The isolation and characterization of ngm2, a mutation that affects nitrosoguanidine mutagenesis in yeast

Summary We have isolated and characterized a new mutant of Saccharomyces cerevisiae, carrying a single mutant allele that we designate ngm2-1, which is defective with respect to induced mutagenesis. This mutant was isolated by screening mutagenized clones for reduced frequencies of reversion of the his1-7 allele, induced by N-methyl-N-nitro-N-nitrosoguanidine. As judged by the reversion of his1-7 and ilv1-92, ngm2-1 mutant strains are also deficient with respect to mutability induced by methyl methane sulfonate, ethyl methane sulfonate and, at least partially, by UV. UV-induced reversion of the ochre mutation arg4-17 and the frameshift mutation his4-38 was not much affected by ngm2-1, however. Like rev3 and rev7 mutations, ngm2-1 also has little influence on the reversion of the proline missense allele, cyc1-115. Ngm2-1 mutants are only at best very slightly more sensitive to the toxicity of the four mutagens used, and homozygous diploids sporulate normally.     

UV-induced reversion of his4 frameshift mutations in rad6, rev1, and rev3 mutants of yeast

Summary The UV-induced reversion of two his4 frameshift alleles was much reduced in rad6 mutants of Saccharomyces cerevisiae, an observation that is consistent with the hypothesis that RAD6 function is required for the induction of all types of genetic alteration in misrepair mutagenesis. The reversion of these his4 alleles, together with two others of the same type, was also reduced in rev1 and rev3 mutant strains; in these, however, the extent of the reduction varied considerably with test allele used, in a manner analogous to the results in these strains for base repair substitution test alleles. The general features of UV-induced frameshift and substitution mutagenesis therefore appear quite similar, indicating that they may depend on related processes. If this conclusion is correct, greater attention must be given to integrating models which account for the production of nucleotide additions and deletions into those concerning misrepair mutagenesis.     

REV7, a new gene concerned with UV mutagenesis in yeast

Summary Three allelic mutations of a new yeast gene, which we have named REV7, have been isolated by testing 313 methyl methane sulfonate sensitive mutants for UV-induced reversion of a lys2 allele. Rev7 mutants are markedly deficient with respect to UV-induced reversion of lys2, are slightly sensitive to UV and appear to be in the RAD6 epistasis group for UV survival. Rev7-1, which is probably an amber mutation, does not appear to affect sporulation in homozygous diploids. The REV7 gene is located about 12 cM distal to HIS5 on chromosome IX.     

Ultraviolet-Induced Reversion of cyc1 Alleles in Radiation Sensitive Strains of Yeast. II. rev2 Mutant Strains

The range of specificity of the rev2-1 mutation, an allele that reduces the frequency of ochre revertants induced by UV in Saccharomyces cerevisiae (LEMONTT 1971a), has been investigated by examining its influence on the reversion of eleven well-defined and contrasting cyc1 mutations. We have shown, in support of a suggestion of LEMONTT (1971a), that the REV2 gene product is concerned only with the reversion of ochre alleles; it plays virtually no role in the reversion of amber, missense or frameshift mutations. We have also shown that its effect is specific and confined to only some highly revertible ochre alleles. The REV2 gene product appears to enhance reversion at these sites by facilitating the conversion of two otherwise nonmutagenic photo-products into a single premutational lesion. UV-induced killing of rev2-1 strains was found to be significantly greater on fermentable rather than on nonfermentable media.     

Involvement of umuDCST genes in nitropyrene-induced -CG frameshift mutagenesis at the repetitive CG sequence in the hisD3052 allele of Salmonella typhimurium

Expression of the umuDC operon is required for UV and most chemical mutagenesis in Escherichia coli. The closely related species Salmonella typhimurium has two sets of umuDC-like operons, umuDC _ST on the chromosome and samAB on a 60-MDa cryptic plasmid. The roles of theumuDC-like operons in chemically induced frameshift mutagenesis of the hisD3052 allele of S. typhimurium were investigated. Introduction of a pBR322-derived plasmid carrying umuDCST increased the rate of reversion of hisD3052, following treatment with 1-nitropyrene (1-NP) or 1,8-dinitropyrene (1,-8DNP) tenfold and fivefold, respectively, whereas it did not substantially increase the rate of reversion induced by other frameshift mutagens, i.e. 2-nitrofluorene (2NF) and 2-amino- 3-methyldipyrido[1,2-a:3 ,2-d]imi-dazole (Glu-P-1). Introduction of a pBR322-derived plasmid carrying samAB did not increase the incidence of reversion of hisD3052 observed with any of the mutagens examined. Deletion of umuDC _STSubstantially lowered the reversion rate induced by l-NP or 1,8-DNP, but it did not affect reversion induced by 2-NF, Glu-P-1 or N-hydroxyacetylaminofluorene (N-OH-AAF). Deletion of samAB had little impact on reversion incidence induced by any of the five frameshift mutagens. DNA amplification using the polymerase chain reaction technique followed by restriction enzyme analysis using BssHII, suggested that the mutations induced by the five frameshift mutagens were all CG deletions at the CGCGCGCG sequence in hisD3052. These results suggest that umuDCST, but not samAB, is involved in the -2 frameshift mutagenesis induced by l-NP and 1,8-DNP at the repetitive CG sequence, whereas neither operon participates in induction of the same type of mutations by 2-NF, Glu-P-1 or N-OH-AAF.     

The Saccharomyces cerevisiae rev6-1 mutation, which inhibits both the lesion bypass and the recombination mode of DNA damage tolerance, is an allele of POL30, encoding proliferating cell nuclear antigen

The rev6-1 allele was isolated in a screen for mutants deficient for UV-induced reversion of the frameshift mutation his4-38. Preliminary testing showed that the rev6-1 mutant was substantially deficient for UV-induced reversion of arg4-17 and ilv1-92 and markedly UV sensitive. Unlike other REV genes, which encode DNA polymerases and an associated subunit, REV6 has been found to be identical to POL30, which encodes proliferating cell nuclear antigen (PCNA), the subunit of the homotrimeric sliding clamp, in which the rev6-1 mutation produces a G178S substitution. This substitution appears to abolish all DNA damage-tolerance activities normally carried out by the RAD6/RAD18 pathway, including translesion replication by DNA polymerase zeta/Rev1 and DNA polymerase eta, and the error-free, recombination-dependent component of this pathway, but has little effect on the growth rate, suggesting that G178S may prevent ubiquitination of lysine 164 in PCNA. We also find that rev6-1 mutation can be fully complemented by a centromere-containing, low copy-number plasmid carrying POL30, despite the presumed occurrence in the mutant of sliding clamp assemblies that contain between one and three G178S PCNA monomers as well as the fully wild-type species.     

Mechanism of gene conversion in Ascobolus immersus. II. The relationships between the genetic alterations in b 1 or b 2 mutants and their conversion spectrum

Summary In order to establish a relationship between the type of genetic alteration occurring in the mutant and the conversion spectrum which is associated with it, an attempt was made to characterize the genetic alterations in a sample of five spore colour mutants by specific reversion.All the mutants revert by back-mutation. Reversion of one of them is possible by mutation in external suppressor gene: at least two of the external suppressors behave like super-suppressors. Reversion of two other mutants by intragenic second-site mutation and study of the intragenic suppressors indicate that they are of the frameshift type. One of the frameshift mutants (ICR₁₇₀ induced) reverts by two alkylating agents suggesting that it originates from base(s) addition. The inhability of ICR₁₇₀ to induce reversion of this mutant suggests the preferential occurrence of base addition in ICR₁₇₀ mutagenesis. Conversly the other frameshift mutant (induced by ethyl methanesulfonate) reverts strongly by ICR₁₇₀. Thus it is concluded that it originates from deletion.These two frameshift mutants and all the ICR₁₇₀ induced mutants give no or very few asci with postmeiotic segregation and either an excess of conversion towards the mutant type allele (addition type mutant) or towards the wild type allele (deletion type mutant).Evidence supports the hypothesis that mutants which give many asci with postmeiotic segregation originate from substitution.These data imply differential recognition of non-pairing and mispairing of bases and in the case of non-pairing, that the direction of conversion is determined by the non-pairing itself.     

Induced mutagenesis in Ustilago maydis

Summary A UV-revertible mutant of the nar1 structural gene for nitrate reductase was isolated in wildtype (nar ⁺ nir ⁺) Ustilago maydis. It proved to be vigorously revertible by gamma rays as well. Genetic analysis revealed that the strain carried a single, nonleaky, recessive allele (nar1-m) with an unusually high spontaneous reversion rate (3×10^-5/div.). Reliable reversion frequencies were determined with a special agar medium that reduced the normally high level of residual growth observed on nitrate minimal agar. Radiation-induced reversion frequencies in the homozygous diploid were approximately twice those in the hapliod. Following crosses to wild type, two revertants (one spontaneous and one UV-induced) were found to map at nar1. Although the molecular basis of nar1-m reversion is not known, available data suggest that some form of point mutation is involved.     

Induction of forward mutations in mutationally defective yeast

Summary The 3 rev loci that reduce ultraviolet light (UV)-induced reversion in S. cerevisiae had a similar effect on forward mutation to auxotrophy induced by a single 400 erg/mm² UV dose: rev1-1, rev2-1 and rev3-1 reduced average frequencies of auxotrophs to 4%, 64% and 4% that in wild type and reduced frequencies of mutants at ade1 or ade2 to 19%, 88% and 2% wild type, respectively. The rev2-1 strain exhibited high frequencies of spontaneous mutation. It is suggested that rev1-1 and rev3-1 block steps in a general UV mutation mechanism controlling forward and reverse mutation throughout the genome. The small effect of rev2-1, compared to the effect of rev1-1 or rev3-1, is consistent with previously obtained data on UV reversion and could be due to a specificity for induced mutation involving only certain types of UV damage or, on the other hand, it may be related to mutator activity. Although rev caused varying degrees of sensitivity to ethylmethanesulfonate (EMS), there was little or no significant effect on mutation induced by a single 30 min. dose of 3% EMS. Auxotroph frequencies were 79%, 109% and 94% wild type, whild frequencies at ade1 or ade2 were 82%, 56% and 51% wild type in the respective strains. It is suggested that steps blocked by rev, although they may participate in repair of lethal EMS damage, do not themselves generate EMS-induced mutations.     

The in vivo characterization of translesion synthesis across UV-induced lesions in Saccharomyces cerevisiae: insights into Pol zeta- and Pol eta-dependent frameshift mutagenesis

UV irradiation, a known carcinogen, induces the formation of dipyrimidine dimers with the predominant lesions being cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone adducts (6-4PPs). The relative roles of the yeast translesion synthesis DNA polymerases Pol zeta and Pol eta in UV survival and mutagenesis were examined using strains deficient in one or both polymerases. In addition, photoreactivation was used to specifically remove CPDs, thus allowing an estimate to be made of the relative contributions of CPDs vs. 6-4PPs to overall survival and mutagenesis. In terms of UV-induced mutagenesis, we focused on the +1 frameshift mutations detected by reversion of the lys2deltaA746 allele, as Pol zeta produces a distinct mutational signature in this assay. Results suggest that CPDs are responsible for most of the UV-associated toxicity as well as for the majority of UV-induced frameshift mutations in yeast. Although the presence of Pol eta generally suppresses UV-induced mutagenesis, our data suggest a role for this polymerase in generating some classes of +1 frameshifts. Finally, the examination of frameshift reversion spectra indicates a hierarchy between Pol eta and Pol zeta with respect to the bypass of UV-induced lesions.     

UV-induced mutation in bacteriophage T4.

Two late gene am mutants of bacteriophage T4 that can be induced to revert by UV were crossed to a temperature-sensitive ligase mutant. In the double mutants, UV-induced reversion was eliminated at a semirestrictive temperature. When the single am mutants were irradiated and then allowed a single passage in a permissive host, the UV-induced reversion frequency was increased by 15- to 25-fold. This increased mutagenesis was also abolished by the presence of the ligase allele. When the UV-irradiated single am mutants multiply infected a permissive host, allowing multiplicity reactivation to occur, the induced reversion frequency was reduced similarly to the reduction in lethality. The mutagenesis that remained was again abolished by the presence of the ligase allele. It is concluded that UV induces mutations in phage T4 through the action of a pathway that includes polynucleotide ligase. The increase in mutation frequency after growth in a permissive host implies that mutagenesis can occur at more than one stage of the infection rather than only in an early stage before expression of the mutant genome. The process of multiplicity reactivation appears to be error-free since it overcomes lethal lesions without inducing new mutations.     

The Saccharomyces Cerevisiae Rad30 Gene, a Homologue of Escherichia Coli Dinb and Umuc, Is DNA Damage Inducible and Functions in a Novel Error-Free Postreplication Repair Mechanism

Damage-inducible mutagenesis in prokaryotes is largely dependent upon the activity of the UmuD'C-like proteins. Since many DNA repair processes are structurally and/or functionally conserved between prokaryotes and eukaryotes, we investigated the role of RAD30, a previously uncharacterized Saccharomyces cerevisiae DNA repair gene related to the Escherichia coli dinB, umuC and S. cerevisiae REV1 genes, in UV resistance and UV-induced mutagenesis. Similar to its prokaryotic homologues, RAD30 was found to be damage inducible. Like many S. cerevisiae genes involved in error-prone DNA repair, epistasis analysis clearly places RAD30 in the RAD6 group and rad30 mutants display moderate UV sensitivity reminiscent of rev mutants. However, unlike rev mutants, no defect in UV-induced reversion was seen in rad30 strains. While rad6 and rad18 are both epistatic to rad30, no epistasis was observed with rev1, rev3, rev7 or rad5, all of which are members of the RAD6 epistasis group. These findings suggest that RAD30 participates in a novel error-free repair pathway dependent on RAD6 and RAD18, but independent of REV1, REV3, REV7 and RAD5.     

Mutational Specificity of Ultraviolet Light in ESCHERICHIA COLI with and without the R Plasmid Pkm101

Plasmid pKM101 provides UV protection and increases the frequency of spontaneous and UV-induced mutations in Escherichia coli. By analyzing reversion patterns of defined trpA alleles, we showed that pKM101 altered the mutational specificity of UV-induced mutations. Certain UV-induced base-pair substitutions were strongly enhanced, while others were decreased in frequency in the presence of pKM101. This result suggests an interaction between cellular misrepair and an error-prone repair function(s) provided by pKM101. We have also examined UV mutational specificity in the absence of pKM101 and found the following: (1) UV preferentially enhances missense, as well as nonsense, intergenic suppressor mutations; (2) UV causes all possible base-pair substitutions as well as frameshift mutations; (3) G·C base pairs are more susceptible to UV mutagenesis than A·T base pairs at the same nucleotide positions; and (4) UV-induced mutations can occur at nucleotide positions that are not part of pyrimidine-pyrimidine sequences.     

Genetic control of AAF-induced mutagenesis at alternating GC sequences: An additional role for RecA

In a previous study, the forward mutation spectrum induced by the chemical carcinogen N-acetoxy-N-2-acetylaminofluorene was determined (Koffel-Schwartz et al. 1984). It was found that 90% of the induced mutations are frameshift mutations located within specific sequences (mutation hot spots). Two classes of mutation hot spots were found: (i) -1 frameshift mutations occurring within runs of guanines (i.e. GGGG----GGG; (ii) -2 frameshift mutations occurring within the NarI recognition sequence (GGCGCC----GGCC). In the present work, we further investigate the genetic requirements of these frameshift events by using specific reversion assays. Like UV-induced mutagenesis, frameshift mutations occurring within runs of G's (also referred to as the "slippage pathway") require the activated form of the RecA protein (RecA*). On the other hand, frameshift mutations occurring at the NarI site (the "NarI mutation pathway") require a LexA-controlled function(s) that is not UmuDC. The LexA-controlled gene(s) that is (are) involved in this pathway remain to be identified. Moreover, this pathway does not require RecA* for the proteolytic processing of a protein other than LexA (like the cleavage of UmuD in UV-induced mutagenesis). An "additional" role of RecA can be defined as follows: (i) The non-activated form of the RecA protein acts as an inhibitor in the NarI mutation pathway. (ii) This inhibition is relieved upon activation of RecA by UV irradiation of the bacteria. (iii) A recA deletion mutant is totally proficient in the NarI mutation pathway provided the SOS system is derepressed [lexA (Def) allele]. Therefore, RecA does not actively participate in the fixation of the mutation. A molecular model for this "additional" role of RecA is proposed.     

HSM2 (HMO1) gene participates in mutagenesis control in yeast Saccharomyces cerevisiae

We have previously reported about a new Saccharomyces cerevisiae mutation, hsm2-1, that results in increase of both spontaneous and UV-induced mutation frequencies but does not alter UV-sensitivity. Now HSM2 gene has been genetically and physically mapped and identified as a gene previously characterized as HMO1, a yeast homologue of human high mobility group genes HMG1/2. We found that hsm2 mutant is slightly deficient in plasmid-borne mismatch repair. We tested UV-induced mutagenesis in double mutants carrying hsm2-1 mutation and a mutation in a gene of principal damaged DNA repair pathways (rad2 and rev3) or in a mismatch repair gene (pms1 and recently characterized in our laboratory hsm3). The frequency of UV-induced mutations in hsm2 rev3 was not altered in comparison with single rev3 mutant. In contrast, the interaction of hsm2-1 with rad2 and pms1 was characterized by an increased frequency of UV-induced mutations in comparison with single rad2 and pms1 mutants. The UV-induced mutation frequency in double hsm2 hsm3 mutant was lower than in the single hsm2 and hsm3 mutants. The role of the HSM2 gene product in control of mutagenesis is discussed.     

Conditions for mutagenesis in the cynanobacterium Aphanocapsa 6714

Conditions for induction of mutants have been studied in the unicellular, blue-green alga, Aphanosapsa 6714. Ethylmethanesulfonate, nitrosomethylurea, an acridine (euflavine) and -rays from ⁶⁰Co induced no significant increase in mutant frequency although they produced classical killing effects. Methyl-nitro-nitrosoguanidine (NTG), at similar lethal doses, led to only a small increase in mutant yields. A useful mutagenic action was obtained only with ultraviolet light. This effect proved to be sensitive to photodependent repair processes, but not to dark excision repair systems. Attempts to adapt enrichment procedures by penicillin selective killing were unsuccessful.Abbreviations UV Ultraviolet light - NTG Methyl nitro nitroso guanidine - NMU Nitro methyl urea - EMS Ethyl methane sulfonate     

Involvement of umuDC ST genes in nitropyrene-induced -CG frameshift mutagenesis at the repetitive CG sequence in the hisD3052 allele of Salmonella typhimurium

Expression of the umuDC operon is required for UV and most chemical mutagenesis in Escherichia coli. The closely related species Salmonella typhimurium has two sets of umuDC-like operons, umuDC _ST on the chromosome and samAB on a 60-MDa cryptic plasmid. The roles of theumuDC-like operons in chemically induced frameshift mutagenesis of the hisD3052 allele of S. typhimurium were investigated. Introduction of a pBR322-derived plasmid carrying umuDCST increased the rate of reversion of hisD3052, following treatment with 1-nitropyrene (1-NP) or 1,8-dinitropyrene (1,-8DNP) tenfold and fivefold, respectively, whereas it did not substantially increase the rate of reversion induced by other frameshift mutagens, i.e. 2-nitrofluorene (2NF) and 2-amino- 3-methyldipyrido[1,2-a:3 ′,2′-d]imi-dazole (Glu-P-1). Introduction of a pBR322-derived plasmid carrying samAB did not increase the incidence of reversion of hisD3052 observed with any of the mutagens examined. Deletion of umuDC _STSubstantially lowered the reversion rate induced by l-NP or 1,8-DNP, but it did not affect reversion induced by 2-NF, Glu-P-1 or N-hydroxyacetylaminofluorene (N-OH-AAF). Deletion of samAB had little impact on reversion incidence induced by any of the five frameshift mutagens. DNA amplification using the polymerase chain reaction technique followed by restriction enzyme analysis using BssHII, suggested that the mutations induced by the five frameshift mutagens were all CG deletions at the CGCGCGCG sequence in hisD3052. These results suggest that umuDCST, but not samAB, is involved in the -2 frameshift mutagenesis induced by l-NP and 1,8-DNP at the repetitive CG sequence, whereas neither operon participates in induction of the same type of mutations by 2-NF, Glu-P-1 or N-OH-AAF.     

MucAB but not UmuDC proteins enhance ?2 frameshift mutagenesis induced by N-2-acetylaminofluorene at alternating GC sequences

N-2-acetylaminofluorene has been shown efficiently to induce both –1 and –2 frameshift mutations in Escherichia coli as well as in mammalian cells. In E. coli, the genetic characteristics of –1 and –2 frameshift mutations were found to be distinct. The –1 frameshift mutation pathway occurs at monotonous runs of G residues (i.e. GGGGG). This pathway exhibits the same genetic requirements as UV light-induced base substitution mutagenesis. Indeed, optimal mutagenesis requires the expression of both UmuDC and the activated form of RecA. The –2 frameshift mutation pathway operates at short alternating GpC sequences, such as the NarI sequence (i.e. GGCGCCGGCC). In contrast to the –1 frameshift mutation pathway, optimal induction does not require the UmuDC and RecA proteins. This pathway involves a LexA-repressed function tentatively called Npf (for NarI processing factor). In this paper, we show that MucAB efficiently stimulates the –2 frameshift mutation pathway. However, unlike the Npf pathway, MucAB-mediated stimulation requires expression of the RecA protein.     

Induction and repair of gene conversion in UV-sensitive mutants of yeast

Summary Photoreactivation effect on UV-induced allelic recombination has been examined using various combinations of leu 1 alleles in UV-sensitive and wild type diploid yeast, Saccharomyces cerevisiae. The frequencies of UV-induced heteroallelic reversion in UV-sensitive strains, presumably lacking dark-repair, are strikingly enhanced compared to those in wild type at the same doses under dark condition. However, these enhanced frequencies of reversion are diminished by photoreactivation almost to the level of those in wild type. The induced frequencies of homoallelic reversion (mutation) of relevant alleles are apparently lower than those of heteroallelic reversion. Phenotypic analysis for linked gene leu 1 on UV-induced heteroallelic revertants has shown that most of the revertants are of the nonreciprocal type recombination (mitotic gene conversion). These results would indicate that most of the dark-repairable damage leading to mitotic gene conversion after UV-light is due to pyrimidine dimers.On leave of absence from Radiation Center of Osaka Prefecture, Shinke-cho Sakai, Osaka, Japan.