Effect of UVM induction on mutation fixation at non-pairing and mispairing DNA lesions

Mutation fixation at an ethenocytosine (εC) residue borne on transfected M13 single-stranded DNA is significantly enhanced in response to pretreatment of Escherichia coli cells with UV, alkylating agents or hydrogen peroxide, a phenomenon that we have called UVM for UV modulation of mutagenesis. The UVM response does not require the E. coli SOS or adaptive responses, and is observed in cells defective for oxyR , an oxidative DNA damage-responsive regulatory gene. UVM may represent either a novel DNA-repair phenomenon, or an unrecognized feature of DNA replication in damaged cells that affects a specific class of non-coding DNA lesions. To explore the range of DNA lesions subject to the UVM effect, we have examined mutation fixation at 3, N  ⁴-ethenocytosine and 1, N  ⁶-ethenoadenine, as well as at O⁶-methylguanine (O⁶mG). M13 viral single-stranded DNA constructs bearing a single mutagenic lesion at a specific site were transfected into cells pretreated with UV or 1-methyl-3-nitro-1-nitrosoguanidine (MNNG). Survival of transfected viral DNA was measured as transfection efficiency, and mutagenesis at the lesion site was analysed by a quantitative multiplex sequence analysis technology. The results suggest that the UVM effect modulates mutagenesis at the two etheno lesions, but does not appear to significantly affect mutagenesis at O⁶mG. Because the modulation of mutagenesis is observed in cells incapable of the SOS response, these data are consistent with the notion that UVM may represent a previously unrecognized DNA damage-inducible response that affects the fidelity of DNA replication at certain mutagenic lesions in Escherichia coli .     

Two distinct phosphorylation pathways have additive effects on Abl family kinase activation

The activities of the related Abl and Arg nonreceptor tyrosine kinases are kept under tight control in cells, but exposure to several different stimuli results in a two- to fivefold stimulation of kinase activity. Following the breakdown of inhibitory intramolecular interactions, Abl activation requires phosphorylation on several tyrosine residues, including a tyrosine in its activation loop. These activating phosphorylations have been proposed to occur either through autophosphorylation by Abl in trans or through phosphorylation of Abl by the Src nonreceptor tyrosine kinase. We show here that these two pathways mediate phosphorylation at distinct sites in Abl and Arg and have additive effects on Abl and Arg kinase activation. Abl and Arg autophosphorylate at several sites outside the activation loop, leading to 5.2- and 6.2-fold increases in kinase activity, respectively. We also find that the Src family kinase Hck phosphorylates the Abl and Arg activation loops, leading to an additional twofold stimulation of kinase activity. The autoactivation pathway may allow Abl family kinases to integrate or amplify cues relayed by Src family kinases from cell surface receptors.     

DNA mismatch repair and mutation avoidance pathways

Unpaired and mispaired bases in DNA can arise by replication errors, spontaneous or induced base modifications, and during recombination. The major pathway for correction of mismatches arising during replication is the MutHLS pathway of Escherichia coli and related pathways in other organisms. MutS initiates repair by binding to the mismatch, and activates together with MutL the MutH endonuclease, which incises at hemimethylated dam sites and thereby mediates strand discrimination. Multiple MutS and MutL homologues exist in eukaryotes, which play different roles in the mismatch repair (MMR) pathway or in recombination. No MutH homologues have been identified in eukaryotes, suggesting that strand discrimination is different to E. coli. Repair can be initiated by the heterodimers MSH2-MSH6 (MutSalpha) and MSH2-MSH3 (MutSbeta). Interestingly, MSH3 (and thus MutSbeta) is missing in some genomes, as for example in Drosophila, or is present as in Schizosaccharomyces pombe but appears to play no role in MMR. MLH1-PMS1 (MutLalpha) is the major MutL homologous heterodimer. Again some, but not all, eukaryotes have additional MutL homologues, which all form a heterodimer with MLH1 and which play a minor role in MMR. Additional factors with a possible function in eukaryotic MMR are PCNA, EXO1, and the DNA polymerases delta and epsilon. MMR-independent pathways or factors that can process some types of mismatches in DNA are nucleotide-excision repair (NER), some base excision repair (BER) glycosylases, and the flap endonuclease FEN-1. A pathway has been identified in Saccharomyces cerevisiae and human that corrects loops with about 16 to several hundreds of unpaired nucleotides. Such large loops cannot be processed by MMR.     

Involvement of DNA lesions and SOS functions in 5-bromouracil-induced mutagenesis

Mutagenesis resulting from incorporation of 5-bromouracil (BU) in the DNA of E. coli K12 proceeds largely (approximately 80%) via misrepair of the lesions resulting from incorporation of the analogue. The premutational lesions are due principally to dehalogenation of incorporated BU residues, leading to formation of uracil residues, and removal of these by uracil-DNA glycosylase with formation of apyrimidinic sites. In the xthA mutant, defective in AP endonuclease, there is a several-fold increase in the frequency of BU-induced mutations, underlining the importance of AP sites in BU-induced mutagenesis. Premutational lesions undergo mutation frequency decline (MFD), which is subject to delay in the xthA mutant, pointing to some role of AP endonuclease in MFD, and further supporting involvement of AP sites in BU-induced mutagenesis. Efficient BU mutagenesis is dependent on the functions of the genes recA and umuC and non-mutated lexA protein.     

SOS mutator DNA polymerase IV functions in adaptive mutation and not adaptive amplification.

Adaptive point mutation and amplification are induced responses to environmental stress, promoting genetic changes that can enhance survival. A specialized adaptive mutation mechanism has been documented in one Escherichia coli assay, but its enzymatic basis remained unclear. We report that the SOS-inducible, error-prone DNA polymerase (pol) IV, encoded by dinB, is required for adaptive point mutation in the E. coli lac operon. A nonpolar dinB mutation reduces adaptive mutation frequencies by 85% but does not affect adaptive amplification, growth-dependent mutation, or survival after oxidative or UV damage. We show that pol IV, together with the major replicase, pol III, can account for all adaptive point mutations at lac. The results identify a role for pol IV in inducible genetic change.     

The myostatin null mutation and clenbuterol administration elicit additive effects in mice

In mice, the myostatin (Mstn) null mutation and treatment with clenbuterol both increase muscle growth and decrease fat mass. Our objective was to determine whether mechanistic overlap exists by administering clenbuterol to Mstn null mice. Male Mstn null and wild-type mice of similar genetic backgrounds received either 0 (control) or 20 p.p.m. clenbuterol in tap water free choice for 14 days. Several traits were measured to estimate muscle and fat growth. The Mstn null mutation resulted in increased body and empty carcass weight, increased muscle weights and decreased fat pad weights. Fat content was reduced and protein content was increased in the empty carcasses of Mstn null mice. Similarly, treatment with clenbuterol resulted in increased body and empty carcass weight, increased muscle weights and reduced fat pad weights. Fat content of empty carcasses and viscera was reduced and protein content of empty carcasses was increased with clenbuterol treatment. A significant interaction of genotype and clenbuterol treatment would indicate an altered responsiveness of Mstn null mice to clenbuterol. However, only the weight of gastrocnemius muscles exhibited a significant (P = 0.01) interaction of genotype and clenbuterol treatment, indicating that Mstn null mice were less responsive to clenbuterol compared with wild-type mice. Thus, for all other traits, the impact of Mstn null mutation and clenbuterol treatment was completely additive. These data suggest that disruption of Mstn function does not alter the response of mice to β-adrenergic agonists.     

Effects of SOS and MucAB functions on reactivation and mutagenesis of M13 replicative form DNA bearing bulky lesions

We have previously determined the specificity of -1 frameshifts induced by aflatoxin-B1-2,3-dichloride (AFB1C12) in phage M13 double-strand replicative form (RF) DNA. The system consists of: (i) in vitro adduction of RF DNA of BK8, a lacZ + 1 frameshift derivative of phage M13mp8; (ii) transfection into unirradiated or UV-irradiated bacterial host cells; (iii) scoring and sequencing of revertants (i.e., -1 frameshifts). The previous data had shown that induction of SOS functions enhanced mutagenesis. However, this increase in mutagenesis is not accompanied by enhanced survival in a majority of the strains tested. Here, we present evidence to show that the lack of SOS reactivation is a specific property of the RF DNA system rather than a specific property of the lesion. A model mechanism based on the replicative strategy of transfected RF DNA can account for these observations. In addition, we have calculated individual Weigle mutagenesis factors at 8 major mutagen induced sites reported previously. Analysis of these data indicates that, within a restricted subset of possible mutational events (i.e., -1 frameshifts), Weigle mutagenesis is affected by both the DNA sequence environment of the mutation site as well as the repair phenotype of the cell.     

Caffeine and heat have additive but not interactive effects on physiologic strain: A factorial experiment

This study tested the interactive effects of heat and caffeine on exercise-induced physiological strain by using a 2x2 within-subjects factorial design. Thirty-five physically fit Caucasians underwent a bout of exercise under four conditions wherein ambient conditions (heat vs no heat) and caffeine (placebo vs caffeine; double-blinded) were manipulated. Exercise consisted of a 60-min walk and 5-min step/squat test while wearing weighted backpack. Primary outcomes include measures of physiologic strain (Core temperature [T_r] and heart rate [HR]). Secondary measures included blood pressure, markers of sweat loss, and creatine kinase (CK). Repeated measures models were created to evaluate the individual and combined effects of heat and caffeine. Key results indicated that heat and caffeine significantly increased T_r and HR after walking and stair-stepping. No significant heat by caffeine interactions were detected, and caffeine's main effects were relatively low (≤0.17 °C for T_r and ≤6.6 bpm for HR). Of note, heat and caffeine exhibited opposite effects on blood pressure: caffeine increased both systolic and diastolic blood pressure (by 6–7 mmHg) and heat decreased them (by 4–6 mm Hg; ps < 0.05). In summary, heat and caffeine affected physiologic strain during exercise but exhibited no synergistic effects. In contrast, neither factor affected muscle damage. Clinical implications for heat illness risk in the military are discussed.     

Personality and body condition have additive effects on motivation to feed in Zebra Finches Taeniopygia guttata

Several hypotheses have been proposed to account for the adaptive evolution of personality, defined as inter‐individual differences in behaviour that are consistent over time and across situations. For instance, the ‘pace‐of‐life syndrome’ hypothesis suggests that personality evolved as a behavioural correlate of life‐history trajectories that vary within populations. Thus, proactivity, corresponding to higher exploratory tendencies or higher boldness levels, has been linked to higher productivity or mortality rates. However, the extent to which proactivity is associated with a higher motivation to forage remains poorly understood. Moreover, although personality and its effects on foraging behaviour are usually considered to be independent of any motivational or nutritional state, few studies so far have challenged this. Here we show that personality traits, both individually or combined using a principal component analysis, and body condition have additive effects on latency to feed following food deprivation in the Zebra Finch Taeniopygia guttata, with personality accounting for 41% and body condition for about 20% of the total variation in latency to feed. In accordance with the pace‐of‐life syndrome hypothesis, latency to feed was negatively related to the degree of proactivity and positively related to body condition. Thus, proactive individuals and individuals in poorer condition were quicker to start feeding after a period of food deprivation. The absence of a significant interaction between personality and body condition further suggests that the effect of personality was independent of body condition. We discuss the relevance of our results in relation to the different factors influencing foraging in birds. Moreover, we place our results within a life‐history framework by emphasizing the correlated evolution of life‐history traits and personality.     

A deafness-associated mitochondrial DNA mutation caused pleiotropic effects on DNA replication and tRNA metabolism

In this report, we investigated the molecular mechanism underlying a deafness-associated m.5783C > T mutation that affects the canonical C50-G63 base-pairing of TΨC stem of tRNA^Cys and immediately adjacent to 5′ end of light-strand origin of mitochondrial DNA (mtDNA) replication (OriL). Two dimensional agarose gel electrophoresis revealed marked decreases in the replication intermediates including ascending arm of Y-fork arcs spanning OriL in the mutant cybrids bearing m.5783C > T mutation. mtDNA replication alterations were further evidenced by decreased levels of PolγA, Twinkle and SSBP1, newly synthesized mtDNA and mtDNA contents in the mutant cybrids. The m.5783C > T mutation altered tRNA^Cys structure and function, including decreased melting temperature, conformational changes, instability and deficient aminoacylation of mutated tRNA^Cys. The m.5783C > T mutation impaired the 5′ end processing efficiency of tRNA^Cys precursors and reduced the levels of tRNA^Cys and downstream tRNA^Tyr. The aberrant tRNA metabolism impaired mitochondrial translation, which was especially pronounced effects in the polypeptides harboring higher numbers of cysteine and tyrosine codons. These alterations led to deficient oxidative phosphorylation including instability and reduced activities of the respiratory chain enzyme complexes I, III, IV and intact supercomplexes overall. Our findings highlight the impact of mitochondrial dysfunction on deafness arising from defects in mitochondrial DNA replication and tRNA metabolism.     

A novel SOS1 mutation in Costello/CFC syndrome affects signaling in both RAS and PI3K pathways

Abstract

Context: Pathological upregulation of the RAS/MAPK pathway causes Costello, Noonan and cardio–facio–cutaneous (CFC) syndrome; however, little is known about PI3K/AKT signal transduction in these syndromes. Previously, we found a novel mutation of the SOS1 gene (T158A) in a patient with Costello/CFC overlapping phenotype. Objective: The aim of this study was to investigate how this mutation affects RAS/MAPK as well as PI3K/AKT pathway signal transduction.

Materials and methods: Wild-type and mutant (T158A) Son of Sevenless 1 (SOS1) were transfected into 293T cells. The levels of phospho- and total ERK1/2, AKT, p70S6K and pS6 were examined under epidermal growth factor (EGF) stimulation. Results: After EGF stimulation, the ratio of phospho-ERK1/2 to total ERK1/2 was highest at 5?min in mutant (T158A) SOS1 cells, and at 15?min in wild-type SOS1 cells. Phospho-AKT was less abundant at 60?min in mutant than in wild-type SOS1 cells. Phosphorylation at various sites in p70S6K differed between wild-type and mutant cells. Eighteen hours after activation by EGF, the ratio of phospho-ERK1/2 to total ERK1/2 remained significantly higher in mutant than in wild-type SOS1 cells, but that of phospho-AKT to total AKT was unchanged. Discussion: T158A is located in the histone-like domain, which may have a role in auto-inhibition of RAS exchanger activity of SOS1. T158A may disrupt auto-inhibition and enhance RAS signaling. T158A also affects PI3K/AKT signaling, probably via negative feedback via phospho-p70S6K. Conclusion: The SOS1 T158A mutation altered the phosphorylation of gene products involved in both RAS/MAPK and PI3K/AKT pathways.     

Host-specificity mutants of Rhizobium meliloti have additive effects in situ on initiation of alfalfa nodules

Pairs of Rhizobium meliloti nod mutants were co-inoculated onto alfalfa (Medicago saliva L.) roots to determine whether one nod mutant could correct, in situ, for defects in nodule initiation of another nod mutant. None of the Tn5 or nod deletion mutants were able to help each other form nodules when co-inoculated together in the absence of the wild-type. However, as previously observed, individual nod mutants significantly increased nodule initiation by low dosages of co-inoculated wild-type cells. Thus, nod mutants do produce certain signal substances or other factors which overcome limits to nodule initiation by the wild-type. When pairs of nod mutants were co-inoculated together with the wild-type, the stimulation of nodulation provided by individual nodABC mutants was not additive. However, clearly additive or synergistic stimulation was observed between pairs of mutants with a defective host-specificity gene (nodE, nodF, or nodH). Each pair of host-specificity mutants stimulated first nodule formation to nearly the maximum levels obtainable with high dosages of the wild-type. Mutant bacteria were recovered from only about 10% of these nodules, whereas the co-inoculated wild-type was present in all these nodules and substantially outnumbered mutant bacteria in nodules occupied by both. Thus, these mutant co-inoculants appeared to help their parent in situ even though they could not help each other. Sterile culture filtrates from wild-type cells stimulated nodule initiation by low dosages of the wild-type, but only when a host-specificity mutant was also present. The results from our studies seem consistent with the possibility that pairs of host-specificity mutants are able to help the wild-type initiate nodule formation by sustained production of complementary signals required for induction of symbiotic host responses.     

Lethal and mutagenic effects of 8-methoxypsoralen-induced lesions on plasmid DNA

The genotoxic effect of 8-methoxypsoralen damages (monoadducts and crosslinks) on plasmid DNA was studied. pBR322 DNA was treated with several concentrations of 8-methoxypsoralen plus fixed UVA light irradiation. After transformation into E. coli cells with different repair capacities (uvrA, recA and wild-type), plasmid survival and mutagenesis in ampicillin- and tetracycline-resistant genes were analysed. Results showed that crosslinks were extremely lethal in all 3 strains; indeed, it seemed that they were not repaired even in proficient bacteria. Monoadducts were also found to be lethal although they were removed to some extent by the excision-repair pathway (uvrA-dependent). Damaged plasmid DNA appeared to induce mutagenic repair, but only in the wild-type strain. In order to study the influence of the SOS response on plasmid recovery, preirradiation of the host cells was also performed. Preirradiation of the uvrA or wild-type strains significantly increased plasmid recovery. Consistent with the expectations of SOS repair, no effect was observed in preirradiated recA cells. Plasmid recovery in the excision-deficient strain was mainly achieved by the mutagenic repair of some fraction of the lesions, probably monoadducts. The greatest increase in plasmid recovery was found in the wild-type strain. This likely involved the repair of monoadducts and some fraction of the crosslinks. We conclude that repair in preirradiated repair-proficient cells is carried out mainly by an error-free pathway, suggesting enhancement of the excision repair promoted by the induction of SOS functions.     

L-DOPA and glia-conditioned medium have additive effects on tyrosine hydroxylase expression in human catecholamine-rich neuroblastoma NB69 cells

The aim of this study was to investigate the effect of L-DOPA and glia-conditioned medium (GCM) on cell viability, tyrosine hydroxylase (TH) expression, dopamine (DA) metabolism and glutathione (GSH) levels of NB69 cells. L-DOPA (200 microM) induced differentiation of NB69 cells of more than 4 weeks in vitro, as shown by phase-contrast microscopy and TH immunocytochemistry, and decreased replication, as shown by 5-bromodeoxyuridine immunostaining. L-DOPA did not increase the number of necrotic or apoptotic cells, as shown by morphological features, Trypan Blue, lactate dehydrogenase activity, bis-benzimide staining and TUNEL assay. Furthermore, L-DOPA (200 microM) increased Bcl-xL protein expression. Incubation of cells with L-DOPA (50, 100, 200 microM) for 24 h resulted in an increase in TH protein levels (174, 196 and 212% versus control). Neither carbidopa, an inhibitor of L-aromatic amino acid decarboxylase enzyme, nor L-buthionine sulfoximine, which inhibits GSH synthesis, or ascorbic acid, an antioxidant, blocked the L-DOPA-induced effect on TH protein expression. L-DOPA (0, 50, 100 and 200 microM) plus GCM further increased the amount of TH protein (346, 446, 472 and 424%). L-DOPA (200 microM) increased TH protein levels to 132, 191 and 245% of controls after incubation for 24, 48 and 72 h. DA metabolism in NB69 cells was increased in cultures treated with either L-DOPA (200-300 microM) or GCM and these two agents had a synergistic effect on DA metabolism. In addition, L-DOPA (200 microM) or/and GCM-treated cells increased their GSH extracellular levels (223, 257, 300% of controls) after 48 h of treatment. The L-DOPA-induced increase of TH protein expression in NB69 cells was independent of DA production, free radicals and GSH up-regulation.     

The balance between pleiotropic mutation and selection,when alleles have discrete effects

The theory of pleiotropic mutation and selection is investigated and developed for a large population of asexual organisms. Members of the population are subject to stabilising selection on Omega phenotypic characters, which each independently affect fitness. Pleiotropy is incorporated into the model by allowing each mutation to simultaneously affect all characters. To expose differences with continuous-allele models, the characters are taken to originate from discrete-effect alleles and thus have discrete genotypic effects. Each character can take the values nxDelta where n=0,+/-1,+/-2, em leader, and the splitting in character effects, Delta, is a parameter of the model. When the distribution of mutant effects is normally distributed around the parental value, and Delta is large, a "stepwise" model of mutation arises, where only adjacent trait effects are accessible from a single mutation. The present work is primarily concerned with the opposite limit, where Delta is small and many different trait effects are accessible from a single mutation.In contrast to what has been established for continuous-effect models, discrete-effect models do not yield a singular equilibrium distribution of genotypic effects for any value of Omega. Instead, for different values of Omega, the equilibrium frequencies of trait values have very different dependencies on Delta. For Omega=1 and 2, decreasing Delta broadens the width of the frequency distribution and hence increases the equilibrium level of polymorphism. For all sufficiently large values of Omega, however, decreasing Delta decreases the width of the frequency distribution and the equilibrium level of polymorphism. The connection with continuous trait models follows when the limit Delta-->0 is considered, and a singular probability density of trait values is obtained for all sufficiently large Omega.     

Two closely related nickel complexes have different effects on DNA damage and cell viability

Nickel is considered a weak carcinogen. It is known to interact with DNA and DNA-binding proteins. The ability of certain nickel compounds to cleave DNA has been exploited mainly for research purposes and less for developing new anticancer drugs. Here we compare the interactions of two closely related nickel complexes, [NiCR]2+ and [Ni(CR-2H)]2+, with DNA. CR stands for 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene. [NiCR]2+ has been used in the past as a structure-specific probe for RNA and DNA oligonucleotides in the presence of oxidizing agent but little is known about the biological effects of either complex. Our results show that [Ni(CR-2H)]2+ can damage DNA in vivo and in vitro in the absence of an added oxidizing agent and has an IC50 of 70 microM in human breast cancer cells whereas [NiCR]2+ and NiCl2 do not exhibit significant cytotoxicity. However, both [NiCR]2+ and [Ni(CR-2H)]2+ bind to the minor groove of double-stranded DNA.     

Light at night and melatonin have opposite effects on breast cancer tumors in mice assessed by growth rates and global DNA methylation

Light-at-night (LAN) is a worldwide problem co-distributed with breast cancer prevalence. We hypothesized that exposure to LAN is coincided with a decreased melatonin (MLT) secretion level, followed by epigenetic modifications and resulted in higher breast cancer tumors growth-rate. Accordingly, we studied the effect of LAN exposure and exogenous MLT on breast cancer tumors growth-rate. 4T1 cells were inoculated into BALB/c short day-acclimated mice, resulting in tumors growth. Growth rates were followed under various light exposures and global DNA methylations were measured. Results demonstrated the positive effect of LAN on tumors growth-rate, reversed by MLT through global DNA methylation.     

Analysis of ssb mutations in vivo implicates SSB protein in two distinct pathways of SOS induction and in recombinational DNA repair

Site-directed mutations in the Escherichia coli ssb gene were tested for the ability to complement a chromosomal ssb deletion for viability, and only the ssb W54→G mutation failed to do so at the pSC101 copy level. Non-aromatic amino acid substitutions for SSB Trp-54 ( ssb W54→L and ssb W54→S) produced the greatest effects on in vivo protein function including altered marker linkage subsequent to generalized transduction, extreme UV sensitivity, and a lack of ability to support SOS induction. Additionally, the ssb-113 ( ssb P176→S) mutation demonstrated the existence of both uvrA -dependent and uvrA -independent components of SOS induction. Although nucleotide excision repair appeared unaffected by alterations in the SSB protein, the mutational analysis suggests a direct role for SSB in recombinational repair.     

Frameshift mutagenesis by ultraviolet and gamma radiations in Salmonella: SOS inducibility and effects of DNA polymerase I

Ultraviolet (UV) and gamma-induced mutagenesis have been studied using a doubly auxotrophic strain of Salmonella typhimurium carrying the amber leuA150 mutation (which reverts by base-pair substitution) and the frameshift hisC3076 marker (which reverts by compensating frameshifts). In the initially constructed LT2 background, both markers were poorly revertible by UV and essentially non-revertible by gamma-radiation. A derivative of this strain carrying the mutation-enhancing plasmid pKM101 was however readily reverted by both UV and gamma, with either Leu+ (base substitution) or His+ (frameshift) revertants being observed on appropriate selective media. Photoreactivation experiments suggested that the lesions leading to formation of the two types of mutagenic event were similar if not identical. Support for this suggestion was obtained when it was found that yields of both types of UV-induced revertant were significantly increased in an excision-deficient background, while no revertants of either type were found in a recA background. Yields of gamma-induced revertants were not greatly altered in a uvrB background, but were also reduced to zero (for both markers) in the recA background. These results are consistent with what has previously been well-documented for UV and gamma-induced base-pair substitution mutagenesis, and serve to emphasize the similarities between base-pair substitution mutagenesis and frameshift mutagenesis by these agents. There are differences, however, since although UV-induced reversion of the leuA150 marker was little affected and gamma-induced reversion of leuA150 was somewhat reduced in the presence of a polA mutation (polA3), the yields of His+ frameshift revertants were significantly increased in the polA3 background following treatment with either UV or gamma. Thus while inducible DNA repair (SOS repair) appears to be involved in generating both types of mutational event following either UV- or gamma-irradiation, at some stage in the processing of premutational lesions the level (or type) of DNA polymerase I activity in the cell seems to have an important role in determining whether or not frameshifts or base-pair substitutions will be produced at a particular frequency.