Mutagenesis and repair of DNA damage caused by nitrogen mustard, N,N′-bis(2-chloroethyl)-N-nitrosourea (BCNU), streptozotocin, and mitomycin C in E. coli

Homologous Recombination-Enzymes and Pathways

Homologous recombination is an ubiquitous process that shapes genomes and repairs DNA damage. The reaction is classically divided into three phases: presynaptic, synaptic, and postsynaptic. In Escherichia coli, the presynaptic phase involves either RecBCD or RecFOR proteins, which act on DNA double-stranded ends and DNA single-stranded gaps, respectively; the central synaptic steps are catalyzed by the ubiquitous DNA-binding protein RecA; and the postsynaptic phase involves either RuvABC or RecG proteins, which catalyze branch-migration and, in the case of RuvABC, the cleavage of Holliday junctions. Here, we review the biochemical properties of these molecular machines and analyze how, in light of these properties, the phenotypes of null mutants allow us to define their biological function(s). The consequences of point mutations on the biochemical properties of recombination enzymes and on cell phenotypes help refine the molecular mechanisms of action and the biological roles of recombination proteins. Given the high level of conservation of key proteins like RecA and the conservation of the principles of action of all recombination proteins, the deep knowledge acquired during decades of studies of homologous recombination in bacteria is the foundation of our present understanding of the processes that govern genome stability and evolution in all living organisms.

Novel illudins from Coprinopsis episcopalis (syn. Coprinus episcopalis), and the distribution of illudin-like compounds among filamentous fungi

The illudins are a family of fungal sesquiterpenes that have been studied as anti-tumor agents, and they also have antibacterial activity. Over a four-year period, 25 304 fungal isolates (approximately 97% ascomycetes and 3% basidiomycetes), were screened for antibacterial activity against methicillin-resistant Staphylococcus aureus. Illudin-like compounds with antibacterial and cytotoxic activity against tumor cell lines were observed in 10 basidiomycete strains. The isolates were recovered from different types of substrata using indirect methods and only formed sterile mycelia in pure culture. The isolates were genetically related but not identical, based on PCR-based fingerprinting techniques. DNA sequencing of the ITS1-5.8 S-ITS2 region of the strains revealed that nine had identical sequences, indicating that they were conspecific. The sequence of the remaining isolate was 96.34% similar, suggesting that it was a closely related species. The D1-D2 region of the 25 S rRNA gene of the two strain types was also sequenced. Both sequences were 99.39% similar, and Coprinopsis gonophylla (syn. Coprinus gonophyllus) was the closest match for both. Strains were grown in pure culture on a rice-based medium that allowed the development of basidiomata from one culture of the main strain type, which was identified as C. episcopalis, a close relative of C. gonophyllus. Both species (or strain types) produced different types of illudin-like compounds. Three novel illudins (I, I2 and J2) were found to be produced by the cultures identified as C. episcopalis, while only illudinic acid was produced by the other Coprinopsis sp. The taxonomical relationships of the Coprinops is species identified in this study with other illudin producers previously reported in the literature are discussed.

Mutational specificity of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea in the Escherichia coli lacl gene of O6-alkylguanine-DNA alkyltransferase-proficient and -deficient strains

Forward mutations induced by 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) in the lacl gene of Escherichia coli were recovered from bacteria proficient (Ogt+ Ada+) and deficient (Ogt- Ada-) in O6-alkylguanine-DNA alkyltransferase activity. A CCNU dose of 1 mM was selected for DNA sequence analysis. A total of 245 induced mutations were characterized. The mutations were almost exclusively (95%) GC-->AT transitions, indicating that CCNU-induced mutations arose in bacteria primarily from misreplication of O6-chloroethylguanine, in total agreement with results obtained for monofunctional alkylating agents. The distribution of CCNU-induced GC-->AT mutations was significantly altered by the presence of DNA alkyltransferase activity (P = 0.01). In the Ogt+ Ada+ mutational spectrum, guanines flanked on both sides by A:T base-pairs were on average 2.8 times more likely to mutate than those flanked by G:C base-pairs on at least one side. This bias disappeared in the Ogt- Ada- genetic background, thereby providing evidence that O6-chloroethylated guanines adjacent to G:C base-pairs are better targets for bacterial alkyltransferase than those not adjacent to G:C base-pairs. We recently reported a similar bias for ethyl methanesulfonate, strengthening the idea that CCNU is acting as a simple ethylating compound. In summary, this paper presents for the first time evidence that DNA repair by O6-alkylguanine-DNA alkyltransferases plays a major role in removing lesions responsible for GC-->AT transitions induced by CCNU, influencing their ultimate distribution with respect to sequence context.

Streptozotocin-induced chromosomal aberrations, SCEs and mutations in CHO-9 parental cells and in EM-C11 mutant cell line

The genotoxic effects induced by the monofunctional nitrosourea derivative streptozotocin (STZ) were investigated in Chinese hamster ovary cells, parental (CHO-9) and its mutant hypersensitive to alkylating agents, designated EM-C11. The ability of this compound to induce chromosomal aberrations, cell killing, sister-chromatid exchanges (SCEs) and mutations was evaluated on these two cell lines. The mutant cells were found to be slightly more sensitive to the killing effects of STZ than the parental cell line. EM-C11 cells also showed higher levels of STZ-induced chromosomal aberrations than CHO-9 cells, but appeared to be equally sensitive to induction of SCEs. The frequencies of STZ-induced mutations, measured as resistant Na+/K(+)-ATPase and HPRT mutants, revealed a higher sensitivity of EM-C11 to the mutagenic effects of this compound.

DNA sequence analysis of spontaneous and gamma-radiation (anoxic)-induced lacId mutations in Escherichia coli umuC122::Tn5: differential requirement for umuC at G.C vs. A.T sites and for the production of transversions vs. transitions

Escherichia coli umuC122::Tn5 cells were gamma-irradiated (137Cs, 750 Gy, under N2), and lac-constitutive mutants were produced at 36% of the wild-type level (the umuC strain was not deficient in spontaneous mutagenesis, and the mutational spectrum determined by sequencing 263 spontaneous lacId mutations was very similar to that for the wild-type strain). The specific nature of the umuC strain's partial radiation mutability was determined by sequencing 325 radiation-induced lacId mutations. The yields of radiation-induced mutation classes in the umuC strain (as a percentage of the wild-type yield) were: 80% for A.T-->G.C transitions, 70% for multi-base additions, 60% for single-base deletions, 53% for A.T-->C.G transversions, 36% for G.C-->A.T transitions, 25% for multi-base deletions, 21% for A.T-->T.A transversions, 11% for G.C-->C.G transversions, 9% for G.C-->T.A transversions, and 0% for multiple mutations. Based on these deficiencies and other factors, it is concluded that the umuC strain is near-normal for A.T-->G.C. transitions, single-base deletions and possibly A.T-->C.G transversions; is generally deficient for mutagenesis at G.C sites and for transversions, and is grossly deficient in multiple mutations. Damage at G.C sites seems more difficult for translesion DNA synthesis to bypass than damage at A.T sites, and especially when trying to produce a transversion. The yield of G.C-->A.T transitions in the umuC strain (36% of the wild-type level) argues that abasic sites are involved in no more than 64% of gamma-radiation-induced base substitutions in the wild-type strain. Altogether, these data suggest that the UmuC and UmuD' proteins facilitate, rather than being absolutely required for, translesion DNA synthesis; with the degree of facilitation being dependent both on the nature of the noncoding DNA damage, i.e., at G.C vs. A.T sites, and on the nature of the misincorporated base, i.e., whether it induces transversions or transitions.

DNA damage and mutagenesis induced by nitrogen mustards

The nitrogen mustards are bifunctional alkylating agents which, although used extensively in cancer chemotherapy, are themselves highly carcinogenic. All nitrogen mustards induce monofunctional guanine-N7 adducts, as well as interstrand N7-N7 crosslinks involving the two guanines in GNC.GNC (5'-->3'/5'-->3') sequences. In addition, the aromatic mustards melphalan and chlorambucil also induce substantial alkylation at adenine N3, while cyclophosphamide forms phosphotriesters with relatively high frequency. Nitrogen mustards are genotoxic in virtually every assay, and produce a wide array of mutations, including base substitutions at both G.C and A.T base pairs, intragenic as well as multilocus deletions, and chromosomal rearrangements. Mutational spectra generated by these agents in various model systems vary widely, and no single lesion has been implicated as being primarily responsible for mustard-induced mutagenesis. On the contrary, adducts of both adenine and guanine, and monofunctional as well as bifunctional adducts, appear to be involved. Further, it is still not known which types of mutation are responsible for mustard-induced cancers, since no genes have yet been identified which are consistently altered in these malignancies.

The scid mutation in mice causes defects in the repair system for both double-strand DNA breaks and DNA cross-links

The sensitivity of scid fibroblasts established from C.B17-scid/scid fetuses to the DNA-damaging agents bleomycin, neocarzinostatin, mechlorethamine, mitomycin C, methyl methanesulfonate, and ultraviolet light, all of which induce different types of DNA damage, was examined. Scid fibroblasts were 2.8-, 3.7-, and 3.0-fold more sensitive to bleomycin, neocarzinostatin, and mechlorethamine, respectively, than wild-type fibroblasts derived from C.B17-+/+ fetuses. These findings indicate that the scid mutation in mice causes defects in repairing both double-strand DNA breaks and DNA cross-links.

Widespread adaptive response against environmental methylating agents in microorganisms

Many bacterial species have adaptive responses which protect against the toxicity and mutagenicity of methylating agents. Induced 3-methyladenine-DNA glycosylase and O6-methylguanine-DNA methyltransferase activities increase the cellular capacity of E. coli, B. subtilis, and M. luteus to repair toxic and mutagenic methylated base derivatives in DNA. The DNA methyltransferase or Ada protein of E. coli regulates the response and is converted into a strong transcriptional activator by self-methylation on repair of a methylphosphotriester in DNA. The multiple functions of the E. coli Ada protein (39 kDa) are split between two proteins, AdaA (24 kDa) and AdaB (20 kDa), in B. subtilis. Proteins (39 kDa) recognised by anti-Ada antibodies are efficiently induced in several enterobacterial species and correlate with increased DNA methyltransferase activities. In contrast, an "Ada-related" protein is only weakly induced in Salmonella typhimurium and no increase in DNA repair activity is detectable. The existence of adaptive responses in diverged bacterial species suggests the frequent occurrence of methylating agents in the environment. Several direct-acting methylating agents which are known to arise in the environment have been shown to induce the response. These include abundantly occurring methyl chloride, the antibiotic streptozotocin, the precursors of the known labile inducers N-methyl-N'-nitrosourea and N-methyl-N'-nitro-N-nitrosoguanidine and as shown in this paper, methyl radicals which may arise by the irradiation or oxidation of methyl compounds.

Role of ruvAB genes in UV- and gamma-radiation and chemical mutagenesis in Escherichia coli

Escherichia coli umuC122::Tn 5 was mutagenized with N-methyl-N'-nitro-N-nitrosoguanidine to isolate mutations that block the residual gamma-radiation mutagenesis observed in umuC strains. Two of these mutations were shown by transductional mapping and plasmid complementation to map in the ruvA and ruvB genes (i.e., ruvA200 and ruvB201). Whereas ruvA200 was complemented by ruvA+ plasmids, the only other known ruvA mutation, ruvA59::Tn10 required both the ruvA+ and ruvB+ genes to show complementation. The ruvA200, ruvB201, ruvA59::Tn10 and ruvB60::Tn10 mutations all reduced gamma-radiation-induced ochre reversion [argE3(Oc)----Arg+] to about 30% of the wild-type level, and they all reduced UV-radiation-induced ochre reversion to about 15% of the wild-type level. The ruvA200 and ruvB201 mutants also showed reduced gamma- and UV-radiation mutagenesis with two other assays [hisG4(Oc)----His+ and Rifs----Rifr]. Streptozotocin mutagenesis (Rifr) was reduced to about half of the wild-type level in ruv strains, but ethyl methanesulfonate mutagenesis was normal. While the umuC strain did not show the oxygen enhancement of gamma-radiation mutagenesis, the ruvA200 strain showed an oxygen effect that was similar to that shown by the wild-type strain. When the ruvA200 mutation was combined with the umuC mutation, gamma-radiation mutagenesis was further reduced to 5% of the wild-type level and cells showed a synergistic sensitization to UV- and gamma-radiation-induced killing. A mutational spectrum analysis indicates a general depression of both umuC-dependent and umuC-independent gamma-radiation mutagenesis in the ruvA strain, which is in contrast with the site-specific reduction in gamma-radiation mutagenesis that is observed in the umuC mutant. The reduced radiation mutagenesis in the ruvA strain could not be correlated with a reduction in transcription of the recA or umuC genes.

DNA repair mechanisms affecting cytotoxicity by streptozotocin in E. coli

Mechanisms underlying cytotoxicity by the monofunctional nitrosourea streptozotocin (STZ) were evaluated in DNA repair-deficient E. coli mutants. Strains not proficient in recombinational repair which lack either RecA protein or RecBC gene products were highly sensitive to STZ. In contrast, cells that constitutively synthesize RecA protein and cannot initiate SOS repair mechanisms because of uncleavable LexA repressor (recAo98 lexA3) were resistant to this drug compared to a lexA3 strain. Further, E. coli cells lacking both 3-methyladenine DNA glycosylases I (tag) and II (alkA) also were highly sensitive to STZ. DNA synthesis was most inhibited by STZ in recA and alkA tag E. coli mutants, but was suppressed less markedly in wild-type and recBC cells. DNA degradation was most extensive in recA E. coli after STZ treatment, while comparable in recBC, alkA tag, and wild-type cells. Although increased single-stranded DNA breaks were present after STZ treatment in recA and recBC mutants compared to the wild type, no significant increase in DNA single-stranded breaks was noted in alkA tag E. coli. Further, DNA breaks in recBC cells were repaired, while those present in recA cells were not. These findings establish the critical importance of both recombinational repair and 3-methyladenine DNA glycosylase in ameliorating cytotoxic effects and DNA damage caused by STZ in E. coli.

Mutational spectrum analysis of umuC-independent and umuC-dependent gamma-radiation mutagenesis in Escherichia coli

gamma-Radiation mutagenesis (oxic versus anoxic) was examined in wild-type, umuC and recA strains of Escherichia coli K-12. Mutagenesis [argE3(Oc)----Arg+] was blocked in a delta (recA-srlR)306 strain at the same doses that induced mutations in umuC122::Tn5 and wild-type strains, indicating that both umuC-independent and umuC-dependent mechanisms function within recA-dependent misrepair. Analyses of various suppressor and back mutations that result in argE3 and hisG4 ochre reversion and an analysis of trpE9777 (+1 frameshift) reversion were performed on umuC and wild-type cells irradiated in the presence and absence of oxygen. While the umuC strain showed the gamma-radiation induction of base substitution and frameshifts when irradiated in the absence of oxygen, the umuC mutation blocked all oxygen-dependent base-substitution mutagenesis, but not all oxygen-dependent frameshift mutagenesis. For anoxically irradiated cells, the yields of GC----AT [i.e., at the supB and supE (Oc) loci] and AT----GC transitions (i.e., at the argE3 and hisG4 loci) were essentially umuC independent, while the yields of (AT or GC)----TA transversions (i.e., at the supC, supL, supM, supN and supX loci) were heavily umuC dependent. These data suggest new concepts about the nature of the DNA lesions and the mutagenic mechanisms that lead to gamma-radiation mutagenesis.

Gene expression in E. coli after treatment with streptozotocin

Gene induction by the methylating agents streptozotocin (STZ), N-methyl-N-nitrosourea (MNU), and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was evaluated in E. coli fusion mutants. These mutants have fusions of the lac operon to genes induced by treatment with sublethal levels of alkylating agents and were previously selected from random insertions of the Mu-dl (Apr lac) phage by screening for induction of beta-galactosidase activity in the presence of methyl methanesulfonate or MNNG. The results demonstrate that STZ differs from MNNG and MNU in failing to induce aidC expression. Further, expression of aidC after exposure to MNU and MNNG occurs only in nonaerated cultures; aeration blocks the induction. Induction of aidD, alkA, aidB, and sfiA expression occurs with all 3 agents although at markedly lower concentrations of MNNG and STZ compared to MNU. alkA and to a lesser extent aidD mutants of E. coli strains were more sensitive to these agents, while no differences were evident between wild-type and aidB or aidC fusion mutants.