Dependence of the mutation spectrum in a shuttle plasmid replicated in human lymphoblasts on dose of gamma radiation

Radical Effects on Mutation Spectra in Lambda Phage

Mutations in the lambda repressor gene cI (710 bp) were induced by 60Co-gamma radiation in dissolved lambda phage DNA. After in vitro DNA packaging to lambda phage particles (pack phage) and phenotypic expression of the mutants, DNA was sequenced directly. Two-thirds of mutations were located in the amino terminus region of the gene without any signs of hotspots. Changes consisted of (+1) insertions (25%) and base substitutions (75%). Transitions were exclusively G/C to A/T. Transversions were mostly G/C to C/G and few G/C to T/A. We did not find A/T to T/A transversions, A/T to G/C transitions, deletions and gross rearrangements. In most of the base substitutions a pre-existing base pair had been replaced by an A/T pair; this might come from 'non-instructional sites' like abasic sites. Several mechanisms for base substitutions are considered.

Mutations Caused by γ-radiation-induced Double-strand Breaks in a Shuttle Plasmid Replicated in Human Lymphoblasts

The mutagenicity of open-circular DNA (containing base damage and single-strand breaks) and linear DNA (containing base damage, single-strand breaks, and one double-strand break) produced in vitro by gamma-irradiation of shuttle vector pZ189, was analysed after the plasmid's repair and replication in the human lymphoblast line, GM606. By comparing the survival, mutation frequency, and types of mutations in descendants from the two DNA forms, the effects of the double-strand break were determined. The percentage of viable plasmids from linear DNA was two-fold lower than that from open-circular DNA, 7.8 versus 14.0 (compared with unirradiated, control DNA). The mutation frequency in progenies of the open-circular plasmid was 4.2 +/- 1.7 x 10(-3), compared with 7.8 +/- 0.1 x 10(-3) in progenies of the linear DNA, again, nearly a two-fold difference. Approximately 59% of the mutations from the linear DNA were deletions and 34% were base substitutions. In contrast, only 13% of mutations from open-circular DNA were deletions, but 87% were base substitutions. All recoverable deletions were small, ranging from 1 to 205 base pairs, and the majority contained direct repeats at the deletion junctions, indicating non-homologous recombinations. Thus, mutations found among descendants from the linear and open-circular DNAs were qualitatively similar but quantitatively different. The data suggests that producing one double-strand break in DNA by ionizing radiation causes a two-fold increase in both lethality and mutation frequency.

Mutagenicity of tamoxifen DNA adducts in human endometrial cells and in silico prediction of p53 mutation hotspots

Tamoxifen elevates the risk of endometrial tumours in women and alpha-(N(2)-deoxyguanosinyl)-tamoxifen adducts are reportedly present in endometrial tissue of patients undergoing therapy. Given the widespread use of tamoxifen there is considerable interest in elucidating the mechanisms underlying treatment-associated cancer. Using a combined experimental and multivariate statistical approach we have examined the mutagenicity and potential consequences of adduct formation by reactive intermediates in target uterine cells. pSP189 plasmid containing the supF gene was incubated with alpha-acetoxytamoxifen or 4-hydroxytamoxifen quinone methide (4-OHtamQM) to generate dG-N(2)-tamoxifen and dG-N(2)-4-hydroxytamoxifen, respectively. Plasmids were replicated in Ishikawa cells then screened in Escherichia coli. Treatment with both alpha-acetoxytamoxifen and 4-OHtamQM caused a dose-related increase in adduct levels, resulting in a damage-dependent increase in mutation frequency for alpha-acetoxytamoxifen; 4-OHtamQM had no apparent effect. Only alpha-acetoxytamoxifen generated statistically different supF mutation spectra relative to the spontaneous pattern, with most mutations being GC-->TA transversions. Application of the LwPy53 algorithm to the alpha-acetoxytamoxifen spectrum predicted strong GC-->TA hotspots at codons 244 and 273. These signature alterations do not correlate with current reports of the mutations observed in endometrial carcinomas from treated women, suggesting that dG-N(2)-tam adduct formation in the p53 gene is not a prerequisite for endometrial cancer initiation in women.

N-methyl-N'-nitro-N-nitrosoguanidine sensitivity, mutator phenotype and sequence specificity of spontaneous mutagenesis in FEN-1-deficient cells

Intact pZ189 DNA was allowed to replicate in FL-FEN-1(-) cell line that was established in this laboratory in which the expression of FEN-1 gene was blocked by dexamethasone-inducible expression of antisense RNA to FEN-1. E. coli MBM7070 was transfected with the replicated plasmid, and those with mutations in the supF gene were identified. The frequency of mutants that did not contain recognizable changes in the electrophoretic mobility of the plasmid DNA was scored. The frequency of such mutants was 19.1 x 10(-4) (34/17781), significantly higher than those of 2.9 x 10(-4) (4/13668) and 3.0 x 10(-4) (3/9857) in the corresponding controls, respectively. Sequence analysis of the supF genes of these mutants showed that all (37/37) the base substitutions occurred at C:G base pairs; 68% (23/37) of the base substitutions were base transversions, while 32% (12/37) were transitions. Approximately 76% (23/37) of these base substitutions occurred frequently at nine positions; two of these sites contain triple pyrimidine (T or C) repeat upstream to the mutated base; four of these sites consist of 5'-TTN1N2 and mutations occurred at N1 site sequence; another two sites have the characteristics of triple A flanked at both 5' and 3' side by TCT, with the base substitution occurring at C in the context sequence. These data suggested that these sites are the hot spot of mutagenesis in plasmid replicated in FEN-1-deficient cells. Besides the mutator phenotype of the FEN-1-deficient cell, it was also demonstrated that FEN-1-deficient cell exhibited an increased N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) sensitive phenotype.

An exploratory analysis of multiple mutation spectra

The last decade has witnessed a remarkable increase in the number of mutations identified both in human disease-related genes and mutation reporter genes including those in mammalian cells and transgenic animals. This has led to the curation of a number of computerised databases, which make mutation data freely available for analysis. A primary interest of both the clinical researcher and the genetic toxicologist is determination of location and types of mutation within a gene of interest. Collections of mutation data observed for a disease-related gene or, for a gene exposed to a particular chemical, permits discovery of regions of sequence along the gene prone to mutagenesis and may provide clues to the origin of a mutation. The principal tool for visualising the distribution pattern of mutant data along a gene is the mutation spectrum: the distribution and frequency of mutations along a nucleotide sequence. In genetic toxicology, the current wealth of mutation data available allows us to construct many mutation spectra of interest to investigate the mutagenic mechanisms and mutational sites for one or a group of mutagens. Using the multivariate statistical methods principal components analysis (PCA) and cluster analysis (CA) we have tested the ability of these methods to establish the underlying patterns within and between 60 UV-induced, mitomycin C-induced and spontaneous mutations in the supF gene. The spectra were derived from human, monkey and mouse cells including both repair efficient and repair deficient cell lines. We demonstrate and support the successful application of multivariate statistical methods for exploring large sets of mutation spectra to reveal underlying patterns, groupings and similarities. The methods clearly demonstrate how different patterns of spontaneous and UV-induced supF mutation spectra can result from variation in plasmid, culture medium, species origin of cell line and whether mutations arose in vivo or in vitro.

DNA adducts formed from 4-hydroxytamoxifen are more mutagenic than those formed by alpha-acetoxytamoxifen in a shuttle vector target gene replicated in human Ad293 cells

The drug tamoxifen, used to treat breast cancer, causes liver cancer in rats and endometrial cancer in women. Tamoxifen forms liver DNA adducts in both short- and long-term dosing of rodents, and DNA adducts have also been reported in tissues of women undergoing tamoxifen therapy. It is not known if the induction of endometrial cancer in women is through these DNA adducts or through the estrogenic nature of the drug. In this study, we have investigated the mutagenicity of two model reactive intermediates of tamoxifen, alpha-acetoxytamoxifen and 4-hydroxytamoxifen quinone methide (4-OHtamQM). These form the same DNA adducts as those found in tamoxifen-treated rats. The two compounds were used to treat the pSP189 plasmid containing the supF gene, which was replicated in Ad293 cells before being screened in indicator bacteria. Plasmid reacted with 4-OHtamQM was more likely to be mutated (2-7-fold increase) than that reacted with alpha-acetoxytamoxifen, despite having a lower level of DNA damage (12-20-fold less), as assayed by (32)P-postlabeling. The two compounds induced statistically different mutation spectra in the supF gene. The majority of mutations in alpha-acetoxytamoxifen-treated plasmid were GC -->TA transversions while GC-->AT transitions were formed in 4-OHtamQM-treated plasmid. 4-OHTamQM-treated DNA induced a larger proportion of multiple mutations and large deletions compared to alpha-acetoxytamoxifen. Sites of mutational hotspots were observed for both compounds. In conclusion, the quantitatively minor DNA adduct of tamoxifen (dG-N(2)-4-hydroxytamoxifen) is more mutagenic than the major tamoxifen DNA adduct (dG-N(2)-tamoxifen).

The importance of using absolute mutant frequencies to compare mutation spectra

Because damage to the cellular DNA is very hazardous for a cell, it is important to identify compounds, which can cause DNA damage. To investigate the mutagenic effect of a particular agent of interest, usually mutation spectra are determined in a selected target gene. The most commonly used method to compare different mutation spectra with each other, is the comparison of the percentages of each type of mutation. In this paper, it is emphasized that comparison of percentages can lead to incorrect conclusions and therefore another determinant, the absolute mutant frequency, should be used.

Lack of dependence on p53 for DNA double strand break repair of episomal vectors in human lymphoblasts

The p53 tumor suppressor gene has been shown to be involved in a variety of repair processes, and recent findings have suggested that p53 may be involved in DNA double strand break repair in irradiated cells. The role of p53 in DNA double strand break repair, however, has not been fully investigated. In this study, we have constructed a novel Epstein-Barr virus (EBV)-based shuttle vector, designated as pZEBNA, to explore the influence of p53 on DNA strand break repair in human lymphoblasts, since EBV-based vectors do not inactivate the p53 pathway. We have compared plasmid survival of irradiated, restriction enzyme linearized, and calf intestinal alkaline phosphatase (CIP)-treated pZEBNA with a Simian virus 40 (SV40)-based shuttle vector, pZ189, in TK6 (wild-type p53) and WTK1 (mutant p53) lymphoblasts and determined that p53 does not modulate DNA double strand break repair in these cell lines.

Mutations induced in the supF gene of pSP189 by hydroxyl radical and singlet oxygen: relevance to peroxynitrite mutagenesis

We previously showed that the oxidant peroxynitrite (ONOO-) was strongly mutagenic in the supF shuttle vector pSP189 replicated in bacteria or human cells. Qualitative characteristics of the mutational spectra induced by ONOO- differed significantly from those reportedly caused by hydroxyl radical (OH.) in other experimental systems but showed similarities to spectra reportedly produced by singlet oxygen (1O2). The molecular mechanisms of ONOO--mediated DNA damage are unknown. The objective of the present set of experiments was to characterize mutational effects induced in the supF gene of pSP189 by OH* and 1O2 to permit direct comparison with mutational spectra induced by ONOO- in this system. Base substitutions were the major form of mutation induced in plasmids replicated in human (AD293) cells by ONOO- (84%) and 1O2 (71%), whereas OH* induced fewer of them (49%). In plasmids replicated in bacteria (Escherichia coli MBL50), frequencies of base substitutions induced by the three treatments were similar. G:C-to-T:A transversions were the most common form of base substitution induced by ONOO- (75% and 67%, respectively, in AD293- and MBL50-replicated plasmids) and 1O2 (68% and 71%); they were induced at lower frequencies by OH. (51% and 47%). G:C-to-C:G transversions or G:C-to-A:T transitions were induced at almost equal frequencies by both ONOO- and 1O2, whereas OH* induced these mutations at different frequencies in the AD293 system. Collectively, our results confirm that in several important respects mutational spectra induced by ONOO- have greater similarity to spectra induced by 1O2 than to those induced by OH* and suggest that genotoxic derivatives of ONOO- are likely to include species that have DNA-damaging properties resembling those of 1O2 in selectivity for guanine but not identical in sequence specificity.

Genotoxicity and mutagenicity of the alpha, beta-unsaturated carbonyl compound crotonaldehyde (butenal) on a plasmid shuttle vector

Crotonaldehyde is an alpha,beta-unsaturated carbonyl compound and an important environmental and industrial toxic substance. Its mutagenic and carcinogenic properties are related to its reactivity to DNA, where it forms different guanine adducts. In order to study the mutagenic consequences of this agent in intact human cells, we treated the shuttle vector plasmid pZ189 with different doses of crotonaldehyde at 37 degrees C for 2 h and then transfected the such damaged plasmid into the normal human lymphoblast cell line GM0621. Within these host cells the guanine adducts are repaired and the plasmids replicated by cellular enzymes. After 2.5 days replicated plasmids were purified from the cells and plasmid survival was quantitated by transformation ability. With increasing doses of crotonaldehyde, we found a significant decline of plasmid survival, reflecting a pronounced genotoxicity of crotonaldehyde-induced DNA damage in intact human cells. Using the plasmid encoded mutagenesis marker gene supF, we were able to screen for mutants and determine mutation frequency in recovered plasmids. A significant increase in mutation frequency with increasing doses of crotonaldehyde reflects mutagenicity of crotonaldehyde-induced DNA damage. Base sequence analysis of recovered mutants revealed 39% point mutations, 46% deletions, and 15% insertions and inversions. Most of the point mutations (82%) were located at G:C base pairs, which is well explained by the DNA damage profile of crotonaldehyde. Among deletions we found a frequent reoccurrence of two hot spot deletions, representing 62% of all deletions. The sites of breakpoints of these deletions hot spots and of other deletions within the plasmid were also found to be sites of DNA breaks, directly induced by crotonaldehyde, as seen in an endlabeled plasmid fragment, treated with crotonaldehyde. Further analysis of the flanking sequences around the deletion breakpoints revealed a high frequency of four different kinds of short sequence homologies of up to eight base pairs.

Peroxynitrite-induced mutation spectra of pSP189 following replication in bacteria and in human cells

Peroxynitrite is a powerful oxidant formed through reaction of nitric oxide with superoxide. Because activated macrophages can produce both nitric oxide and superoxide, it has been proposed that peroxynitrite may contribute to cytotoxicity and increased cancer risks associated with the inflammatory response during chronic infections. We therefore investigated mutagenicity of peroxynitrite in the supF gene of the pSP189 shuttle vector as a mutation target. The plasmid was exposed to 2.5 mM peroxynitrite in vitro, then replicated in Eschericia coli MBL50 and in human AD293 cells. Mutation frequency increased 21-fold in pSP189 replicated in E. coli and 9-fold in plasmid replicated in human cells. Mutations were clustered within the 5' region of the supF gene in plasmids replicated in bacteria. The hot spots were located at positions 108, 113, 116, 124, 126 and 141; more than 25% of all mutations occurred at position 124. Following replication in human cells, mutations were more widely distributed over the gene, with hot spots at positions 113, 124, 133, 156 and 164; 15% occurred at position 124. In both systems, the majority of mutations occurred at G:C base pairs, predominantly involving G:C-->T:A transversions (65% when replication was in bacteria and 63% when in human cells). G:C-->C:G transversions were observed at lower frequency (28% in MBL50 and 11% in AD293 cells), and 11% of mutations found in vectors replicated in AD293 cells were G:C-->A:T transitions. A greater number of large deletions, insertions, tandem and multiple mutations occurred in plasmid replicated in AD293 cells. Differences in mutation spectra following replication in the two systems may be attributable to differences in recognition and repair of the lesions and/or properties of the replication apparatus.

Induction of mutagenic DNA damage by chromium (VI) and glutathione

Certain chromium (Cr) compounds are known to be carcinogenic in humans and mutagenic in cell culture. However, the mechanism of Cr mutagenesis is not well understood. It appears that intracellular reduction of Cr by agents such as glutathione plays a role in the induction of DNA damage. We have used a simian virus 40-based shuttle vector to investigate the relationship between chromium-induced DNA damage and Cr mutagenicity. The treatment of the plasmid pZ189 with Cr(VI) plus glutathione (GSH) induced DNA strand breaks and reduced the plasmid biological activity, whereas Cr(III) treatment with or without GSH did not give rise to such DNA damage. When Cr(VI)/GSH- or Cr(III)/GSH-treated pZ189 was replicated in mammalian cells, a dose-dependent increase in mutant frequency was observed with Cr(VI)/GSH-treated pZ189, but not with Cr(III)/GSH-treated plasmid. About 43% of the mutants from Cr(VI)/GSH-treated pZ189 were deletion mutants. The remainder were base substitution mutants, mostly GC-->AT transitions and GC-->TA transversions. This pattern of mutagenesis is similar to that observed with other agents that cause oxidative DNA damage such as ionizing radiation and H2O2. These results support the hypothesis that Cr mutagenesis can be induced by the generation of reactive oxygen intermediates during the reduction of Cr(VI) by glutathione.

Effects of vacuum UV and UVC radiation on dry Escherichia coli plasmid pUC19. II. Mutational specificity at the lacZ gene

The mutational spectra at the lacZ gene, induced either by vacuum UV at 160 nm or UVC at 254 nm in vacuum-dried preparations of Escherichia coli plasmid pUC19 DNA, have been characterized from 72 E. coli-propagated mutants by DNA sequencing. In plasmids irradiated in vacuum, vacuum UV is five times more mutagenic than UVC. In the UV-induced mutants, base substitutions largely predominate, with GC-->AT (G, guanine; C, cytosine; A, adenine; T, thymine) transitions being the most abundant type of base change for vacuum UV (61%) and UVC (47%). Most of the GC-->AT transitions appear to occur at dipyrimidine sites, which are located at the non-transcribed DNA strand. Some, but not all, hot spots for GC-->AT transitions are identical for vacuum UV and UVC. Frameshifts, resulting from a loss of the thymine residue, are specific for UVC (22%), and were not detected after treatment with vacuum UV. They occur predominantly at thymine runs of the transcribed DNA strand. Only a few deletions were detected following irradiation with vacuum UV (7.5%) and UVC (2%); however, their frequency is not enhanced compared with the spontaneous mutation spectrum. The data confirm the important role of base substitution mutations in UV-induced mutagenesis, which is not only valid for the UVC range, but extends towards the vacuum UV range.

Assessment of genotoxicity and mutagenicity of 1,2-dioxetanes in human cells using a plasmid shuttle vector

1,2-Dioxetanes are efficient sources of triplet excited carbonyl compounds on thermal decomposition. They cause photochemical and photobiological transformations in the dark. In order to study the genotoxicity and mutagenicity of 1,2-dioxetanes, the replicating shuttle vector pZ189 was damaged with 3,3,4-trimethyl-1,2-dioxetane (TrMD) or 3-hydroxymethyl-3,4,4-trimethyl-1,2-dioxetane (HTMD) in vitro and subsequently transfected into normal human lymphoblasts. We found a dose-dependent increase of genotoxicity (decrease of plasmid survival) and increase of mutation frequency with both dioxetanes. However, TrMD was less mutagenic than HTMD at similar genotoxicity. Sequence analysis of the supF gene revealed more point mutations with TrMD and 100% with HTMD were G:C to T:A and G:C to C:G transversions. These are the typical mutations following 7,8-dihydro-8-oxoguanine (8-oxo-G) formation, the main DNA lesion induced by TrMD and HTMD. Only with TrMD we found 5.4% G:C to A:T transitions, probably reflecting the more pronounced ability of TrMD to form some pyrimidine dimers. Our results indicate that 8-oxo-G is also the most relevant modification in in vivo mutagenesis.

DNA sequence analysis of gamma-radiation (anoxic)-induced and spontaneous lacId mutations in Escherichia coli K-12

An extensive spectrum of ionizing radiation mutagenesis was determined by sequencing 318 137Cs gamma-radiation (anoxic)-induced episomal lacId mutations in Escherichia coli strain NR9102. The most commonly found radiation-induced mutations were base substitutions (44% transversions and 41% transitions). The radiation-induced spectrum consisted of: 23% G.C-->A.T, 18% A.T-->G.C, 17% G.C-->T.A, 14% G.C-->C.G, 8% A.T-->T.A, 6% A.T-->C.G, 8% single-base deletions, 5% multiple mutations, 3% multi-base deletions, and essentially no single- or multi-base additions. This spectrum compared better with spectra for other systems obtained by in vivo irradiation than with one obtained by in vitro irradiation. Multiple mutations, which were unique to the radiation-induced spectrum, generally consisted of one active and one closely linked silent mutation, and are suggested to result from an altered replication complex of reduced fidelity. Mutation rates were 4.1 x 10(-8) lac-constitutive mutations/gene/Gy and 1.2 x 10(-10) base substitutions/base pair/Gy. Thirty-two percent more radiation-induced mutations occurred at G.C vs. A.T base pairs. A strand asymmetry was noted for G.C-->C.G and A.T-->T.A transversions. A nearest-neighbor analysis showed that C (vs. A, G, or T), on either side of the mutation site, substantially enhanced most types of base substitutions. Similarly, G and C flanked both sides of single-base deletion sites twice as frequently as would be expected from the base composition of the mutation target. For comparative purposes, we sequenced 411 spontaneous lac-constitutive mutants of which 269 were lacId mutants, and there was good agreement between these and previously published mutational spectra. The spontaneous and radiation-induced mutational spectra differed substantially for virtually every class of mutation. For example, the set of spontaneous dominant lac-constitutive mutations contained many more mutations that did not map in the normal region for lacId mutations (i.e., 35% vs. 3%) and were presumed to be lacO-constitutive mutations. A sampling of these presumptive lacOc mutations was also sequenced: 17/22 (spontaneous) and 1/9 (radiation) were found to be lacOc long deletions, one from each set were base substitutions, and the remaining mutations showed the wild-type lacO sequence. Like the radiation-induced spectrum, the spontaneous spectrum showed enhanced mutagenesis at G.C sites, strand asymmetry, and enhanced mutagenesis when G or C were the nearest neighbors.

Alteration of irradiated shuttle vector processing by exposure of human lymphoblast host cells to single or split gamma-ray doses

The repair of damaged DNA by mammalian cells exposed to single or split doses of radiation was probed with shuttle vector pZ189. Human lymphoblast hosts who received a single 120 cGy dose 2 h before transfection with 2500 cGy-damaged pZ189 yielded a two-fold higher frequency of progeny plasmids with mutations in their supF-tRNA target genes than did unirradiated host cells. Delaying transfection for 12 h, however, reduced the mutation frequency by half versus unirradiated controls. Plasmid survival was also affected by the time between host cell irradiation and transfection. Splitting doses of 50-500 cGy into two equal fractions separated by 4 h lowered mutation frequency and increased plasmid survival compared with equivalent acute doses; increasing the interval between dose fractions to 8 h, however, lowered plasmid survival compared with acute doses. Sequence analyses of the target gene in mutant plasmids revealed increased multiple-base substitution mutations among progenies recovered from irradiated hosts, indicating enhanced excision repair. These findings support modulation of mammalian cell DNA repair by ionizing radiation, disclose the transient nature of the effect of radiation on DNA repair, and demonstrate a quantitative difference in the effectiveness of single and split doses.

Mutational specificity of oxidative DNA damage

In this paper we describe our studies on the mutagenic consequences of oxidative DNA damage introduced by radiation-induced OH radicals (.OH) and by exposure to singlet oxygen (1O2), released by thermo-dissociation of the endoperoxide 3,3'-(1,4-naphthalidene) dipropionate (NDPO2). We have made use of M13mp10 bacteriophage and pUC18 plasmid DNA, containing a 144 base pair (bp) insert in the lacZ alpha gene. This 144 bp insert was used as a mutational target sequence. When dilute aqueous solutions of double-stranded (ds) M13mp10 (plus 144 bp insert) were gamma-irradiated in the presence of oxygen (O2; 100% .OH) or nitrous oxide (N2O; 90% .OH, 10% .H), very specific mutation spectra were found. Mainly bp substitutions were observed, of which C/G to G/C transversions are the predominant type. Moreover, the mutations are for the most part concentrated into two mutational hot spots: a minor and major one. Differences between the oxic (O2) and anoxic (N2O) mutation spectra could also be observed. Under N2O-1 bp deletions were detected, which are absent in the presence of O2, and in the anoxic spectrum more C/G to A/T transversions are present. To investigate whether these differences were due to the small amount of H radicals, which are formed under N2O, ds M13mp10 (plus 144 bp insert) was exposed to gamma-rays in phosphate buffer under nitrogen (55% .H, 45% .OH). Under these conditions a remarkable shift was observed from C/G-->G/C to C/G-->A/T transversions, while the mutations were far more scattered along the 144 bp sequence and no -1 bp deletions were detected. These results strongly suggest that H radicals do not cause -1 bp deletions, but may be responsible for the observed C/G to A/T transversions. The kind of bp substitution not only appeared to be dependent on the type of the water radicals, but also appeared to be strongly influenced by the replicon in which the target sequence is incorporated. When an oxygenated solution of pUC18 plasmid DNA (plus 144 bp insert) is irradiated, mainly C/G to A/T transversions were found at the same major hot spot instead of C/G to G/C transversions when the 144 bp sequence is part of M13mp10 DNA. Finally, in agreement with the observation that 1O2 reacts preferentially with guanine in DNA, a guanine is involved in most of the mutations scored after exposure of single-stranded (ss) M13mp10 DNA to NDPO2-generated 1O2.(ABSTRACT TRUNCATED AT 400 WORDS)

Published data on mutagenesis by ionizing radiation of plasmids in solution probably reflect in part the specificity of adventitious transition metal ions complexed to the DNA

A number of recent papers show that single base changes induced by mutagenesis with ionizing radiation of genes on plasmids in solution, followed by transfection into mammalian or bacterial cells for assay, are mostly at G:C base pairs, with mutagenic hot spots. Genes irradiated in mammalian or bacterial cells, on the other hand, have comparable numbers of base changes at all sites, with no evidence for hot spots. The differences are ascribed to induction of many base change mutations in vitro by reactions catalyzed by adventitious transition metal ions complexed to the DNA. Reasons are given why this process should play a much smaller role in vivo.