromodeoxyuridine relative to the wild type strain, pointing to involvement of these enzymes in repair of bromouracil-induced lesions in DNA. Mutants defective in DNA polymerase I, either in polymerizing activity (polAl-) or (5' leads to 3')-exonuclease activity (polA107-) exhibit unusually high sensitivity (including marked lethality) in the presence of 5-bromodeoxyuridine. The results indicate that DNA polymerase I, and its associated (5'--3')-exonuclease activity, are involved in repair of bromouracil-induced lesions and are not readily replaced, if at all, by DNA polymerases II and III. Thermosensitive mutant in DNA ligase gene (lig ts7) shows high sensitivity towards 5-bromodeoxyuridine at 42 degrees C indicating the role of the enzyme in repair of bromouracil-induced lesions in DNA. Involvement of DNA uracil N-glycosidase, and endonuclease active against apurinic/apyrimidinic sites in recognition and repair of 5-bromouracil-induced damage permits of some inferences regarding the nature of this damage (lesions), in particular dehalogenation of incorporated bromouracil to uracil residues.

Bromouracil mutagenesis and mismatch repair in mutator strains of Escherichia coli

A screening procedure based on the formation of papillae on individual bacterial colonies was used to isolate mutants of Escherichia coli with high mutation rates in the presence of bromouracil. Most of the mutants obtained had high spontaneous mutation rates and mapped close to the previously known mutators mutT, mutS, mutR, uvrE and mutL. Except for mutants of mutT type, these mutators also showed high mutability by bromouracil. Transfection experiments were performed with heteroduplex lambda DNA to test for mismatch repair. The results suggest a reduced efficiency of repair of mismatched bases in mutators mutS, mutR, uvrE and mutL, whereas mutants mapping as mutT appear normal. The results support a connection between spontaneous and bromouracil-induced mutability and repair of mismatched bases in DNA.

Induced mutagenesis in dam- mutants of Escherichia coli: a role for 6-methyladenine residues in mutation avoidance

E. coli strains carrying the dam-3 and dam-4 mutations resulting in reduced levels of 6-methyladenine in the DNA have been found to be more sensitive to base analogue mutagenesis than dam+ strains. Mutagenesis by EMS was also found to be enhanced in dam- strains. Dam- mutants however were not found to be hypermutable by UV light. It is concluded that the dam- strains are deficient in the correct repair of mispairing lesions. The data are consistent with the hypothesis that 6-methyladenine residues in the DNA are involved in strand discrimination during mismatch correction.

The relation of repair phenomena to mutation induction in bacteria

The relation of various processes to mutation induction by radiation and chemicals is discussed for for various species of bacteria. A variety of repair processes have been identified at the molecular level that can eliminate many kinds of potentially mutagenic lesions before they can be converted to final mutation. Fixation often but not always occurs at replication. A number of mutagens, including UV light, ionizing radiation, and a number of chemicals, induce an error-prone process, perhaps a modification of the proof-reading system, that allows bacteria to survive after potentially lethal damage at the expense of making errors. Some mutagens, notably monofunctional alkylating agents and base analogues, produce mutations by other processes. Even in these cases, repair processes play an important role. There is some evidence that error-free as well as error-prone repair processes can be induced. A brief discussion is given of the relation of these findings to the practical problems of hazards estimations.