Mutagenesis induced by mono- and bi-functional alkylating agents in yeast mutants sensitive to photo-addition of furocoumarins (pso)

Novel method for reduction of virus load in blood plasma by sonication

BACKGROUND

Aim of the present study is the evaluation of ultrasound as a physical method for virus inactivation in human plasma products prior to transfusion. Our study is focused on achieving a high level of virus inactivation simultaneously leaving blood products unaltered, measured by the level of degradation of coagulation factors, especially in third world countries where virus contamination of blood products poses a major problem. Virus inactivation plays an important role, especially in the light of newly discovered or unknown viruses, which cannot be safely excluded via prior testing.

METHODS

Taking into account the necessary protection of the relevant coagulation activity for plasma, the basis for a sterile virus inactivation under shielding gas insufflation was developed for future practical use. Influence of frequency and power density in the range of soft and hard cavitation on the inactivation of transfusion-relevant model viruses for Hepatitis-(BVDV = bovine diarrhea virus), for Herpes-(SFV = Semliki Forest virus, PRV = pseudorabies virus) and Parvovirus B19 (PPV = porcine parvovirus) were examined. Coagulation activity was examined via standard time parameters to minimize reduction of functionality of coagulation proteins. A fragmentation of coagulation proteins via ultrasound was ruled out via gel electrophoresis. The resulting virus titer was examined using end point titration.

RESULTS

Through CO₂ shielding gas insufflation-to avoid radical emergence effects-the coagulation activity was less affected and the time window for virus inactivation substantially widened. In case of the non-lipidated model virus (AdV-luc = luciferase expressing adenoviral vector), the complete destruction of the virus capsid through hard cavitation was proven via scanning electron microscopy (SEM). This can be traced back to microjets and shockwaves occurring in hard cavitation. The degree of inactivation seems to depend on size and compactness of the type of viruses. Using our pre-tested and subsequently chosen process parameters with the exception of the small PPV, all model viruses were successfully inactivated and reduced by up to log 3 factor. For a broad clinical usage, protection of the coagulation activities may require further optimization.

CONCLUSIONS

Building upon the information gained, an optimum inactivation can be reached via raising of power density up to 1200 W and simultaneous lowering of frequency down to 27 kHz. In addition, the combination of the two physical methods UV treatment and ultrasound may yield optimum results without the need of substance removal after the procedure.

REV1 and DNA polymerase zeta in DNA interstrand crosslink repair

DNA interstrand crosslinks (ICLs) are covalent linkages between two strands of DNA, and their presence interferes with essential metabolic processes such as transcription and replication. These lesions are extremely toxic, and their repair is essential for genome stability and cell survival. In this review, we will discuss how the removal of ICLs requires interplay between multiple genome maintenance pathways and can occur in the absence of replication (replication-independent ICL repair) or during S phase (replication-coupled ICL repair), the latter being the predominant pathway used in mammalian cells. It is now well recognized that translesion DNA synthesis (TLS), especially through the activities of REV1 and DNA polymerase zeta (Polζ), is necessary for both ICL repair pathways operating throughout the cell cycle. Recent studies suggest that the convergence of two replication forks upon an ICL initiates a cascade of events including unhooking of the lesion through the actions of structure-specific endonucleases, thereby creating a DNA double-stranded break (DSB). TLS across the unhooked lesion is necessary for restoring the sister chromatid before homologous recombination repair. Biochemical and genetic studies implicate REV1 and Polζ as being essential for performing lesion bypass across the unhooked crosslink, and this step appears to be important for subsequent events to repair the intermediate DSB. The potential role of Fanconi anemia pathway in the regulation of REV1 and Polζ-dependent TLS and the involvement of additional polymerases, including DNA polymerases kappa, nu, and theta, in the repair of ICLs is also discussed in this review.

Strategies for DNA interstrand crosslink repair: insights from worms, flies, frogs, and slime molds

DNA interstrand crosslinks (ICLs) are complex lesions that covalently link both strands of the DNA double helix and impede essential cellular processes such as DNA replication and transcription. Recent studies suggest that multiple repair pathways are involved in their removal. Elegant genetic analysis has demonstrated that at least three distinct sets of pathways cooperate in the repair and/or bypass of ICLs in budding yeast. Although the mechanisms of ICL repair in mammals appear similar to those in yeast, important differences have been documented. In addition, mammalian crosslink repair requires other repair factors, such as the Fanconi anemia proteins, whose functions are poorly understood. Because many of these proteins are conserved in simpler metazoans, nonmammalian models have become attractive systems for studying the function(s) of key crosslink repair factors. This review discusses the contributions that various model organisms have made to the field of ICL repair. Specifically, it highlights how studies performed with C. elegans, Drosophila, Xenopus, and the social amoeba Dictyostelium serve to complement those from bacteria, yeast, and mammals. Together, these investigations have revealed that although the underlying themes of ICL repair are largely conserved, the complement of DNA repair proteins utilized and the ways in which each of the proteins is used can vary substantially between different organisms.

RNR4 mutant alleles pso3-1 and rnr4Delta block induced mutation in Saccharomyces cerevisiae

The PSO3 gene of Saccharomyces cerevisiae was molecularly cloned by complementing the cold-sensitivity phenotype of a pso3-1 mutant and was found to be allelic to RNR4, encoding one of the two DNA damage-inducible small subunits of the ribonucleotide reductase (RNR) complex. Compared to a rnr4Delta mutant that allows only very little mutation induction at very low doses of 254(nm) ultraviolet light (UVC), the pso3-1 mutant allele confers leakiness in that it permits some DNA damage-induced mutagenesis at low doses of UVC. Similarly, the pso3 mutant is slightly less sensitive to UVC than an rnr4Delta mutant. Cloning and sequencing of the RNR4 locus of the pso3-1 mutant revealed that its intermediate phenotype is attributable to a G --> A transition at nucleotide 352, leading to replacement of glycine by arginine [G118R] in the mutant's protein. Both RNR4 mutant alleles confer significantly less sensitivity to UVC than mutant alleles of non-UVC-mutable REV3, indicating that, apart from nucleotide excision repair, RAD6-dependent error-free DNA repair may still be functional. The phenotype of a strongly reduced UVC-induced mutagenesis for rnr4 mutant alleles has not yet been described; it suggests the importance of this gene for a fully functional RNR providing correct amounts of DNA precursor molecules, thereby, allowing translesion synthesis (error-prone) of UVC-damaged DNA. Stationary phase cells of the rnr4Delta mutant, but not of the original pso3-1 mutant, are swollen with a fourfold to eightfold increase in volume. The central role of RNR in DNA precursor metabolism and its complex regulation allow for several modes of suppression that may influence the phenotypes of RNR4 mutants, especially those containing the leaky pso3-1 mutant allele.

DNA interstrand cross-link repair in the Saccharomyces cerevisiae cell cycle: overlapping roles for PSO2 (SNM1) with MutS factors and EXO1 during S phase

Pso2/Snm1 is a member of the beta-CASP metallo-beta-lactamase family of proteins that include the V(D)J recombination factor Artemis. Saccharomyces cerevisiae pso2 mutants are specifically sensitive to agents that induce DNA interstrand cross-links (ICLs). Here we establish a novel overlapping function for PSO2 with MutS mismatch repair factors and the 5'-3' exonuclease Exo1 in the repair of DNA ICLs, which is confined to S phase. Our data demonstrate a requirement for NER and Pso2, or Exo1 and MutS factors, in the processing of ICLs, and this is required prior to the repair of ICL-induced DNA double-strand breaks (DSBs) that form during replication. Using a chromosomally integrated inverted-repeat substrate, we also show that loss of both pso2 and exo1/msh2 reduces spontaneous homologous recombination rates. Therefore, PSO2, EXO1, and MSH2 also appear to have overlapping roles in the processing of some forms of endogenous DNA damage that occur at an irreversibly collapsed replication fork. Significantly, our analysis of ICL repair in cells synchronized for each cell cycle phase has revealed that homologous recombination does not play a major role in the direct repair of ICLs, even in G2, when a suitable template is readily available. Rather, we propose that recombination is primarily involved in the repair of DSBs that arise from the collapse of replication forks at ICLs. These findings have led to considerable clarification of the complex genetic relationship between various ICL repair pathways.

Role of PSO genes in repair of DNA damage of Saccharomyces cerevisiae

Photoactivated psoralens used in treatment of skin diseases like Psoriasis and Vitiligo cause DNA damage, the repair of which may lead to mutations and thus to higher risk to have skin cancer. The simple eukaryote Saccharomyces cerevisiae was chosen to investigate the cells' genetic endowment with repair mechanisms for this type of DNA damage and to study the genetic consequences of such repair. Genetic studies on yeast mutants sensitive to photoactivated psoralens, named pso mutants, showed their allocation to 10 distinct loci. Cloning and molecular characterization allowed their grouping into three functional classes: (I) the largest group comprises seven PSO genes that are either generally or specifically involved in error-prone DNA repair and thus affect induced mutability and recombination; (II) one PSO gene that represents error-free excision repair, and (III) two PSO genes encoding proteins not influencing DNA repair but physiological processes unrelated to nucleic acid metabolism. Of the seven DNA repair genes involved in induced mutagenesis three PSO loci [PSO1/REV3, PSO8/RAD6, PSO9/MEC3] were allelic to already known repair genes, whereas three, PSO2/SNM1, PSO3/RNR4, and PSO4/PRP19 represent new genes involved in DNA repair and nucleic acid metabolism in S. cerevisiae. Gene PSO2 encodes a protein indispensable for repair of interstrand cross-link (ICL) that are produced in DNA by a variety of bi- and polyfunctional mutagens and that appears to be important for a likewise repair function in humans as well. In silico analysis predicts a putative endonucleolytic activity for Pso2p/Snm1p in removing hairpins generated as repair intermediates. The absence of induced mutation in pso3/rnr4 mutants indicates an important role of this subunit of ribonucleotide reductase (RNR) in regulation of translesion polymerase zeta in error-prone repair. Prp19p/Pso4p influences efficiency of DNA repair via splicing of pre-mRNAs of intron-containing repair genes but also may function in the stability of the nuclear scaffold that might influence DNA repair capacity. The seventh gene, PSO10 which controls an unknown step in induced mutagenesis is not yet cloned. Two genes, PSO6/ERG3 and PSO7/COX11, are responsible for structural elements of the membrane and for a functional respiratory chain (RC), respectively, and their function thus indirectly influences sensitivity to photoactivated psoralens.

Schizosaccharomyces pombe checkpoint response to DNA interstrand cross-links

Drugs that produce covalent interstrand cross-links (ICLs) in DNA remain central to the treatment of cancer, but the cell cycle checkpoints activated by ICLs have received little attention. We have used the fission yeast, Schizosaccharomyces pombe, to elucidate the checkpoint responses to the ICL-inducing anticancer drugs nitrogen mustard and mitomycin C. First we confirmed that the repair pathways acting on ICLs in this yeast are similar to those in the main organisms studied to date (Escherichia coli, budding yeast, and mammalian cells), principally nucleotide excision repair and homologous recombination. We also identified and disrupted the S. pombe homologue of the Saccharomyces cerevisiae SNM1/PSO2 ICL repair gene and found that this activity is required for normal resistance to cross-linking agents, but not other forms of DNA damage. Survival and biochemical analysis indicated a key role for the "checkpoint Rad" family acting through the chk1-dependent DNA damage checkpoint in the ICL response. Rhp9-dependent phosphorylation of Chk1 correlates with G(2) arrest following ICL induction. In cells able to bypass the G(2) block, a second-cycle (S-phase) arrest was observed. Only a transient activation of the Cds1 DNA replication checkpoint factor occurs following ICL formation in wild-type cells, but this is increased and persists in G(2) arrest-deficient mutants. This likely reflects the fraction of cells escaping the G(2) damage checkpoint and arresting in the subsequent S phase due to ICL replication blocks. Disruption of cds1 confers increased resistance to ICLs, suggesting that this second-cycle S-phase arrest might be a lethal event.

Repair of DNA interstrand cross-links

DNA interstrand cross-links (ICLs) are very toxic to dividing cells, because they induce mutations, chromosomal rearrangements and cell death. Inducers of ICLs are important drugs in cancer treatment. We discuss the main properties of several classes of ICL agents and the types of damage they induce. The current insights in ICL repair in bacteria, yeast and mammalian cells are reviewed. An intriguing aspect of ICLs is that a number of multi-step DNA repair pathways including nucleotide excision repair, homologous recombination and post-replication/translesion repair all impinge on their repair. Furthermore, the breast cancer-associated proteins Brca1 and Brca2, the Fanconi anemia-associated FANC proteins, and cell cycle checkpoint proteins are involved in regulating the cellular response to ICLs. We depict several models that describe possible pathways for the repair or replicational bypass of ICLs.

Increased DNA double strand breakage is responsible for sensitivity of the pso3-1 mutant of Saccharomyces cerevisiae to hydrogen peroxide

Escherichia coli endonuclease III (endo III) is the key repair enzyme essential for removal of oxidized pyrimidines and abasic sites. Although two homologues of endo III, Ntgl and Ntg2, were found in Saccharomyces cerevisiae, they do not significantly contribute to repair of oxidative DNA damage in vivo. This suggests that an additional activity(ies) or a regulatory pathway(s) involved in cellular response to oxidative DNA damage may exist in yeast. The pso3-1 mutant of S. cerevisiae was previously shown to be specifically sensitive to toxic effects of hydrogen peroxide (H2O2) and paraquat. Here, we show that increased DNA double strand breakage is very likely the basis of sensitivity of the pso3-1 mutant cells to H2O2. Our results, thus, indicate an involvement of the Pso3 protein in protection of yeast cells from oxidative stress presumably through its ability to prevent DNA double strand breakage. Furthermore, complementation of the repair defects of the pso3-1 mutant cells by E. coli endo III has been examined. It has been found that expression of the nth gene in the pso3-1 mutant cells recovers survival, decreases mutability and protects yeast genomic DNA from breakage following H2O2 treatment. This might suggest some degree of functional similarity between Pso3 and Nth.

Excision repair of nitrogen mustard-DNA adducts in Saccharomyces cerevisiae

The bifunctional alkylating anticancer drug nitrogen mustard forms a variety of DNA lesions, including monoadducts and intrastrand and interstrand crosslinks. Although it is known that nucleotide excision repair (NER) is important in processing these adducts, the role of the other principal excision repair pathway, base excision repair (BER) is less well defined. Using isogenic Saccharomyces cerevisiae strains disrupted for a variety of NER and BER genes we have examined the relative importance of the two pathways in the repair of nitrogen mustard adducts. As expected, NER defective cells (rad4 and rad14 strains) are extremely sensitive to the drug. One of the BER mutants, a 3-methyladenine glycosylase defective (mag1) strain also shows significant hypersensitivity. Using a rad4/mag1 double mutant it is shown that the two excision repair pathways are epistatic to each other for nitrogen mustard sensitivity. Furthermore, both rad14 and mag1 disruptants show elevated levels of nitrogen mustard-induced forward mutation. Measurements of repair rates of nitrogen mustard N-alkylpurine adducts in the highly transcribed RPB2 gene demonstrate defects in the processing of mono-adducts in rad4, rad14 and mag1 strains. However, there are differences in the kinetics of adduct removal in the NER mutants compared to the mag1 strain. In the mag1 strain significant repair occurs within 1 h with evidence of enhanced repair on the transcribed strand. Adducts however accumulate at later times in this strain. In contrast, in the NER mutants repair is only evident at times greater than 1 h. In a mag1/rad4 double mutant damage accumulates with no evidence of repair. Comparison of the rates of repair in this gene with those in a different genomic region indicate that the contributions of NER and BER to the repair of nitrogen mustard adducts may not be the same genome wide.

A Chlamydomonas reinhardtii UV-sensitive mutant uvs15 is impaired in a gene involved in several repair pathways

In this report, three DNA repair-deficient mutants of Chlamydomonas reinhardtii (uvs13, uvs14, uvs15) were characterized by using genetic, mutational and biochemical analyses. The mutant strain uvs15 belongs to the most sensitive repair-deficient mutants following exposure to all agents used. It is deficient in the nuclear excision-repair pathway, whereas uvs13 and uvs14 are not blocked in removal of pyrimidine dimers. Mutation study also revealed differences among strains. The mutant uvs15 does not mutate after UV and X-ray irradiation, and there is very low mutation rate after MNNG. These findings might indicate the involvement of UVS15 gene product in regulation of several repair pathways. Contrary to this, uvs14 showed higher mutation frequency, both spontaneous and induced after UV and MNNG treatments. Tetrad dissection proved that the uvs13 and uvs14 genes are located on the right arm of the linkage group I in the vicinity of the previously mapped uvs10 gene. Both mutants belong to the same repair pathway, which is different from that of uvs10 and uvs15.

Role of PSO genes in the repair of photoinduced interstrand cross-links and photooxidative damage in the DNA of the yeast Saccharomyces cerevisiae

Recent progress in elucidating the molecular structure of the PSSO genes PSO2 to PSO7 is presented. Their role in DNA repair and mutagenesis is discussed in the light of the putative proteins encoded in the respective ORFs and with the knowledge of recent progress in biological and biochemical experimentation. The role of the RecA protein in some steps of DNA repair in Saccharomyces cerevisiae is presented and discussed.

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 E. coli recA gene can restore the defect in mutagenesis of the pso4-1 mutant of S. cerevisiae

The E. coli recA gene was introduced into the pso4-1 mutant of S. cerevisiae and transformants were treated with 8-MOP+UVA and 254-nm UV light. The results showed that the recA gene increased the resistance to the toxic effect of 8-MOP+UVA and restored the frequency of reversion of the pso4-1 mutants after both treatments. The presence of the recA gene stimulated expression of the small subunit of the ribonucleotide reductase (Rnr2) in the pso4-1 mutants. Thus the E. coli recA gene is functional in yeast. Moreover, it was shown that the pso4-1 mutant is epistatic to pso1-1 and rad6-1, which belong to a mutagenic repair pathway. We propose here that the PSO4 gene has some role in the control of mutagenic repair in yeast.

Genetic effects of photoactivated psoralens during meiosis in DNA repair mutant pso3-1 of Saccharomyces cerevisiae

The influence of the DNA repair gene PSO3 on photoactivated psoralen-induced meiotic recombination, gene conversion, reverse mutation, and on survival, was assayed in diploid strains of Saccharomyces cerevisiae homozygous for the wild-type or the pso3-1 mutant allele. Sporulation was normal in the pso3-1 diploid. Wild-type and mutant strains had the same sensitivity to photoactivated monofunctional psoralen (3-CPs + UVA) in meiosis-uncommitted and meiosis-committed stages. The mutant showed higher sensitivity to photoactivated bifunctional psoralen (8-MOP + UVA) during all stages of the meiotic cycle. Mutation induction by 3-CPs + UVA or 8-MOP + UVA in meiosis-committed cells revealed no significant differences between wild-type and the pso3-1 mutant. The status of the PSO3 gene has no influence on the kinetics of induction of gene conversion and crossing-over after 3-CPs + UVA treatment in meiosis-committed cells: gene conversion was blocked while recombination was induced. After treatment with 8-MOP + UVA gene conversion was also blocked in both strains while crossing-over could only be observed in meiosis-committed wild-type cells.

Antimutagenicity in yeast

In recent years there has been increasing interest in antimutagenesis, and studies have been done using both prokaryotic and eukaryotic systems. In eukaryotic systems the first studies were performed with different strains of Schizosaccharomyces pombe. In particular, caffeine and L-methionine were investigated. Different strains of Saccharomyces cerevisiae were employed in studies of a wide variety of compounds, including acridine, saccharin, salts, tumor promoters and co-carcinogens. Strain D7 was widely employed and antimutagenic activity of spermine, chlorophyllin, cobaltous chloride and fermented milk is reported.

The DNA repair gene PSO3 of Saccharomyces cerevisiae belongs to the RAD3 epistasis group

The mutant allele pso3-1 of Saccharomyces cerevisiae confers sensitivity to treatment with UV365nm (UVA) light-activated mono- and bi-functional psoralens. When pso3-1 is combined in double mutants with selected rad and pso mutant alleles and subjected to 8-MOP + UVA treatment, epistatic interaction with regard to survival is observed with pso1, pso2, and rad3. With the same treatment the combination of pso3-1 with rad6 and rad52 leads to synergistic interaction. For the monofunctional agent 3-carbethoxypsoralen (3-CPs) the analysis of double mutants yields the same results as with the bifunctional 8-methoxypsoralen (8-MOP) with the exception of the pso1-1pso3-1 double mutant. Here we find an additive interaction, i.e., the sensitivities of both parental strains are summed in the double mutant, which indicates a different substrate specificity of the repair activity encoded by the PSO1 and PSO3 genes.

Repair of exogenous (plasmid) DNA damaged by photoaddition of 8-methoxypsoralen in the yeast Saccharomyces cerevisiae

The contribution of different repair pathways to the repair of 8-methoxypsoralen (8-MOP) plus UVA induced lesions on a centromeric plasmid (YCp50) was investigated in the yeast Saccharomyces cerevisiae using the lithium acetate transformation method. The pathways of excision-resynthesis (RAD1) and recombination (RAD52) were found to be involved in the repair of exogenous as well as of genomic DNA. Mutants in RAD6 and PSO2 genes showed the same transformation efficiency with 8-MOP plus UVA treated plasmid as wild-type cells suggesting that these latter pathways involved in mutagenesis are not operating on plasmid DNA although required for the repair of 8-MOP photoadducts induced in genomic DNA. These results indicate that DNA-repair gene products may be differently involved in the repair of exogenous and endogenous DNA depending on the repair system and the nature of the DNA damage considered.

Cross-sensitivity of gamma-ray-sensitive hamster mutants to cross-linking agents

A range of hamster cell mutants, which have been characterised as sensitive to ionising radiation, were examined for their cross-sensitivity to four DNA-DNA cross-linking agents and the protein-DNA cross-linking agent, camptothecin. The mutants represent 7 distinct complementation groups. Two complementation groups were identified as having a major sensitivity to cross-linking damage, more marked than their sensitivity to ionising radiation (irs1, irs1SF). These two mutants also show sensitivity to UV-irradiation. Two of the remaining complementation groups (xrs and XR-1) have a defect in rejoining DNA double-strand breaks, and these exhibit sensitivity to 3 of the 4 DNA-DNA cross-linking agents. The results with these mutants suggest an involvement of double-strand break rejoining in the repair of certain cross-link damage. Two mutants were also notably sensitive to the topoisomerase I inhibiting anticancer drug, camptothecin. One of these mutants was sensitive to the DNA cross-linking agents examined (irs1SF), but the other was not at all sensitive to this class of drug (EM9).

The PSO3 gene is involved in error-prone intragenic recombinational DNA repair in Saccharomyces cerevisiae

The induction of gene conversion and mitotic crossing-over by photoaddition of psoralens, 254 nm ultraviolet radiation, and nitrogen mustards was determined in diploid cells homozygous for the pso3-1 mutation and in the corresponding wild type of Saccharomyces cerevisiae. For these different agents, the frequency of non-reciprocal events (conversion) is reduced in the pso3-1 mutant compared to the wild type. In contrast, the frequency of reciprocal events (crossing-over) is increased at a range of doses. These observations, together with the block in induced mutagenesis for both reverse and forward mutations previously reported for the pso3-1 mutant, suggest that the PSO3 gene product plays a role in mismatch repair of short patch regions. The block in gene conversion in the pso3 homozygous diploid leads, in the case of nitrogen mustards, to specific repair intermediates which are lethal to the cells.

PSO4: a novel gene involved in error-prone repair in Saccharomyces cerevisiae

The haploid xs9 mutant, originally selected for on the basis of a slight sensitivity to the lethal effect of X-rays, was found to be extremely sensitive to inactivation by 8-methoxypsoralen (8MOP) photoaddition, especially when cells are treated in the G2 phase of the cell cycle. As the xs9 mutation showed no allelism with any of the 3 known pso mutations, it was now given the name of pso4-1. Regarding inactivation, the pso4-1 mutant is also sensitive to mono- (HN1) or bi-functional (HN2) nitrogen mustards, it is slightly sensitive to 254 nm UV radiation (UV), and shows nearly normal sensitivity to 3-carbethoxypsoralen (3-CPs) photoaddition or methyl methanesulfonate (MMS). Regarding mutagenesis, the pso4-1 mutation completely blocks reverse and forward mutations induced by either 8MOP or 3CPs photoaddition, or by gamma-rays. In the cases of UV, HN1, HN2 or MMS treatments, while reversion induction is still completely abolished, forward mutagenesis is only partially inhibited for UV, HN1, or MMS, and it is unaffected for HN2. Besides severely inhibiting induced mutagenesis, the pso4-1 mutation was found to be semi-dominant, to block sporulation, to abolish the diploid resistance effect, and to block induced mitotic recombination, which indicates that the PSO4 gene is involved in a recombinational pathway of error-prone repair, comparable to the E. coli SOS repair pathway.

Reassessing the genotoxic potential of 8-MOP + UVA-induced DNA damage in the yeast Saccharomyces cerevisiae

Two different UVA irradiation systems were initially biologically calibrated with two haploid yeast strains proficient and deficient, respectively, in nucleotide excision repair. The number of DNA lesions introduced into the cell's genome by the photoactivated bifunctional furocoumarin 8-MOP was then calculated by means of the applied UVA exposure doses. At LD37 the repair-proficient wild type had about 14 ICL and 34 furan-side monoadducts in its DNA, while doubly blocked repair mutant rad3-12 pso1-1 had 2 ICL and 3 monoadducts. Locus-specific reversion of lys1-1 followed two-hit kinetics in the repair-proficient wild type and one-hit kinetics in an excision-deficient rad2-20 mutant, as would be expected if ICL was the main type of mutagenic lesion in the wild type and monoadducts the main mutagenic lesion type in the excision-deficient strain. Quantitative comparison of 8-MOP + UVA-induced ICL with those induced by bifunctional mustard revealed the former to have a much higher genotoxicity.

Saccharomyces cerevisiae mutants with enhanced induced mutation and altered mitotic gene conversion

We have developed a method to isolate yeast (Saccharomyces cerevisiae) mutants with enhanced induced mutagenesis based on nitrous acid-induced reversion of the ade2-42 allele. Six mutants have been isolated and designated him (high induced mutagenesis), and 4 of them were studied in more detail. The him mutants displayed enhanced reversion of the ade2-42 allele, either spontaneous or induced by nitrous acid, UV light, and the base analog 6-N-hydroxylaminopurine, but not by gamma-irradiation. It is worth noting that the him mutants turned out not to be sensitive to the lethal effects of the mutagens used. The enhancement in mutation induced by nitrous acid, UV light, and 6-N-hydroxylaminopurine has been confirmed in a forward-mutation assay (induction of mutations in the ADE1, ADE2 genes). The latter agent revealed the most apparent differences between the him mutants and the wild-type strain and was, therefore, chosen for the genetic analysis of mutants, him mutations analyzed behaved as a single Mendelian trait; complementation tests indicated 3 complementation groups (HIM1, HIM2, and HIM3), each containing 1 mutant allele. Uracil-DNA glycosylase activity was determined in crude cell extracts, and no significant differences between the wild-type and him strains were detected. Spontaneous mitotic gene conversion at the ADE2 locus is altered in him1 strains, either increased or decreased, depending on the particular heteroallelic combination. Genetic evidence strongly suggests him mutations to be involved in a process of mismatch correction of molecular heteroduplexes.

The PSO4 gene is responsible for an error-prone recombinational DNA repair pathway in Saccharomyces cerevisiae

The induction of mitotic gene conversion and crossing-over in Saccharomyces cerevisiae diploid cells homozygous for the pso4-1 mutation was examined in comparison to the corresponding wild-type strain. The pso4-1 mutant strain was found to be completely blocked in mitotic recombination induced by photoaddition of mono- and bifunctional psoralen derivatives as well as by mono- (HN1) and bifunctional (HN2) nitrogen mustards or 254 nm UV radiation in both stationary and exponential phases of growth. Concerning the lethal effect, diploids homozygous for the pso4-1 mutation are more sensitive to all agents tested in any growth phase. However, this effect is more pronounced in the G2 phase of the cell cycle. These results imply that the ploidy effect and the resistance of budding cells are under the control of the PSO4 gene. On the other hand, the pso4-1 mutant is mutationally defective for all agents used. Therefore, the pso4-1 mutant has a generalized block in both recombination and mutation ability. This indicates that the PSO4 gene is involved in an error-prone repair pathway which relies on a recombinational mechanism, strongly suggesting an analogy between the pso4-1 mutation and the RecA or LexA mutation of Escherichia coli.

Survival and mutagenic responses of mitomycin C-sensitive mouse lymphoma cell mutants to other DNA cross-linking agents

Mitomycin C-sensitive mutants MCN 151 (complementation group I) and MCE 50 (complementation group II) derived from mouse lymphoma L5178Y cells were found to be also highly sensitive to the lethal effects of other DNA cross-linking agents, such as photoaddition of 8-methoxypsoralen (8-MOP) and cis-diamminedichloroplatinum II (cis-DDP). They were less sensitive to the monofunctional derivative 3-carbethoxypsoralen (3-CPs) and to trans-DDP to trans-DDP than their bifunctional counterparts. Incorporation levels of labeled 8-MOP or 3-CPs in wild-type cells and 2 mutants were almost the same, indicating that the sensitivity is not caused by differential incorporation of the agents. The rates of photoinduced mutations to 6-thioguanine resistance in the mutants, per unit dose of 8-MOP, were about 4 times higher for MCN 151 and 3 times higher for MCE 50 than that in L5178Y cells. However, the rates of induced mutations per viable cells in the mutants were nearly equal to those in wild-type cells. Cross-link repair was compared between mutants and wild-type cells by using the alkaline sucrose-gradient sedimentation technique. The results show that normal cells and both mutants are able to incise the cross-linked DNA, which is the first step of cross-link repair.

Allelism between pso1-1 and rev3-1 mutants and between pso2-1 and snm1 mutants in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, allelism between the pso1-1 and the rev3-1 mutants on the one hand and the pso2-1 and snm1 mutants on the other, is demonstrated by the comparison of phenotypes, complementation tests and meiotic segregation analysis.

Induction of polypeptides in Saccharomyces cerevisiae after ultraviolet irradiation

Alterations in the synthesis of proteins following exposure of Saccharomyces cerevisiae to UV light were investigated using radioactive labelling and two dimensional electrophoresis. UV-irradiation induced the synthesis of various proteins. Among them the analogue of the RecA protein of Escherichia coli (Angulo et al. 1985) and two other polypeptides (34 Kd and 35 Kd, pI 5.8) were observed in all four strains analyzed namely two DNA-repair deficient (rad-) strains: (rad6-1 and pso2-1) and their isogenic wild type RAD+ strains.

A yeast protein analogous to Escherichia coli RecA protein whose cellular level is enhanced after UV irradiation

In Saccharomyces cerevisiae, a protein was recognized by polyclonal antibodies raised against homogeneous Escherichia coli K 12 RecA protein. The cellular level of the yeast protein called RecAsc (molecular weight 44 kDa, pI 6.3), was transiently enhanced after UV irradiation. Protease inhibitors were required to minimize degradation of the RecAsc protein during cell lysis. The RecAsc protein exhibited similar basal levels and similar kinetics of increase after UV irradiation in DNA-repair proficient (RAD+) strains carrying mitochondrial DNA or not (rho0). This was also true for the following DNA-repair deficient (rad-) strains: rad2-6 rad6-1 rad52-1, a triple mutant blocked in three major repair pathways; rad6-delta, a mutant containing an integrative deletion in a gene playing a central role in mutagenesis; pso2-1, a mutant that exhibits a reduced rate of mutagenesis and recombination after exposure to DNA cross-linking agents.

Proteolytic activities in yeast after UV irradiation. II. Variation in proteinase levels in mutants blocked in DNA-repair pathways

When the levels of three common yeast proteinases in exponentially growing cells of mutants blocked in different repair pathways are compared to that of isogenic wild-type cells, it can be seen that the level of proteinase B is enhanced in the mutants whereas the levels of leucin aminopeptidase (Leu.AP) and lysine aminopeptidase (Lys.AP) are similar in all strains. As in its corresponding wild type, the level of proteinase B activity is further enhanced after UV-irradiation in a mutant blocked in excision-repair (rad1-3). In contrast, following the same treatment the level of proteinase B remains almost constant in a mutant blocked in a general error-prone repair system (rad6-1) and in a mutant defective in a more specific mutagenic repair pathway (pso2-1). Cycloheximide, an inhibitor of protein synthesis, blocks the post-UV enhancement in proteinase B activity observed in rad1-3 indicating that, as in the wild-type cells, an inducible process is involved. The levels of Lys.AP and Leu.AP are, respectively, either unaffected or only moderately increased following UV-treatment of the repair defective mutants, as in wild-type strains. It is obvious that the induction of protease B activity following UV-treatment in Saccharomyces cannot be equated to the induction of the recA protein in Escherichia coli. However the correlation found between the block in mutagenic repair and the lack of UV-induction of protease B activity leads to questions on the possible role of certain protease activities in mutagenic repair in eucaryotic cells.