Apurinic endonuclease from Drosophila melanogaster: reduced enzymatic activity in excision-deficient mutants of the mei-9 and mus(2)201 Loci

Mutational spectra induced under distinct excision repair conditions by the 3 methylating agents N-methyl-N-nitrosourea, N-methyl-N'-nitro-N-nitrosoguanidine and N-nitrosodimethylamine in postmeiotic male germ cells of Drosophila

This paper describes the analysis of mutations induced at the vermilion locus in postmeiotic male germ cell stages of Drosophila exposed to 3 different N-methyl-N-nitroso compounds: N-methyl-N-nitrosourea (MNU); N-methyl-N'-nitro-N-nitrosoguanidine (MNNG); and N-nitrosodimethylamine (DMN). With MNU and DMN, the impact of DNA nucleotide excision repair (NER) on the spectra of mutations was studied. Mutants were isolated from F1 (mutations fixed before the first mitotic replication after fertilization) and F2 (mutations fixed following one or more mitotic replications; mosaics in F1) generations. The vermilion system enables the analysis of both intra- and inter-locus DNA changes for which several techniques have been adapted: (1) amplification of the vermilion gene by PCR, cloning of the fragment and sequence analysis of ssDNA; (2) Southern blot hybridization; and (3) cytological analysis of polytene chromosomes. In total, 49 MNU (26 from the exr+ genotype and 23 from the exr- genotype), 47 DMN (28 from the exr+ genotype and 19 from the exr- genotype) and 16 MNNG-induced mutations were characterized. The F1 spectra of all 3 agents contained base-pair changes and deletions (intra- and multi-locus) in a ratio of roughly 1 to 1, indicating a significant contribution of nitrogen DNA adducts to the spectra. In all F2 spectra the levels of base-pair changes were significantly higher compared to those in the F1 spectra, a finding also made for methyl methanesulfonate-induced mutations in earlier studies. There is an increase of mutations of, especially, the transversion types of mutations under exr- conditions in comparison to the exr+ situation. The induced transversions, clearly present in all spectra (exr+ and exr-), are presumably caused by N-methyl DNA adducts, which upon release from the DNA backbone lead to apurinic sites in a time-related process. Regarding the occurrence of transitions, it turned out for all 3 mutagens that the AT-->GC type strongly dominated the GC-->AT transitions. This suggest that O6-methylguanine is efficiently repaired, in contrast to O4-methylthymine. Based on the data obtained in the vermilion system with ENU, we propose, in addition, that the Drosophila alkyltransferase system repairs O6-methylguanine more efficiently than O6-ethylguanine.

DNA damage and repair in somatic and germ cells in vivo

Alkylation-induced germ cell mutagenesis in the mouse versus Drosophila is compared based on data from forward mutation assays (specific-locus tests in the mouse and in Drosophila and multiple-locus assays in the latter species) but not including assays for structural chromosome aberrations. To facilitate comparisons between mouse and Drosophila, forward mutation test results have been grouped into three categories. Representatives of the first category are MMS (methyl methanesulfonate) and EO (ethylene oxide), alkylating agents with a high s value which predominantly react with ring nitrogens in DNA. ENU (N-ethyl-N-nitrosourea), MNU (N-methyl-N-nitrosourea), PRC (procarbazine), DEN (N-nitrosodiethylamine), and DMN (N-nitrosodimethylamine) belong to the second category. These agents have in common a considerable ability for modification at oxygens in DNA. Cross-linking agents (melphalan, chlorambucil, hexamethylphosphoramide) form the third category. The most unexpected, but encouraging outcome of this study is the identification of common features for three vastly different experimental indicators of genotoxicity: hereditary damage in Drosophila males, genetic damage in male mice, and tumors (TD50 estimates) in rodents. Based on the above three category classification scheme the following tentative conclusions are drawn. Monofunctional agents belonging to category 1, typified by MMS and EO, display genotoxic effects in male germ cell stages that have passed meiotic division. This phenomenon seems to be the consequence of a repair deficiency during spermiogenesis for a period of 3-4 days in Drosophila and 14 days in the mouse. We suggest that the reason for the high resistance of premeiotic stages, and the generally high TD50 estimates observed for this class in rodents, is the efficient error-free repair of N-alkylation damage. If we accept this hypothesis, then the increased carcinogenic potential in rodents, seen when comparing category 2 (ENU-type mutagens) to category 1 (MMS-type mutagens), along with the ability of category 2 genotoxins to induce genetic damage in premeiotic stages, must presumably be due to their enhanced ability for alkylations at oxygens in DNA; it is this property that actually distinguishes the two groups from each other. In contrast to category 1, examination of class 2 genotoxins (ENU and DEN) in premeiotic cells of Drosophila gave no indication for a significant role of germinal selection, and also removal by DNA repair was less dramatic compared to MMS. Thus category 2 mutagens are expected to display activity in a wide range of both post- and premeiotic germ cell stages. A number of these agents have been demonstrated to be among the most potent carcinogens in rodents.(ABSTRACT TRUNCATED AT 400 WORDS)

Co-mutagenic effect of tannic acid on ring-X chromosome loss induced by mitomycin C in sperm cells of Drosophila melanogaster

To investigate the effects of tannic acid (TA) on ring-X chromosome loss, Drosophila melanogaster females exposed to different TA concentrations were crossed with untreated, methyl methanesulfonate (MMS)- or mitomycin C (MMC)-treated males which carried a ring-X chromosome. Progeny were analyzed for loss of the ring-X. The results of this in vivo study showed that TA had no suppressing effect on chromosome loss occurring spontaneously or after induction by MMS in mature spermatozoa. In contrast, TA caused a significant increase in the frequency of MMC-induced ring-X loss. The increase caused by this co-mutagenic effect reached values of 34, 33 and 40% at TA concentrations of 10, 25 and 50 mM, respectively. These increments may reflect the action of TA on a uvrABC-type enzyme which, by increasing the double-strand breaks (DSBs), somehow interferes with the post-replicational repair responsible for the final DSB correction.

Acrolein genotoxicity in Drosophila melanogaster. II. Influence of mus201 and mus308 mutations

The influence of mus201 and mus308 mutants on acrolein mutagenicity was analyzed with the Drosophila melanogaster sex-linked recessive lethal test (SLRL), using the maternal approach, to further study the mechanisms of action of this chemical. The hypermutability indices obtained were 2.59 for mus201 and 0.52 for mus308 conditions. Statistical analysis indicates that whereas part of the acrolein-induced lesions are repaired by excision mechanism, as expected for a cyclic agent, there is no demonstrable influence of the mus308 locus on the mutagenicity of this chemical.

Apurinic endonuclease activity from wild-type and repair-deficient mei-9 Drosophila ovaries

An endonuclease which acts on apurinic (AP) sites in DNA was partially purified from Drosophila ovaries. The enzyme present in the female germ line has a molecular weight of 63,000 daltons, is Mg++ dependent, and produces a site upon cleaving depurinated DNA that supports DNA repair synthesis. Although the same characteristics are shared by the enzyme present in the excision-deficient mutant mei-9, specific activity for the AP endonuclease is reduced 98% when compared with that found for its wild-type counterpart. Moreover, cross-reactivity toward an antibody that recognizes the wild-type AP endonuclease protein is reduced roughly 90% for partially purified preparations from mei-9. Mixing experiments between extracts of mei-9 and wild type suggest that the mei-9 structural gene somehow alters or influences the levels of the AP endonuclease protein, but in view of the complex phenotype of this mutant the endonuclease is probably not the product of the gene itself.

Isolation and characterization of second chromosome mutagen-sensitive mutations in Drosophila melanogaster

We have undertaken the study of a collection of 32 Drosophila melanogaster mus strains selected on the basis of developmental sensitivity to the DNA-damaging agents, methyl methanesulfonate (MMS), N-acetyl-2-aminofluorene (AAF), nitrogen mustard (HN2), and gamma-radiation. In total, 18 of these strains are sensitive to MMS. In turn, 14 of these exhibit unconditional MMS sensitivity (one of the latter mutants is lethal at 29 degrees C), whereas the other 4 are sensitive to MMS only at higher temperatures. Detailed analysis of the 7 strongest MMS-sensitive strains reveals that they identify 4 new second chromosome mus loci. Two mus loci are each represented by two alleles. One mutant (mus205B1) is allelic to a previously characterized mus locus. Different MMS-sensitive mutants display patterns of mutagen cross-sensitivity (to AAF, HN2, benzo[a]pyrene (BP), and gamma-rays) that parallel the range of responses seen in previously recovered X-linked and autosomal mus loci. In general, mutations that are strongly sensitive to MMS are also sensitive to one or both of the procarcinogens, AAF and BP, as opposed to HN2 and gamma-radiation. In contrast, the moderately MMS-sensitive mutations are sensitive to HN2 and gamma-rays, but not to AAF or BP. Of the 14 mus strains that are not sensitive to MMS, 5 are sensitive to AAF, another 5 are sensitive to HN2, and the remaining 4 are sensitive to gamma-rays.

Studies on mutagen-sensitive strains of Drosophila melanogaster. IV. Modification of genetic damage induced by X-irradiation of spermatozoa and spermatids in N2 or O2 by mei-9a, mei-41D5 and mus(1)101D1

The influence of defects in DNA repair processes on X-ray-induced genetic damage in post-meiotic male germ cell stages of Drosophila melanogaster was studied using the 'maternal effects approach'. Basc males were irradiated in N2, air or O2 either as 48-h-old pupae (to sample spermatids) or as 3-4-day-old adults (to sample mature spermatozoa) and mated to females of 3 repair-deficient strains (mei-9a: excision-repair-deficient; mei-41D5: post-replication-repair-deficient; mus(1)101D1: post-replication-repair-deficient and impaired in DNA synthesis). Simultaneous controls involving mating of males to repair-proficient females (mei+) were run. The frequencies of sex-linked recessive lethals and of autosomal translocations were determined following standard genetic procedures. The responses elicited in the different crosses with repair-deficient females were compared with those in mei+ crosses. The main findings are the following: with mei-9 females, the frequencies of recessive lethals are higher after irradiation of spermatids in N2, but not after irradiation in air of O2 (relative to those in the mei+ crosses); this result is different from that obtained in earlier work with spermatozoa, in which cell stage, higher yields of recessive lethals were obtained after irradiation of males in either N2 or air; in the mei-9 crosses, there are no significant differences in response (relative to mei+) after irradiation of either spermatozoa or spermatids in O2; the translocation frequencies in the mei-9 crosses are similar to those in the mei+ crosses, irrespective of the treated germ cell stage or the irradiation atmosphere; irradiation of either spermatozoa or spermatids in N2, air or O2 does not result in any differential recovery of recessive lethals in the mei-41 relative to mei+ crosses; irradiation of spermatids in N2 and of spermatozoa in air leads to a higher recovery of translocations in the mei-41 crosses; and after irradiation of spermatids or spermatozoa in any of the gaseous atmospheres, the frequencies of recessive lethals and of translocations are lower in the mus-101 crosses. The differences in responses (between cell stages, in different gaseous atmospheres and with different repair-deficient females) are explained on the basis of both qualitative and quantitative differences in the composition of the initial lesions and the extent to which their repair may be affected by the defects present in the different repair-deficient females. Several discrepancies between expectations based on biochemical results and the genetic results are pointed out.(ABSTRACT TRUNCATED AT 400 WORDS)

Apurinic/apyrimidinic-specific endonuclease activities from Dictyostelium discoideum

Two apurinic/apyrimidinic- (AP-) specific endonuclease activities have been isolated from the cells of Dictyostelium discoideum by fractionation on DEAE-cellulose, CM-cellulose and Sephadex G-75. These activities, designated A and B, have apparent molecular weights of 49000 and 40000, respectively. Although their precise reaction optima differ somewhat, both A and B quantitatively nick AP DNA best at pH 8.0-8.5 in low salt (less than 100 mM NaCl); both require Mg2+. These activities are apparently specific only for AP sites in DNA. The low activities observed on heavily ultraviolet-irradiated DNA, gamma-irradiated DNA and osmium tetroxide-treated DNA are consistent with the small numbers of secondary AP sites expected in these DNAs. Both A and B produce single-strand nicks in AP DNA that result in termini that serve as good primers for Escherichia coli polymerase I. Hence, A and B appear to be Class II AP endonucleases which yield 3'-OH termini at nicks on the 5' side of baseless sugars. It is unclear whether A and B are independently coded proteins, different post-translational modifications of the same gene product, or whether one is an artifact arising from the isolation. Many of the properties of these D. discoideum AP endonuclease activities are similar to those of the predominant AP endonucleases observed in bacterial, plant and animal cells. They will be of use in the characterization of excision repair in this organism.

Apurinic endonuclease activity remains constant during early Drosophila development

An endonuclease activity that acts on alkali-labile lesions in x-irradiated PM2 DNA and recognizes apurinic lesions in heat/acid treated DNA has been partially purified from Drosophila melanogaster embryos and its specific activity monitored throughout early development. The enzyme activity also showed a low level of activity on UV-irradiated DNA. The saturation kinetics observed with both x-irradiated and apurinic PM2 DNA substrates were similar. The endonuclease activity exhibited a broad pH optimum between pH 6 and 8.5 and was almost completely inhibited by 100 mM NaCl, 0.1 mM EDTA, 2 mM CaCl2 and 10 mM NEM. The reaction was not completely dependent on the presence of Mg++cation, but optimum activity was obtained at a concentration of 0.1 mM; concentrations greater than 1 mM Mg++ were inhibitory. The specific activity of the apurinic endonuclease, partially purified from several stages of embryonic and early larval development, remained the same. Unfertilized eggs exhibited a reduced level of this presumptive repair activity.

Partial purification and characterization of the major AP endonuclease from Chlamydomonas reinhardi

The major AP endonuclease from Chlamydomonas reinhardi has been partially purified and characterized. The enzyme has a molecular weight of about 38 000 as measured by molecular sieving. There is an absolute requirement for a divalent cation, with magnesium being better than manganese. The activity is stimulated by dithiothreitol and Triton X-100. The activity is sensitive to ionic strength, as 50 mM NaCl or KCl results in 70% inhibition. The enzyme is specific for apurinic and apyrimidinic (AP) sites and does not cleave DNA that has been damaged by ultraviolet light, methyl methanesulfonate, osmium tetroxide or sodium bisulfite. There is no deficiency in the AP endonuclease activity in extracts prepared from two mutants of Chlamydomonas that are sensitive to both ultraviolet light and methyl methanesulfonate. There was no evidence for induction of AP endonuclease after exposure of the cells to methyl methanesulfonate.

The induction of chromosome aberrations by cis-platinum(II) diamminodichloride in Drosophila melanogaster

We have determined the in vivo effects of cis-platinum(II)diamminodichloride (cis-PDD) treatment on the induction of chromosome aberrations in Drosophila melanogaster germ cells. cis-PDD treatment induces significant increases in chromosome breakage in all stages of spermatogenesis in a battery of test systems using ring or rod-X males and repair-proficient or deficient females. Since no increase in nondisjunction was induced by cis-PDD in either male or female germ cells, any aneuploidy inducing effects of this compound should result from its clastogenic action. We also find that mei-9 excision repair function is involved in the repair of cis-PDD-induced DNA lesions in a manner that provides additional evidence that partial and ring chromosome losses are not completely homologous.