Paradoxical behaviour of pKM101; inhibition of uvr-independent crosslink repair in Escherichia coli by muc gene products

In strains of Escherichia coli deficient in excision repair (uvrA or uvrB), plasmid pKM101 muc+ but not pGW219 mucB::Tn5 enhanced resistance to angelicin monoadducts but reduced resistance to 8-methoxy-psoralen interstrand DNA crosslinks. Thermally induced recA-441 (= tif-1) bacteria showed an additional resistance to crosslinks that was blocked by pKM101. Plasmid-borne muc+ genes also conferred some additional sensitivity to gamma-radiation and it is suggested that a repair step susceptible to inhibition by muc+ gene products and possibly involving double-strand breaks may be involved after both ionizing radiation damage and psoralen crosslinks.     

An alteration leading to loss of ability to support phleomycin mutagenesis in the pKM101-derived plasmid pGW16 is located in or close to the mucAB genes

The differences between the plasmid pKM101 and its derivative pGW16, which has lost the ability to support muc-dependent phleomycin mutagenesis, while retaining other muc-dependent phenotypes, have been further investigated. Deletion derivatives which retain only 10.8 kb (approximately one third) of the pKM101 genome but retain the muc genes have been constructed from both pKM101 and pGW16. The deletion derivatives confer protection and mutagenesis-enhancing properties similar to those of their respective parents, indicating that the lesion in the mutant plasmid pGW16 lies in or close to the muc genes. Differences in the muc-dependent phenotypes of strains containing pKM101 or pGW16 suggest that the pGW16 lesion results in either differential loss of function in the muc gene products, or constitutive expression of the muc gene products.     

Comparison of the Escherichia coli umu+-encoded function with plasmid R46-mediated error-prone repair in DNA-damaged cells

Escherichia coli strain TK701 umu+ was more resistant than strain TK702 umu when tested against bleomycin (BLM), cis-platinum(II) diamminodichloride (PDD), ultraviolet light and methyl methanesulphonate (MMS), which produce single-strand DNA damage. However, the umu mutant was no more sensitive to mitomycin C (MTC) or proflavine (PF), which cause double-strand DNA binding. Strain TK702 umu was nonmutable by any of the agents, whereas mutations were induced in the wild-type strain by PDD, UV, MMS and MTC. The E. coli umu+ function therefore mimics plasmid R46-mediated error-prone repair in protecting only against single-strand DNA damage, whilst enhancing mutagenesis by both single- and double-strand damaging agents. Comparison of plasmid R46-mediated protection and mutagenesis in umu+ and umu strains indicated that the plasmid confers a greater error-prone DNA-repair activity in the mutant. Results are discussed in terms of analogy between host umu+ and plasmid muc+ functions.     

Segregation of the mutator property of plasmid R46 from its ultraviolet-protecting property

Summary Plasmid R46 (an R factor conferring resistance to ampicillin, sulfonamides, streptomycin and tetracycline) reduces the bactericidal effect of UV irradiation but increases its mutagenic effect (reversion of hisG46), and raises the frequency of spontaneous reversion (mutator effect). Putative deletion mutants of R46 were obtained by transduction of the plasmid, then two successive conjugal transfers. Plasmids of five of six deletion classes, each with a different combination of drug resistance traits, retained conjugative ability and the UV-protecting, mutagenesis-enhancing and mutator effects of R46. (pKM101, used in the Ames system to enhance responsiveness to chemical mutagens, is one such mutant of R46.) Plasmids of a sixth class, represented by pKM115, conferred resistance only to streptomycin and were non-conjugative. All of several such plasmids (of independent origin) had a much stronger mutator effect than did R46, but lacked UV-protecting ability and did not enhance the mutagenic effect of UV irradiation. We infer that R46 possesses: (i) a gene, uvp, which increases capacity for error-prone repair of UV-damaged DNA, and thus causes both UV protection and enhancement of UV mutagenesis; (ii) gene(s) whose action in the absence of gene uvp greatly increases the frequency of spontaneous reversion of hisG46. A plasmid of another incompatibility group, pLS51, has UV-protecting and mutagenesis-enhancing effect but lacks the mutator property; introduction of pLS51 into a clone of hisG46 carrying a pKM115-type plasmid greatly reduced its spontaneous reversion rate, as expected if pLS51 also has a uvp gene able to modulate the mutator effect of R46-derived gene(s) in the pKM115-type plasmid.     

Survival and repair of single-stranded DNA breaks after gamma-irradiation of Escherichia coli cells depending on the presence of plasmids pKN101 and COIIB-P9

Plasmids, pKM101 and ColIb-P9, present in an autonomous state in E. coli AB 1157, JC5519, and P3478 cells at the stationary and logarithmic phases of growth, somewhat sensitize the cells to the lethal effect of gamma-radiation and do not influence the radiosensitivity of B/r, Bs-1 gamma R, Bs-1, and W3110 cells. The efficiency of repair of gamma-ray-induced DNA single-strand breaks in AB1157 and P3478 cells containing plasmids is somewhat lower than that in the same non-plasmid strains.     

Introduction of the plasmid pKM101-associated muc genes into Saccharomyces cerevisiae

Bacteria-yeast shuttle plasmids containing the pKM101-associated muc genes were constructed by cloning an ARS TRP fragment into the plasmid pGW270 in both possible orientations. The insertion of Saccharomyces cerevisiae DNA into pGW270 had no effect on the mutator and protective phenotypes associated with the plasmid in Escherichia coli. Two such recombinant plasmids, pAA90 and pAA91 , were capable of efficient transformation of S. cerevisiae and were stably maintained in this organism. Hybridization experiments suggest that muc-specific mRNA was present in transformed yeast cells and a small amount was polyadenylated. The RNAs were not of a discrete size, all being smaller than the muc genes. The presence of the plasmid pAA91 , and to a lesser extent, pAA90 , in yeast resulted in a detectable increase in the reversion frequencies of three markers and in ultraviolet protection. These results are discussed in terms of studying the relationship of error-prone repair in bacteria and yeast and of developing improved yeast tester strains.     

Physical mapping of the plasmid R205 and identification of a site responsible for increased UV-induced mutability of the bacterial host

A physical map of the conjugative IncN plasmid R205 (56.1 kb) was constructed. The distribution of cleavage sites for investigated restriction enzymes is asymmetric. It was found that R205 suppresses the mutant phenotype of E. coli K12 umuC or umuD strains deficient in UV-induced mutagenesis. A mini-derivative of R205, designated pMU4 (15.1 kb) preserves the ability of the parent plasmid to increase the survival and induced mutagenesis of UV-irradiated host cells. A region of R205 located between 0 and 2.0 kb-on the plasmid map seems to contain information necessary for complementation of mutation in the host genes umuD/C, Hybridization between this region of pMU4 and plasmid pGW1700 bearing mucAB genes of pKM101 was observed.     

LexA-independent expression of a mutant mucAB operon.

pKM101 is a naturally occurring plasmid that carries mucAB, an analog of the umuDC operon, the gene products of which are required for the SOS-dependent processing of damaged DNA necessary for most mutagenesis. Genetic studies have indicated that mucAB expression is controlled by the SOS regulatory circuit, with LexA acting as a direct repressor. pGW16 is a pKM101 derivative obtained by N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis that was originally identified on the basis of its ability to cause a modest increase in spontaneous mutation rate. In this report, we show that pGW16 differs from pKM101 in being able to enhance methyl methanesulfonate mutagenesis and to confer substantial resistance to UV killing in a lexA3 host. The mutation carried by pGW16 is dominant and was localized to a 2.4-kb region of pGW16 that includes the mucAB coding region and approximately 0.6 kb of the 5'-flanking region. We determined the sequence of a 119-bp fragment containing the region upstream of mucAB and identified a single-base-pair change in that region, a G.C-to-A.T transition that alters a sequence homologous to known LexA-binding sites. DNA gel shift experiments indicate that LexA protein binds poorly to a 125-bp fragment containing this mutation, whereas a fragment containing the wild-type sequence is efficiently bound by LexA. This mutation also alters an overlapping sequence that is homologous to the -10 region of Escherichia coli promoters, moving it closer to the consensus sequence. The observation that the synthesis of pGW16-encoded mucAB proteins in maxicells is increased relative to that of pKM101-encoded mucAB proteins even in the absence of a lexA+ plasmid suggests that this mutation also increases the activity of the mucAB promoter.     

Frameshift mutagenesis by acridines in wild-type, uvrB and polA strains of Salmonella typhimurium with and without plasmid pKM101

A large range of acridines, including several anilinoacridines which are active as antitumour agents, have been studied for their ability to revert derivatives of Salmonella typhimurium strains carrying the frameshift marker hisC3076. The strains used all carried deep-rough (rfa) mutations, and were either wild-type with respect to DNA-repair capacity or carried uvrB, polA1 or polA3 (amber) mutations. Derivatives with and without the mutation-enhancing N group plasmid pKM101 were also used. 9-Aminoacridine and other acridines appeared similar to the anilinoacridines for the most part, in that frameshift mutagenesis and toxicity appeared to be unaffected by the uvrB mutation or by the presence of plasmid pKM101. Exceptions were ICR191, 3-NO2-acridine and 1- or 3-NO2-anilinoacridine derivatives in which mutagenesis was increased in uvrB strains and also when pKM101 was present. These compounds were slightly more toxic in the uvrB background, but less toxic when pKM101 was present in either the uvrB or wild-type backgrounds. Mutagenesis by most compounds was reduced by the polA1 mutation and virtually eliminated (except in the case of ICR191) by the polA3 mutation. Plasmid pKM101 occasionally enhanced mutagenesis in the polA1 strain, whereas in the polA3 it appeared to have no effect whatsoever. Again, there were no obvious differences in toxicity between Pol+ and Pol- strains.     

Inducible reactivation of bacteriophage T7 damaged by methyl methanesulfonate or UV light.

We examined the effects of host mutations affecting "SOS"-mediated UV light reactivation on the survival of bacteriophage T7 damaged by UV light or methyl methanesulfonate (MMS). Survival of T7 alkylated with MMS was not affected by the presence of plasmid pKM101 or by a umuC mutation in the host. The survival of UV light-irradiated T7 was similar in umuC+ and umuC strains but was slightly enhanced by the presence of pKM101. When phage survival was determined on host cells preirradiated with a single inducing dose of UV light, these same strains permitted higher survival than that seen with noninduced cells for both UV light- and MMS-damaged phage. The extent of T7 reactivation was approximately proportional to the UV light inducing dose inflicted upon each bacterial strain and was dependent upon phage DNA damage. Enhanced survival of T7 after exposure to UV light or MMS was also observed after thermal induction of a dnaB mutant. Thus, lethal lesions introduced by UV light or MMS are apparently repaired more efficiently when host cells are induced for the SOS cascade, and this inducible reactivation of T7 is umuC+ independent.     

Weakening of bacteriophage lambda EcoK DNA restriction in the presence of plasmid pKM101 ard+. I. General characteristics and genetic localization

The host-controlled K-restriction of unmodified phage lambda is ten to hundred-fold alleviated in the E. coli K12 strain, carring plasmid pKM101 of N-incompatibility group. By restriction mapping Tn5 insertion in pKM101, which reduced pKM101-mediated alleviation of K-restriction, was shown to by located within BglII-B-fragment approximately 9 kb anticlockwise from the EcoRI-site of pKM101. We have termed the gene(s) promoting the alleviation of K-restriction ARD (Alleviation of Restriction of DNA). It was shown that (i) plasmid pKM101-mediated alleviation of K-restriction did not depend on bacterial genes LexA, RecBC, umuC and plasmid gene muc; (ii) ard gene did not mediate EcoK type modification of DNA and did not enhance the modification activity of EcoK system in a way similar to that observed with RAL gene of phage lambda. Action of Ard gene of plasmid pKM101 is highly specific: alleviation of restriction of DNA lambda takes place only in K-strains of E. coli and is practically absent in B-strains and also in E. coli strains which have restricting enzymes of 11 type, EcoRI and EcoRIII.     

Frameshift mutagenesis by nitracrine analogues in wild-type, uvrB, polA and recA strains of Salmonella typhimurium, with and without plasmid pKM101

The mutagenic potential of 9-[(3-dimethylaminopropyl)amino]-acridine and its 1-, 2-, 3- and 4-nitro derivatives was studied in several strains of Salmonella typhimurium carrying the frameshift marker hisC3076. The strains all carried deep rough (rfa) mutations, and were either wild-type with respect to DNA repair capacity or carried recA, uvrB, polA1 or polA3 (amber) mutations. Derivatives with and without plasmid pKM101 were also studied. The des-nitro compound resembled 9 aminoacridine and other simple intercalating compounds. Both toxicity and mutagenesis were apparently unaffected by the uvrB and recA mutations or by the presence of plasmid pKM101. However, mutagenicity was reduced by the polA1 mutation, and virtually eliminated by the polA3 mutation. The drug was substantially more toxic in the latter, slightly more toxic in the former, of these polA- strains. Plasmid pKM101 enhanced mutagenesis and protected from toxicity in both polA1- and polA3- strains, although it did not restore either of these parameters to the level in the wild-type strain. The 2-nitro compound was generally similar to the des-nitro compound, except that it was considerably more toxic and apparently non-mutagenic in the recA-bearing strain. By contrast, mutagenicity of the 3- and 4-nitro compounds was enhanced by the uvrB mutation and by the presence of the plasmid. These compounds were highly toxic but non-mutagenic in the recA- strain, and showed some increased toxicity in polA1- and polA3- strains. The 1-nitro compound has been previously found to cross-link DNA. Unlike well-characterised cross-linkers such as mitomycin C it was highly mutagenic in the uvrB- strain, and this mutagenesis was enhanced by plasmid pKM101, but eliminated by the recA mutation. At high doses, where the drug was completely toxic towards uvrB- or recA-carrying strains, it became mutagenic in the DNA-repair-proficient strains. This 'high-dose' mutagenesis was enhanced by plasmid pKM101, but reduced by the polA1 mutation and almost eliminated by the polA3 mutation. Although there are several possible interpretations of these data, they are compatible with the suggestion that the lesion induced by high doses (but not by low doses) of nitracrine is a cross-link, but that this is not the major mutagenic lesion.     

Alleviation of type I restriction in the presence of plasmids of group inc I. General characteristics and molecular cloning of the ard gene

The capability of a number of plasmids of incN and incI groups to alleviate an action of type I EcoK, EcoB, EcoD, and EcoA restriction endonucleases on the unmodified DNA was revealed. The efficiency of EcoK action on lambda 0 DNA is alleviated about 10 divided by 100 fold in E. coli K12 AB 1157 bacteria containing the plasmid of incN group (pKM101, N3, pJA4733) or incI group (R144, R648; R621a; ColIb-P9). We have cloned ard gene of ColIb-P9 plasmid (SalI-C fragment) in pBR322 multicopying vector. A hybrid clone abolishing the EcoK restriction has been received. Ard gene activity is independent of the recA, recBc, recF, lexA, umuC, lon bacterial genes activity. Ard gene's product does not inhibit the EcoK restriction endonuclease action as well as ocr protein (phage T7) and does not increase the process of methylation of DNA as well as ral protein of phage lambda.     

DNA sequence analysis of the imp UV protection and mutation operon of the plasmid TP110: identification of a third gene.

The sequence of the imp operon of the plasmid TP110 (which belongs to the Incl1 incompatibility group) has been determined, and is shown to contain three open reading frames. This operon, involved in UV protection and mutation, is functionally analogous to the umuDC operon of E. coli and the mucAB operon of the plasmid pKM101, which belongs to the quite unrelated IncN incompatibility group. The umu and muc operons however contain only two open reading frames, coding for proteins of approximately 16kD and 46kD. The high degree of homology between the two 16kD proteins (UmuD and MucA) and between the two 46kD proteins (UmuC and MucB) clearly shows their relatedness. This is shown also to extend to the imp gene products, with ImpA sharing homology with UmuD and MucA, and ImpB sharing homology with UmuC and MucB. However, the two imp genes are preceded in the operon by a third gene, impC, which encodes a small protein of 9.5kD and which has no equivalent in the umu and muc operons.     

Effects of DNA-repair processes on the induction of genetic duplications in bacteria by ultraviolet light

A straightforward positive selection for genetic duplication is possible in strains of Salmonella typhimurium that carry the aroC321 allele. Strains with a single copy of this allele require phenylalanine, tyrosine and tryptophan for growth. Such strains give rise to tryptophan prototrophs, which still require phenylalanine and tyrosine, through the formation of a duplication that includes about 30% of the chromosome. We have constructed strains that permit the simultaneous study of duplications and mutations and have used these strains to explore the effects of DNA repair processes on the induction of duplications by ultraviolet light (UV). UV causes dose-dependent increases in the frequency of duplications in bacteria. The exposure required to induce duplications is much less in a delta uvrB strain than in repair-proficient strains, suggesting that duplications result from DNA lesions that are subject to excision repair. The photoreversibility of UV-induced preduplication lesions implicates pyrimidine dimers in the induction of duplications. Unlike its effect on the induction of mutations, the error-prone repair process associated with plasmid pKM101 does not enhance the induction of duplications. The prevention of duplication-formation by a recA mutation suggests that the formation of duplications involves recombinational events. Taken together, the data indicate that the same DNA lesions can be mutagenic and recombinagenic in bacteria, but that the two effects involve different pathways of processing DNA damage.     

Effects of the antimutagen cinnamaldehyde on reversion and survival of selected Salmonella tester strains

The antimutagenic activity of trans-cinnamaldehyde (C6H5CH = CHCHO) on chemically induced mutagenesis has been shown in E. coli. Using the Ames Salmonella typhimurium tester strains TA1535 (hisG46 uvrB rfa) and TA100 (TA1535/pKM101), the effects of cinnamaldehyde on spontaneous reversions and reversions induced by 4-nitroquinoline-N-oxide(4NQO) and ethyl methanesulfonate (EMS) have been examined. To observe the effect of cinnamaldehyde in the absence of functional muc genes, a third strain, TA1535/pGW201 (pKM101 muc140: :Tn5) was included in the testing. Modifications of the standard Ames test procedures and direct-plating techniques were employed to study the "antimutagenic" response exerted by cinnamaldehyde. In all strains tested, concentrations of cinnamaldehyde up to 25 micrograms/ml slightly decreased the number of spontaneous reversions and induced reversions were more markedly reduced. The decreases in the numbers of 4NQO-induced revertants were greater than those decreases which occurred for EMS-induced reversions. There was no effect on viability in 1% (v/v) nutrient broth supplemented minimal medium containing 5-25 micrograms/ml of cinnamaldehyde. Cinnamaldehyde did not display any mucAB dependent or independent specificity against the mutagens used. On minimal medium supplemented with histidine and biotin, concentrations of cinnamaldehyde above 10 micrograms/ml were lethal for the strains tested. When the test medium was supplemented with 1-5% (v/v) liquid nutrient broth, viability was not affected at concentrations up to 25 micrograms/ml. For both TA100 and TA1535 the presence of 20 micrograms/ml of cinnamaldehyde in 1% (v/v) liquid nutrient broth-supplemented minimal glucose broth extended the lag phase for 2-4 h with no effect on survival. Depending on the test procedure employed, decreases in numbers of revertants may reflect lethality rather than antimutagenesis. When used to test for antimutagenesis rather than mutagenesis, modifications of the standard Ames test procedure may mimic an antimutagenic response due to a decrease in the total number of revertants seen even though enough cells survive to produce a background lawn.     

Effect of SOS-induced Pol II, Pol IV, and Pol V DNA polymerases on UV-induced mutagenesis and MFD repair in Escherichia coli cells

Irradiation of organisms with UV light produces genotoxic and mutagenic lesions in DNA. Replication through these lesions (translesion DNA synthesis, TSL) in Escherichia coli requires polymerase V (Pol V) and polymerase III (Pol III) holoenzyme. However, some evidence indicates that in the absence of Pol V, and with Pol III inactivated in its proofreading activity by the mutD5 mutation, efficient TSL takes place. The aim of this work was to estimate the involvement of SOS-inducible DNA polymerases, Pol II, Pol IV and Pol V, in UV mutagenesis and in mutation frequency decline (MFD), a mechanism of repair of UV-induced damage to DNA under conditions of arrested protein synthesis. Using the argE3-->Arg(+) reversion to prototrophy system in E. coli AB1157, we found that the umuDC-encoded Pol V is the only SOS-inducible polymerase required for UV mutagenesis, since in its absence the level of Arg(+) revertants is extremely low and independent of Pol II and/or Pol IV. The low level of UV-induced Arg(+) revertants observed in the AB1157mutD5DumuDC strain indicates that under conditions of disturbed proofreading activity of Pol III and lack of Pol V, UV-induced lesions are bypassed without inducing mutations. The presented results also indicate that Pol V may provide substrates for MFD repair; moreover, we suggest that only those DNA lesions which result from umuDC-directed UV mutagenesis are subject to MFD repair.     

Enhancement of UV survival, UV- and MMS-mutability, precise excision of Tn10 and complementation of umuC by plasmids of different incompatibility groups

29 conjugative resistance and colicin plasmids from 19 different incompatibility (Inc) groups were examined for their ability to enhance post-ultraviolet (UV) survival and UV- and methyl methanesulfonate(MMS)-induced mutability in Salmonella typhimurium LT2 strains. 14 Muc+ plasmids enhanced each of the survival and mutation-related properties tested, while 14 Muc- plasmids showed no enhancing effects in any tests. One Muc+ plasmid, pRG1251 (IncH1), enhanced post-UV survival and each of the mutation-related properties tested, except MMS-induced mutagenesis. Two further noteworthy plasmids, R391 (IncJ) and R394 (IncT), produced apparent strain-dependent effects in S. typhimurium which differed from those reported to have been found in Escherichia coli. Plasmid R391 enhanced post-UV survival in S. typhimurium, in contrast to its UV-sensitizing effects in E. coli. In both hosts plasmid R391 enhanced UV- and MMS-induced mutagenesis. Plasmid R394 had no enhancing effects on UV survival or UV- and MMS-induced mutagenesis in S. typhimurium, in contrast to its reported enhancement of MMS-induced mutagenesis in E. coli. Conjugal transfer of R394 to E. coli strain AB1157 and assays of mutagenesis-related traits supported results observed in S. typhimurium. Muc+ plasmids were found to enhance the frequency of precise excision of the transposon Tn10 when inserted within hisG or trpA in S. typhimurium strains. Precise excision could be further enhanced in S. typhimurium by UV-irradiation. Analysis of Tn10 mutants with altered IS10 ends indicated that intact inverted repeats at the ends of Tn10 were required for efficient enhancement of precise excision.     

UV inducible UV protection and mutation functions on the I group plasmid TP110

Summary The UV protection and mutation properties of the I group plasmid TP110 have been investigated. It is demonstrated that the genes responsible for these effects are able to complement the deficiency in umuC36 mutants of E. coli, as are the similar genes carried by the B group plasmid R16. Mu-lac inserts into TP110 have been isolated which abolish the UV protection and mutation functions. Restriction mapping of these inserts locates them within a single region of the genome. A comparison of the restriction sites of this region with the muc region of pKM101 reveals very little similarity. Expression of -galactosidase in those Mu-lac inserts in which the lacZ gene is fused to the promoter for the protection and mutation functions is inducible by DNA damaging agents, and induction in mutant strains suggests that these genes are under the direct control of the lexA repressor.     

Influence of DNA repair on mutation spectra in Salmonella

This paper reviews the influence of DNA repair on spontaneous and mutagen-induced mutation spectra at the base-substitution (hisG46) and -1 frameshift (hisD3052) alleles present in strains of the Salmonella (Ames) mutagenicity assay. At the frameshift allele (mostly a CGCGCGCG target), ΔuvrB influences the frequency of spontaneous hotspot mutations (−CG), duplications, and deletions, and it also shifts the sites of deletions and duplications. Cells with pKM101+ΔuvrB spontaneously produce complex frameshifts (frameshifts with an adjacent base substitution). The spontaneous frequency of 1-base insertions or concerted (templated) mutations is unaffected by DNA repair, and neither mutation is inducible by mutagens. Glu-P-1, 1-nitropyrene (1NP), and 2-acetylaminofluorene (2AAF) induce only hotspot mutations and are unaffected by pKM101, whereas benzo(a)pyrene and 4-aminobiphenyl induce only hotspot in pKM101⁻, and hotspot plus complex in pKM101⁺. At the base-substitution allele (mostly a CC/GG target), the ΔuvrB allele increases spontaneous transitions in the absence of pKM101 and increases transversions in its presence. The frequency of suppressor mutations is decreased 4× by ΔuvrB, but increased 7.5× by pKM101. Both repair factors cause a shift in the proportion of mutations to the second position of the CC/GG target. With UV light and γ-rays, the ΔuvrB allele increases the proportion of transitions relative to transversions. pKM101 is required for mutagenesis by Glu-P-1 and 4-AB, and the types and positions of the substitutions are not altered by the addition of the ΔuvrB allele. Changes in DNA repair appear to cause more changes in spontaneous than in mutagen-induced mutation spectra at both alleles. There is a high correlation (r²=0.8) between a mutagen's ability to induce complex frameshifts and its relative base-substitution/frameshift mutagenic potency. A mutagen induces the same primary class of base substitution in TA100 (ΔuvrB, pKM101) as it does in Escherichia coli, mammalian cells, or rodents as well as in the p53 gene of human tumors associated with exposure to that mutagen. Thus, a mutagen induces the same primary class of base substitution in most organisms, reflecting the conserved nature of DNA replication and repair processes.