8-Methoxypsoralen plus 365 nm light effects and repair in yeast

Psoralen-sensitive mutant pso9-1 of Saccharomyces cerevisiae contains a mutant allele of the DNA damage checkpoint gene MEC3

Complementation analysis of the pso9-1 yeast mutant strain sensitive to photoactivated mono- and bifunctional psoralens, UV-light 254 nm, and nitrosoguanidine, with pso1 to pso8 mutants, confirmed that it contains a novel pso mutation. Molecular cloning via the reverse genetics complementation approach using a yeast genomic library suggested pso9-1 to be a mutant allele of the DNA damage checkpoint control gene MEC3. Non-complementation of several sensitivity phenotypes in pso9-1/mec3Delta diploids confirmed allelism. The pso9-1 mutant allele contains a -1 frameshift mutation (deletion of one A) at nucleotide position 802 (802delA), resulting in nine different amino acid residues from that point and a premature termination. This mutation affected the binding properties of Pso9-1p, abolishing its interactions with both Rad17p and Ddc1p. Further interaction assays employing mec3 constructions lacking the last 25 and 75 amino acid carboxyl termini were also not able to maintain stable interactions. Moreover, the pso9-1 mutant strain could no longer sense DNA damage since it continued in the cell cycle after 8-MOP + UVA treatment. Taken together, these observations allowed us to propose a model for checkpoint activation generated by photo-induced adducts.

DNA repair defects channel interstrand DNA cross-links into alternate recombinational and error-prone repair pathways

The repair of psoralen interstrand cross-links in the yeast Saccharomyces cerevisiae involves the DNA repair groups nucleotide excision repair (NER), homologous recombination (HR), and post-replication repair (PRR). In repair-proficient yeast cells cross-links induce double-strand breaks, in an NER-dependent process; the double-strand breaks are then repaired by HR. An alternate error-prone repair pathway generates mutations at cross-link sites. We have characterized the repair of plasmid molecules carrying a single psoralen cross-link, psoralen monoadduct, or double-strand break in yeast cells with deficiencies in NER, HR, or PRR genes, measuring the repair efficiencies and the levels of gene conversions, crossing over, and mutations. Strains with deficiencies in the NER genes RAD1, RAD3, RAD4, and RAD10 had low levels of cross-link-induced recombination but higher mutation frequencies than repair-proficient cells. Deletion of the HR genes RAD51, RAD52, RAD54, RAD55, and RAD57 also decreased induced recombination and increased mutation frequencies above those of NER-deficient yeast. Strains lacking the PRR genes RAD5, RAD6, and RAD18 did not have any cross-link-induced mutations but showed increased levels of recombination; rad5 and rad6 cells also had altered patterns of cross-link-induced gene conversion in comparison with repair-proficient yeast. Our observations suggest that psoralen cross-links can be repaired by three pathways: an error-free recombinational pathway requiring NER and HR and two PRR-dependent error-prone pathways, one NER-dependent and one NER-independent.

Repair of intermediate structures produced at DNA interstrand cross-links in Saccharomyces cerevisiae

Bifunctional alkylating agents and other drugs which produce DNA interstrand cross-links (ICLs) are among the most effective antitumor agents in clinical use. In contrast to agents which produce bulky adducts on only one strand of the DNA, the cellular mechanisms which act to eliminate DNA ICLs are still poorly understood, although nucleotide excision repair is known to play a crucial role in an early repair step. Using haploid Saccharomyces cerevisiae strains disrupted for genes central to the recombination, nonhomologous end-joining (NHEJ), and mutagenesis pathways, all these activities were found to be involved in the repair of nitrogen mustard (mechlorethamine)- and cisplatin-induced DNA ICLs, but the particular pathway employed is cell cycle dependent. Examination of whole chromosomes from treated cells using contour-clamped homogenous electric field electrophoresis revealed the intermediate in the repair of ICLs in dividing cells, which are mostly in S phase, to be double-strand breaks (DSBs). The origin of these breaks is not clear since they were still efficiently induced in nucleotide excision and base excision repair-deficient, mismatch repair-defective, rad27 and mre11 disruptant strains. In replicating cells, RAD52-dependent recombination and NHEJ both act to repair the DSBs. In contrast, few DSBs were observed in quiescent cells, and recombination therefore seems dispensable for repair. The activity of the Rev3 protein (DNA polymerase zeta) is apparently more important for the processing of intermediates in stationary-phase cells, since rev3 disruptants were more sensitive in this phase than in the exponential growth phase.

Differential repair and recombination of psoralen damaged plasmid DNA in Saccharomyces cerevisiae

Psoralen photoreaction with DNA produces interstrand crosslinks, which require the activity of excision and recombinational pathways for repair. Yeast replicating plasmids, carrying the HIS3, TRP1, and URA3 genes, were photoreacted with psoralen in vitro and transfected into Saccharomyces cerevisiae cells. Repair was assayed as the relative transformation efficiency. A recombination-deficient rad52 strain was the least efficient in the repair of psoralen-damaged plasmids; excision repair-deficient rad1 and rad3 strains had repair efficiencies intermediate between those of rad52 and RAD cells. The level of repair also depended on the conditions of transformant selection; repair was more efficient in medium lacking tryptophan than in medium from which either histidine or uracil was omitted. The plasmid repair differential between these selective media was greatest in rad1 cells, and depended on RAD52. Plasmid-chromosome recombination was stimulated by psoralen damage, and required RAD52 function. Chromosome to plasmid gene conversion was seen most frequently at the HIS3 locus. In RAD and rad3 cells, the majority of the conversions were associated with plasmid integration, while in rad1 cells most were non-crossover events. Plasmid to chromosome gene conversion was observed most frequently at the TRP1 locus, and was accompanied by plasmid loss.

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.

Repair of 8-methoxypsoralen photoinduced cross-links in yeast. Analysis by alkaline step-elution and electron microscopy

The repair of interstrand cross-links induced by 8-methoxypsoralen plus UVA (365 nm) radiation DNA was analyzed in diploid strains of the yeast Saccharomyces cerevisiae. The strains employed were the wild-type D7 and derivatives homozygous for the rad18-1 or the rad3-12 mutation. Alkaline step-elution and electron microscopy were performed to follow the process of induction and removal of photoinduced cross-links. In accordance with previous reports, the D7 rad3-12 strain failed to remove the induced lesions and could not incise cross-links. The strain D7 rad18-1 was nearly as efficient in the removal of 8-MOP photoadducts after 2 h of post-treatment incubation as the D7 RAD+ wild-type strain. However, as demonstrated by alkaline step-elution and electron microscopic analysis, the first incision step at DNA cross-links was three times more effective in D7 rad18-1 than in D7 RAD+. This is consistent with the hypothesis that the RAD18 gene product is involved in the filling of gaps resulting from persistent non-informational DNA lesions generated by the endonucleolytic processing of DNA cross-links. Absence of this gene product may lead to extensive strand breakage and decreased recognition of such lesions by structural repair systems.

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.

Molecular cloning of SNM1, a yeast gene responsible for a specific step in the repair of cross-linked DNA

We have isolated yeast gene SNM1 via complementation of sensitivity towards bi- and tri-functional alkylating agents in haploid and diploid yeast DNA repair-deficient snm1-1 mutants. Four independent clones of plasmid DNA containing the SNM1 locus were isolated after transformation with a YEp24-based yeast gene bank. Subcloned SNM1-containing DNA showed (i) complementation of the repair-deficiency phenotype caused by either one of the two different mutant alleles snm1-1 and snm1-2ts; (ii) complementation in haploid and diploid yeast snm1-1 mutants by either single or multiple copies of the SNM1 locus; and (iii) that the SNM1 gene is at most 2.4 kb in size. Expression of SNM1 on the smallest subclone, however, was under the control of the GAL1 promotor. Gene size and direction of transcription was further verified by mutagenesis of SNM1 by Tn10-LUK transposon insertion. Five plasmids containing Tn10-LUK insertions at different sites of the SNM1-containing DNA were able to disrupt the function of genomic SNM1 after gene transplacement. Correct integration of the disrupted SNM1::Tn10-LUK at the genomic site of SNM1 was verified via tetrad analysis of the sporulated diploid obtained after mating of the SNM1::Tn10-LUK transformant to a haploid strain containing the URA3 SNM1 wild-type alleles. The size of the poly(A)+ RNA transcript of the SNM1 gene is 1.1 kb as determined by Northern analysis.

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.

A complex pattern of sensitivity to simple monofunctional alkylating agents exists amongst the rad mutants of Saccharomyces cerevisiae

The radiation-sensitive rad mutants of the yeast Saccharomyces cerevisiae exhibit a complex pattern of sensitivity to simple monofunctional alkylating agents. The RAD1, RAD2, RAD4 and RAD14 genes of the RAD3 epistasis group are implicated in the repair of ethylations to DNA. The RAD3, RAD10 and RAD16 genes of this group are not involved. The RAD4 and RAD14 genes have a particular role in repair following exposure to those ethylating agents that preferentially alkylate oxygen, but not to those that preferentially ethylate nitrogen. The RAD1 and RAD2 genes are involved in the repair of damage induced by all the ethylating agents used except EMS. The mutants in this group that are sensitive to ENU were not sensitive to MNU, suggesting that nucleotide excision operates on ethylations but not on methylations. In the RAD6 group, the RAD6 and RAD18 genes are involved in DNA repair after exposure to all the alkylating agents tested, whereas RAD8 appears to have a role in the repair of O-alkylations but not N-alkylations. RAD9 operates in the repair of methylations and ethylations, but does not influence events after exposure to EMS. In the RAD52 group, the mutants tested were sensitive to ENU and DES. Thus some members of all three epistasis groups are involved in the repair of alkylations to DNA.

Thermal aspects of biological effects of microwaves in Saccharomyces cerevisiae

The formation of zygotes between two haploid strains of yeast (Saccharomyces cerevisiae) was determined under treatment with microwaves of 9.4 and 17 GHz at power levels up to 50 and 60 mW/cm2 and a specific absorption rate below 24 mW/g, or with conventional heating. Microwave treatments at 9.4 GHz or 17 GHz at a power density of 10 mW/cm2 produced an increase in zygote formation equivalent to that produced by conventional heating in an incubator, i.e. equivalent to a rise in temperature of 0.5 or 1 degrees C. At higher power densities zygote formation was slightly increased by microwaves at 17 GHz as compared to microwaves at 9.4 GHz probably due to the higher absorption of microwaves at 17 GHz by intracellular water molecules. Under these conditions, microwaves had no effect on cell survival or the induction of cytoplasmic 'petite' mutations.

Fate of photo-induced 8-methoxypsoralen mono-adducts in yeast. Evidence for bypass of these lesions in the absence of excision repair

A fraction of UVA-induced 8-methoxypsoralen (8-MOP) mono-adducts can be transformed by a second UVA (365 nm) irradiation procedure into lethal cross-links in Saccharomyces cerevisiae. To follow the fate of cross-linkable mono-adducts, cells were incubated in complete medium between the two UVA doses and survival was measured. The killing effect of the second UVA dose decreases rapidly in haploid wild-type as well as in strains blocked in mutagenic (RAD6+ type) or in recombinogenic (RAD52+ type) repair pathways. This is also true in the pso1-1 and pso2-1 strains selected for sensitivity to 8-MOP plus UVA treatment. In contrast, persistence of mono-adducts is observed in strains blocked in the excision-resynthesis repair pathway. In other words, cross-linkable mono-adducts are repaired by the excision process. The use of the cell-cycle conditional mutant strain (cdc14-1) permitted us to apply the second dose at a specific cell-cycle stage (post-G2 phase) after a 'priming' UVA treatment on stationary (G1) phase cells. Such experiments showed a bypass of mono-adducts in an excision-deficient context for at least one round of DNA replication.

Isolation and characterization of mitomycin-C-sensitive mouse lymphoma cell mutants

26 mutants with increased sensitivity to the lethal effects of mitomycin C (MMC) were isolated from mouse lymphoma L5178Y cells by a replica-plating technique. Most of them were about 5-10 times more sensitive in terms of D37 values to MMC than were parental cells. 5 of the MMC-sensitive mutants isolated from independently mutagenized cell populations were further analyzed. They were highly sensitive to the killing by decarbamoyl (DC) MMC, a monofunctional derivative of MMC, but were not sensitive to ultraviolet radiation, X-rays, 4-nitroquinoline-1-oxide or methyl methanesulfonate. These 5 mutants were classified into at least 2 genetic complementation groups. The implication of these mutations in cross-link and mono-adduct repair of DNA damage induced by MMC and DCMMC is discussed.

Genetic control of excision of Saccharomyces cerevisiae interstrand DNA cross-links induced by psoralen plus near-UV light

Excision of interstrand DNA cross-links induced by 4,5',8-trimethyl psoralen plus 360-nm light was examined in wild type (RAD+) and various radiation-sensitive (rad) mutants of Saccharomyces cerevisiae known to be defective in the excision of UV light-induced pyrimidine dimers. Alkaline sucrose sedimentation of DNA after incubation of psoralen-plus-light-treated cells indicated little or no nicking of cross-linked DNA in rad1-2, rad2-5, rad3-2, rad4-4, rad10-2, and mms19-1 mutants. In the rad14-2 mutant, substantial nicking was observed but to a much lesser extent than in the RAD+ strains, whereas the rad16-1 mutant was as proficient in nicking as the RAD+ strain. Removal of cross-links was also examined in RAD+, rad3-2, and rad14-2 strains by determining the sensitivity of alkali-denatured and -neutralized DNA to hydrolysis by S1 nuclease. No cross-link removal was observed in the rad3-2 mutants, and the rad14-2 mutant was much less efficient than the RAD+ strain in removing cross-links.

Genetic control of excision of Saccharomyces cerevisiae interstrand DNA cross-links induced by psoralen plus near-UV light

Excision of interstrand DNA cross-links induced by 4,5',8-trimethyl psoralen plus 360-nm light was examined in wild type (RAD+) and various radiation-sensitive (rad) mutants of Saccharomyces cerevisiae known to be defective in the excision of UV light-induced pyrimidine dimers. Alkaline sucrose sedimentation of DNA after incubation of psoralen-plus-light-treated cells indicated little or no nicking of cross-linked DNA in rad1-2, rad2-5, rad3-2, rad4-4, rad10-2, and mms19-1 mutants. In the rad14-2 mutant, substantial nicking was observed but to a much lesser extent than in the RAD+ strains, whereas the rad16-1 mutant was as proficient in nicking as the RAD+ strain. Removal of cross-links was also examined in RAD+, rad3-2, and rad14-2 strains by determining the sensitivity of alkali-denatured and -neutralized DNA to hydrolysis by S1 nuclease. No cross-link removal was observed in the rad3-2 mutants, and the rad14-2 mutant was much less efficient than the RAD+ strain in removing cross-links.

The fate of 8-methoxypsoralen photoinduced crosslinks in nuclear and mitochondrial yeast DNA: comparison of wild-type and repair-deficient strains

In Saccharomyces cerevisiae, after 8-methoxypsoralen [8-(OMe)Ps] photoaddition, more crosslinks are induced per unit dose in mitochondrial DNA than in nuclear DNA. In wild-type cells treated in the exponential phase of growth, single- and double-strand breaks are produced during crosslink removal and then are rejoined upon postexposure incubation. The incision step is almost blocked in the rad 3-2 mutant, which is also defective in excision-repair of UV-induced (254 nm) pyrimidine dimers. The cutting of crosslinks from nuclear DNA is depressed in wild-type stationary-phase cells. This is correlated with a higher sensitivity of such cells to 8-(OMe)Ps photoinduced cell killing. The incision of crosslinks is dramatically reduced in mitochondrial DNA. The rejoining of single- and double-strand breaks is not only dependent on the product of the RAD51 gene (as shown by others) but also of the PSO2 gene. A correlation was found between the ability to recombine and strand rejoining. Therefore, as in bacteria, both the excision and the recombinational repair systems are involved in crosslink repair in yeast. However, double-strand breaks in yeast constitute repair intermediates which are not detected in Escherichia coli. The LD37 (dose necessary to induce a mean of one lethal hit per cell) corresponds to about 120 crosslinks per genome in exponential-phase cells of the wild type and to 1-2 crosslinks in the pso2-1 mutant.

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

The inactivation and the induction of forward and reverse mutations by a mono- and a bifunctional nitrogen mustard in 3 pso mutants of Saccharomyces cerevisiae, initially selected for their sensitivity to psoralen photo-addition, were compared with that of the wild-type. The pso1-1 mutant was very sensitive to both alkylating agents, and the mutagenicity was abolished. This correlates with the defect in the error-prone repair capacity for lesions induced by psoralen photo-addition and radiations already observed for this mutant. Therefore it appears that the PSO1+ gene product acts on a spectrum of DNA lesions. The pso2-1 mutant was highly sensitive to the lethal effect of the bifunctional nitrogen mustard and was only slightly sensitive to the monofunctional one. For both agents a reduction in induced mutagenesis was seen. The same was true for mono- and bifunctional psoralen derivatives. The pso2-1 mutant having the same sensitivity as the wild-type to UV and ionizing radiations, it is suggested that the PSO2+ gene product is predominantly necessary for the repair of cross-links irrespective of their molecular nature. In contrast with psoralen photo-induced inactivation the pso3-1 mutant had the same sensitivity as the wild-type to alkylating agents. However, a reduction in induced mutagenesis was seen in both cases. This response was modulated according to dose and type of mutation. Consequently, it appeared that the PSO3+ gene product acts specifically on psoralen photo-induced sub-lethal lesions and on a fraction of premutagenic lesions independently of their structure.

Repair of interstrand cross-links in DNA of Saccharomyces cerevisiae requires two systems for DNA repair: the RAD3 system and the RAD51 system

We have studied the role of the excision-repair system and the recombination-repair system in the removal of cross-links and monoadducts caused by furocoumarins plus 360 nm radiation in yeast DNA by neutral and alkaline sucrose gradients and by a fluorometric procedure which detects cross-linked DNA molecules. We found that the excision-repair system, represented by the rad3 mutations, is required both for the removal of monoadducts, causing single-strand break formation, and for the removal of cross-links, causing double-strand break formation. The recombination-repair system, represented by the rad51 mutation, is necessary for double-strand break repair following cross-link removal, but it has no role in the repair of monoadducts. It can be concluded, that at least some of the same enzymes are used in yeast for both the excision of pyrimidine dimers and the excision of cross-links or monoadducts caused by furocoumarins plus light. The RAD3 and RAD51 repair systems, which act independently in the repair of UV-induced lesions, are part of a single system for the repair of cross-links.

UV-induced reactivation and mutagenesis of lambda-phages after treatment with 8-methoxypsoralen or thiopyronine and light

Lesions, which were produced on lambda-phages DNA by the photosensitization effect of 8-methoxypsoralen (8-MOP) can be repaired by UV-induced repair systems (W-reactivation) in Escherichia coli wild type host cells. By optimum induction of the repair system, about 45% of the 8-MOP lesions are repaired. The survival of lambda-phages inactivated by the photodynamic action of thiopyronine (TP) is only negligibly increased by the same UV-induced repair systems. However, the frequencies of clear plaque mutations of 8-MOP treated as well as TP treated lambda-phages increase in similar fashion if the host cells of wild type have been irradiated with UV. These results show the different capacities of induced repair systems in repairing different types of lesions. They also suggest that some types of base damages are repaired more error-prone than other DNA-lesions.

Studies on possible genetic effects of microwaves in procaryotic and eucaryotic cells

The biological effects of microwaves in the hyperfrequency range, 9,4 GHz, and 70-75 GHz were investigated in bacteria and yeast. At power densities below 60 mW/cm2 and SAR values not exceeding 28 mW/g no significant effects on survival of repair competent an deficient strains were observed in Escherichia coli and Saccharomyces cerevisiae. In addition, microwaves of 17 GHz did not induce mutations in E. coli B/r WP2 trp- uvr- above the spontaneous level, and the induction of nuclear reversions, cytoplasmic "petite" mutations and mitotic recombination as well as the efficiency of sporulation was not affected in yeast.

Determination of a thermal equivalent of millimeter microwaves in living cells

Recent microwave experiments have shown frequency dependent influences on the growth rate of bacteria. To determine whether microwaves are able to affect growth (or to induce lesions in cellular DNA of yeast cells), experiments were performed with millimeter microwaves at frequencies between 70 and 75 GHz. Saccharomyces cerevisiae cells were irradiated on millipore filter discs placed on agar plates in open petri dishes. A diploid strain of yeast (D5, Zimmerman), that is sensitive to genetic insult was used to study the effects of temperature and of microwave irradiation on cell survival, induction of mitotic recombination, and induction of cytoplasmic "petite" mutations. No evidence of altered survival, impaired function, or structural injury was seen at either frequency, even at power densities as high as 60 mW/cm2. Conventional heating had no deleterious effects until temperatures of specimens exceeded 50 degrees C. In addition, two haploid strains of yeast of opposite mating type were compared with respect to temperature and microwave treatment for formation of zygotes. The elevation of temperature due to the microwave treatment at 60 mW/cm2 and 2 mm distance was estimated to correspond to 3 degrees C.

Genetic effect of 3-carbethoxypsoralen, angelicin, psoralen and 8-methoxypsoralen plus 365-nm irradiation in Saccharomyces cerevisiae: induction of reversions, mitotic crossing-over, gene conversion and cytoplasmic "petite" mutations

The genetic effects of two mono-functional photosensitizing furocoumarins, 3-carbethoxypsoralen (3-CPs) and angelicin, were compared with those of two bi-functional furocoumarins, 8-methoxypsoralen and psoralen in Saccharomyces cerevisiae. A drug concentration of 5 X 10(-5) M plus various doses of 365-nm irradiation at a dose rate of 1.2 kJ m-2 min-1 were used. Per dose of 365-nm irradiation, the frequency of induced nuclear events such as gene mutation and mitotic recombination (conversion and crossing-over) is higher for the bi-functional than for the mono-functional compounds. The higher efficiency of the bi-functional furocoumarins is also evident when the frequency of mutants is expressed as a function of survival. However, the photo-addition of the 4 furocoumarins studied leads to the same response for the induction of recombinational events per viable cell. Amongst genetically altered colonies induced in the diploid strains D5 and D7, the colonies corresponding to the induction of crossing-over are effectively produced by bi-functional furocoumarins, but are rare (D7) or even absent (D5) after treatment with monofunctional furocoumarins. This suggests a certain specificity of genetic alterations produced by the bi-functional agents. 3-CPs is the most effective inducer on the cytoplasmic "petite" mutation in stationary phase cells per unit irradiation dose or per viable cell.

Biological effects and repair of damage photoinduced by a derivative of psoralen substituted at the 3,4 reaction site: photoreactivity of this compound and lethal effect in yeast

A newly synthesized linear psoralen derivative, 3-carbethoxypsoralen is shown to bind to yeast nucleic acids after 365 nm light treatment. As compared to 8-methoxypsoralen, a well-known bifunctional furocoumarin, 3-carbethoxypsoralen exhibits a high photoaffinity for DNA in vivo. Both compounds bind and photoreact more efficiently in vivo than in vitro. In contrast to 8-methoxypsoralen, 3-carbethoxypsoralen does not form cross-links in yeast DNA as demonstrated by heat denaturation-reassociation studies at least in the range of doses used. Thus 3-carbethoxypsoralen reacts as a monofunctional compound. Wild-type cells of Saccharomyces cerevisiae are 6 times more resistant to 3-carbethoxypsoralen than to 8-methoxypsoralen plus 365 nm light treatment in terms of lethal effect. In comparison to angelicin, another monofunctional (but angular) furocoumarin, 3-carbethoxypsoralen is more photoreactive. When the photoaffinity for DNA of 8-methoxypsoralen and 3-carbethoxypsoralen are considered in relation to photoinduced cell killing, it is clear that monoadducts are very efficiently repaired in wild-type cells. In contrast to the additivity obtained with 8-methoxypsoralen, a synergistic interaction of the two different repair pathways blocked by the rad2 and the rad9 mutation is observed after 3-carbethoxypsoralen plus 365 nm light. Dark holding experiments show that the excision repair function which is present in wild-type and rad9-4 cells is important for dark recovery.

Dose-rate effects of 8-methoxypsoralen plus 365-NM irradiation on cell killing in Saccharomyces cerevisiae

Tow types of dose-rate effect that alter the survival response of haploid yeast cells to 8-methoxypsoralen (8-MOP) plus treatment with irradiation at 365 nm were studied. (1) When the concentration of 8-MOP was varied between 9.2 X 10(-5) and 2.3 X 10(-8) M and the dose rate of 365-nm irradiation kept constant, the efficiency of the irradiation for killing increased relatively to that of 8-MOP whe the concentration of 8-MOP decreased. This indicated that there was no strict reciprocity between radiation dose and concentration of drug. (2) When the dose rate of radiation was varied between 0.66 X 10(3) and 108 X 10(3) J m-2 h-1 and the concentration of 8-MOP was kept constant, the survival of wild-type cells increased strikingly at low dose rates of radiation as compared with high dose rates. Cells responded more to changes at low dose rates than to equal changes a high dose rates. The high resistance of wild-type cells to 8-MOP plus radiation delivered at low dose rates absent from rad 1-3 cells defective in excision-repair. This suggests that the dose-rate effect seen in wild-type cells depended at least in part on an active excision-repair function. At low dose rates of radiation, the shoulder of the survival curve for rad1-3 cells, i.e. the ability to accumulate sub-lethal damage, was increased by a factor of about 2 when compared with that seen at a high dose rate. Thus it is likely that at low dose rates a repair function other than excision-resynthesis may operate in rad1-3 cells.

Mitochondrial genetic damage induced in yeast by a photoactivated furocoumarin in combination with ethidium bromide or ultraviolet light

Ethidium bromide (EB) and ultraviolet light (UV) in combination are known to produce a synergistic induction of "petite" mutants in yeast. Two other agents were combined with EB, 3-Carbethoxypsoralene (3 CPs) activated by 365 nm light or gamma rays. EB in combination with 3 CPs also resulted in an enhanced production of "petite" mutants. After the photoaddition of 3 CPs in exponential phase cells, recovery of the "petite" mutation during dark liquid holding was inhibited by the presence of EB producing an enhanced number of "petite" mutants. The behavior of mitochondrial antibiotic resistance markers after individual and combined treatments with EB and 3 CPs indicates a random loss of markers after EB and a preferential loss of a certain region for the 3 CPs photoaddition. The combination of the two agents leads to an additivity of total drug marker losses rather than a synergistic loss. The combination of EB with gamma rays produced no enhancement in "petite" induction. A combination of UV and 3 CPs showed a synergistic interaction for "petite" induction. These results indicate that the three agents, EB, UV and 3 CPs photoaddition may share a common repair step for mitochondrial lesions.