Co-localization in replication foci and interaction of human Y-family members, DNA polymerase pol eta and REVl protein

The progress of replicative DNA polymerases along the replication fork may be impeded by the presence of lesions in the genome. One way to circumvent such hurdles involves the recruitment of specialized DNA polymerases that perform limited incorporation of nucleotides in the vicinity of the damaged site. This process entails DNA polymerase switch between replicative and specialized DNA polymerases. Five eukaryotic proteins can carry out translesion synthesis (TLS) of damaged DNA in vitro, DNA polymerases zeta, eta, iota, and kappa, and REV1. To identify novel proteins that interact with hpol eta, we performed a yeast two-hybrid screen. In this paper, we show that hREV1 interacts with hpol eta as well as with hpol kappa and poorly with hpol iota. Furthermore, cellular localization analysis demonstrates that hREV1 is present, with hpol eta in replication factories at stalled replication forks and is tightly associated with nuclear structures. This hREV1 nuclear localization occurs independently of the presence of hpol eta. Taken together, our data suggest a central role for hREV1 as a scaffold that recruits DNA polymerases involved in TLS.     

Mutational specificity of gamma-radiation-induced guanine-thymine and thymine-guanine intrastrand cross-links in mammalian cells and translesion synthesis past the guanine-thymine lesion by human DNA polymerase eta

Comparative mutagenesis of gamma- or X-ray-induced tandem DNA lesions G[8,5-Me]T and T[5-Me,8]G intrastrand cross-links was investigated in simian (COS-7) and human embryonic (293T) kidney cells. For G[8,5-Me]T in 293T cells, 5.8% of progeny contained targeted base substitutions, whereas 10.0% showed semitargeted single-base substitutions. Of the targeted mutations, the G --> T mutation occurred with the highest frequency. The semitargeted mutations were detected up to two bases 5' and three bases 3' to the cross-link. The most prevalent semitargeted mutation was a C --> T transition immediately 5' to the G[8,5-Me]T cross-link. Frameshifts (4.6%) (mostly small deletions) and multiple-base substitutions (2.7%) also were detected. For the T[5-Me,8]G cross-link, a similar pattern of mutations was noted, but the mutational frequency was significantly higher than that of G[8,5-Me]T. Both targeted and semitargeted mutations occurred with a frequency of approximately 16%, and both included a dominant G --> T transversion. As in 293T cells, more than twice as many targeted mutations in COS cells occurred in T[5-Me,8]G (11.4%) as in G[8,5-Me]T (4.7%). Also, the level of semitargeted single-base substitutions 5' to the lesion was increased and 3' to the lesion decreased in T[5-Me,8]G relative to G[8,5-Me]T in COS cells. It appeared that the majority of the base substitutions at or near the cross-links resulted from incorporation of dAMP opposite the template base, in agreement with the so-called "A-rule". To determine if human polymerase eta (hpol eta) might be involved in the mutagenic bypass, an in vitro bypass study of G[8,5-Me]T in the same sequence was carried out, which showed that hpol eta can bypass the cross-link incorporating the correct dNMP opposite each cross-linked base. For G[8,5-Me]T, nucleotide incorporation by hpol eta was significantly different from that by yeast pol eta in that the latter was more error-prone opposite the cross-linked Gua. The incorporation of the correct nucleotide, dAMP, by hpol eta opposite cross-linked T was 3-5-fold more efficient than that of a wrong nucleotide, whereas incorporation of dCMP opposite the cross-linked G was 10-fold more efficient than that with dTMP. Therefore, the nucleotide incorporation pattern by hpol eta was not consistent with the observed cellular mutations. Nevertheless, at and near the lesion, hpol eta was more error-prone compared to a control template. The in vitro data suggest that translesion synthesis by another Y-family DNA polymerase and/or flawed participation of an accessory protein is a more likely scenario in the mutagenesis of these lesions in mammalian cells. However, hpol eta may play a role in correct bypass of the cross-links.     

Adenovirus mediated transduction of the human DNA polymerase eta cDNA

Xeroderma pigmentosum (XP) is an autosomal recessive photosensitive disorder with an extremely high incidence of skin cancers. Seven complementation groups, corresponding to seven proteins involved in nucleotide excision repair (NER), are associated with this syndrome. However, in XP variant patients, the disorder is caused by defects in DNA polymerase eta; this error prone polymerase, encoded by POLH, is involved in translesion DNA synthesis (TLS) on DNA templates damaged by ultraviolet light (UV). We constructed a recombinant adenovirus carrying the human POLH cDNA linked to the EGFP reporter gene (AdXPV-EGFP) and infected skin fibroblasts from both XPV and XPA patients. Twenty-four hours after infection, the DNA polymerase eta-EGFP fusion protein was detected by Western blot analysis, demonstrating successful transduction by the adenoviral vector. Protein expression was accompanied by reduction in the high sensitivity of XPV cells to UV, as determined by cell survival and apoptosis-induction assays. Moreover, the pronounced UV-induced inhibition of DNA synthesis in XPV cells and their arrest in S phase were attenuated in AdXPV-EGFP infected cells, confirming that the transduced polymerase was functional. However, over-expression of polymerase eta mediated by AdXPV-EGFP infection did not result in enhancement of cell survival, prevention of apoptosis, or higher rate of nascent DNA strand growth in irradiated XPA cells. These results suggest that TLS by DNA polymerase eta is not a limiting factor for recovery from cellular responses induced by UV in excision-repair deficient fibroblasts.     

Translesion replication in cisplatin-treated xeroderma pigmentosum variant cells is also caffeine-sensitive: features of the error-prone DNA polymerase(s) involved in UV-mutagenesis

Patients with xeroderma pigmentosum variant (XP-V) have a higher risk to skin cancer and XP-V cells are extremely mutable by ultraviolet (UV). The defective gene encodes a DNA polymerase (Poleta) which catalyzed relatively accurate translesion synthesis past the cyclobutane dimer of UV-lesions instead of the replicative polymerase(s) that stalled just before the lesion. Pulse-chase studies have shown that translesion replication in XP-V cells is delayed, but does not completely cease. Taking these results together, error-prone polymerase(s) are plausively involved in the UV-mutagenesis in XP-V devoid of Poleta. However, less is known about the polymerase(s) in vivo. Using an alkaline sucrose density gradient centrifugation (ASDG) technique, translesion replication is detected in the two XP-V strains XP30RO and XP115LO. As reported by Lehmann et al. [Proc. Natl. Acad. Sci. U.S.A. 72 (1975): 219] in XP-V; (i) smaller replication products were accumulated after UV irradiation; (ii) the elongation of these products was delayed; (iii) the elongation was markedly inhibited by caffeine. XP-V cells UV-irradiated at mid-S phase were normally S-arrested, and no "override" by caffeine (i.e. abrogation of the S-checkpoint) was observed by flow cytometry, suggesting that caffeine does not act via cdc kinase here; (iv) butylphenyldeoxyguanosine (BuPGdR) inhibited elongation of replication products only in UV-irradiated XP-V cells; (v) dideoxycytidine or dideoxyinosine had no effect on this process in either normal or XP-V cells. Next, similar phenomena to UV (all of above i to v) were observed also in cisplatin-treated XP-V cells. Pol eta was indicated to participate in cisplatin-induced translesion replication in normal cells. Summing up the above results, the polymerase(s) which work in translesion replication in XP-V are probably BuPGdR-sensitive, insensitive to dideoxynucleotides and can bypass also cisplatin-lesions. To date, several polymerases capable of lesion-bypass synthesis have been isolated. The features presented here are quite useful for identifying the error-prone polymerase(s) involved in UV-mutagenesis.     

Caffeine and human DNA metabolism: the magic and the mystery

The ability of caffeine to reverse cell cycle checkpoint function and enhance genotoxicity after DNA damage was examined in telomerase-expressing human fibroblasts. Caffeine reversed the ATM-dependent S and G2 checkpoint responses to DNA damage induced by ionizing radiation (IR), as well as the ATR- and Chk1-dependent S checkpoint response to ultraviolet radiation (UVC). Remarkably, under conditions in which IR-induced G2 delay was reversed by caffeine, IR-induced G1 arrest was not. Incubation in caffeine did not increase the percentage of cells entering the S phase 6-8h after irradiation; ATM-dependent phosphorylation of p53 and transactivation of p21(Cip1/Waf1) post-IR were resistant to caffeine. Caffeine alone induced a concentration- and time-dependent inhibition of DNA synthesis. It inhibited the entry of human fibroblasts into S phase by 70-80% regardless of the presence or absence of wildtype ATM or p53. Caffeine also enhanced the inhibition of cell proliferation induced by UVC in XP variant fibroblasts. This effect was reversed by expression of DNA polymerase eta, indicating that translesion synthesis of UVC-induced pyrimidine dimers by DNA pol eta protects human fibroblasts against UVC genotoxic effects even when other DNA repair functions are compromised by caffeine.     

Replication of damaged DNA: molecular defect in xeroderma pigmentosum variant cells.

Individuals with Xeroderma pigmentosum (XP) syndrome have a genetic predisposition to sunlight-induced skin cancer. Genetically different forms of XP have been identified by cell fusion. Cells of individuals expressing the classical form of XP (complementation groups A through G) are deficient in the nucleotide excision repair (NER) pathway. In contrast, the cells belonging to the variant class of XP (XPV) are NER-proficient and are only slightly more sensitive than normal cells to the killing action of UV light radiation. The XPV fibroblasts replicate damaged DNA generating abnormally short fragments either in vivo [A.R. Lehmann, The relationship between pyramidine dimers and replicating DNA in UV-irradiated human fibroblasts, Nucleic Acids Res. 7 (1979) 1901-1912; S.D. Park, J.E. Cleaver, Postreplication repair: question of its definition and possible alteration in Xeroderma pigmentosum cell strains, Proc. Natl. Acad. Sci. U.S.A. 76 (1979) 3927-3931.] or in vitro [S.M. Cordeiro, L.S. Zaritskaya, L.K. Price, W.K. Kaufmann, Replication fork bypass of a pyramidine dimer blocking leading strand DNA synthesis, J. Biol. Chem. 272 (1997) 13945-13954; D.L. Svoboda, L.P. Briley, J.M. Vos, Defective bypass replication of a leading strand cyclobutane thymine dimer in Xeroderma pigmentosum variant cell extracts, Cancer Res. 58 (1998) 2445-2448; I. Ensch-Simon, P.M. Burgers, J.S. Taylor, Bypass of a site-specific cis-syn thymine dimer in an SV40 vector during in vitro replication by HeLa and XPV cell-free extracts, Biochemistry 37 (1998) 8218-8226.], suggesting that in XPV cells, replication has an increased probability of being blocked at a lesion. Furthermore, extracts from XPV cells were found to be defective in translesion synthesis [A. Cordonnier, A.R. Lehmann, R.P.P. Fuchs, Impaired translesion synthesis in Xeroderma pigmentosum variant extracts, Mol. Cell. Biol. 19 (1999) 2206-2211.]. Recently, Masutani et al. [C. Masutani, M. Araki, A. Yamada, R. Kusomoto, T. Nogimori, T. Maekawa, S. Iwai, F. Hanaoka, Xeroderma pigmentosum variant (XP-V) correcting protein from HeLa cells has a thymine dimer bypass DNA polymerase activity, EMBO J. 18 (1999) 3491-3501.] have shown that the XPV defect can be corrected by a novel human DNA polymerase, homologue to the yeast DNA polymerase eta, which is able to replicate past cyclobutane pyrimidine dimers in DNA templates. This review focuses on our current understanding of translesion synthesis in mammalian cells whose defect, unexpectedly, is responsible for the hypermutability of XPV cells and for the XPV pathology.     

Role of DNA polymerase eta in the UV mutation spectrum in human cells

In humans, inactivation of the DNA polymerase eta gene (pol eta) results in sunlight sensitivity and causes the cancer-prone xeroderma pigmentosum variant syndrome (XP-V). Cells from XP-V individuals have a reduced capacity to replicate UV-damaged DNA and show hypermutability after UV exposure. Biochemical assays have demonstrated the ability of pol eta to bypass cis-syn-cyclobutane thymine dimers, the most common lesion generated in DNA by UV. In most cases, this bypass is error-free. To determine the actual requirement of pol eta in vivo, XP-V cells (XP30RO) were complemented by the wild type pol eta gene. We have used two pol eta-corrected clones to study the in vivo characteristics of mutations produced by DNA polymerases during DNA synthesis of UV-irradiated shuttle vectors transfected into human host cells, which had or had not been exposed previously to UV radiation. The functional complementation of XP-V cells by pol eta reduced the mutation frequencies both at CG and TA base pairs and restored UV mutagenesis to a normal level. UV irradiation of host cells prior to transfection strongly increased the mutation frequency in undamaged vectors and, in addition, especially in the pol eta-deficient XP30RO cells at 5'-TT sites in UV-irradiated plasmids. These results clearly show the protective role of pol eta against UV-induced lesions and the activation by UV of pol eta-independent mutagenic processes.     

DNA polymerase eta reduces the gamma-H2AX response to psoralen interstrand crosslinks in human cells

DNA interstrand crosslinks are processed by multiple mechanisms whose relationships to each other are unclear. Xeroderma pigmentosum-variant (XP-V) cells lacking DNA polymerase eta are sensitive to psoralen photoadducts created under conditions favoring crosslink formation, suggesting a role for translesion synthesis in crosslink repair. Because crosslinks can lead to double-strand breaks, we monitored phosphorylated H2AX (gamma-H2AX), which is typically generated near double-strand breaks but also in response to single-stranded DNA, following psoralen photoadduct formation in XP-V fibroblasts to assess whether polymerase eta is involved in processing crosslinks. In contrast to conditions favoring monoadducts, conditions favoring psoralen crosslinks induced gamma-H2AX levels in both XP-V and nucleotide excision repair-deficient XP-A cells relative to control repair-proficient cells; ectopic expression of polymerase eta in XP-V cells normalized the gamma-H2AX response. In response to psoralen crosslinking, gamma-H2AX as well as 53BP1 formed coincident foci that were more numerous and intense in XP-V and XP-A cells than in controls. Psoralen photoadducts induced gamma-H2AX throughout the cell cycle in XP-V cells. These results indicate that polymerase eta is important in responding to psoralen crosslinks, and are consistent with a model in which nucleotide excision repair and polymerase eta are involved in processing crosslinks and avoiding gamma-H2AX associated with double-strand breaks and single-stranded DNA in human cells.     

Palm mutants in DNA polymerases alpha and eta alter DNA replication fidelity and translesion activity

We isolated active mutants in Saccharomyces cerevisiae DNA polymerase alpha that were associated with a defect in error discrimination. Among them, L868F DNA polymerase alpha has a spontaneous error frequency of 3 in 100 nucleotides and 570-fold lower replication fidelity than wild-type (WT) polymerase alpha. In vivo, mutant DNA polymerases confer a mutator phenotype and are synergistic with msh2 or msh6, suggesting that DNA polymerase alpha-dependent replication errors are recognized and repaired by mismatch repair. In vitro, L868F DNA polymerase alpha catalyzes efficient bypass of a cis-syn cyclobutane pyrimidine dimer, extending the 3' T 26000-fold more efficiently than the WT. Phe34 is equivalent to residue Leu868 in translesion DNA polymerase eta, and the F34L mutant of S. cerevisiae DNA polymerase eta has reduced translesion DNA synthesis activity in vitro. These data suggest that high-fidelity DNA synthesis by DNA polymerase alpha is required for genomic stability in yeast. The data also suggest that the phenylalanine and leucine residues in translesion and replicative DNA polymerases, respectively, might have played a role in the functional evolution of these enzyme classes.     

Role of DNA polymerases eta, iota and zeta in UV resistance and UV-induced mutagenesis in a human cell line

Genes coding for DNA polymerases eta, iota and zeta, or for both Pol eta and Pol iota have been inactivated by homologous recombination in the Burkitt's lymphoma BL2 cell line, thus providing for the first time the total suppression of these enzymes in a human context. The UV sensitivities and UV-induced mutagenesis on an irradiated shuttle vector have been analyzed for these deficient cell lines. The double Pol eta/iota deficient cell line was more UV sensitive than the Pol eta-deficient cell line and mutation hotspots specific to the Pol eta-deficient context appeared to require the presence of Pol iota, thus strengthening the view that Pol iota is involved in UV damage translesion synthesis and UV-induced mutagenesis. A role for Pol zeta in a damage repair process at late replicative stages is reported, which may explain the drastic UV-sensitivity phenotype observed when this polymerase is absent. A specific mutation pattern was observed for the UV-irradiated shuttle vector transfected in Pol zeta-deficient cell lines, which, in contrast to mutagenesis at the HPRT locus previously reported, strikingly resembled mutations observed in UV-induced skin cancers in humans. Finally, a Pol eta PIP-box mutant (without its PCNA binding domain) could completely restore the UV resistance in a Pol eta deficient cell line, in the absence of UV-induced foci, suggesting, as observed for Pol iota in a Pol eta-deficient background, that TLS may occur without the accumulation of microscopically visible repair factories.     

Mechanisms of accurate translesion synthesis by human DNA polymerase eta

The XPV (xeroderma pigmentosum variant) gene encodes human DNA polymerase eta (pol eta), which is involved in the replication of damaged DNA. Pol eta catalyzes efficient and accurate translesion synthesis past cis-syn cyclobutane di-thymine lesions. Here we show that human pol eta can catalyze translesion synthesis past an abasic (AP) site analog, N-2-acetylaminofluorene (AAF)-modified guanine, and a cisplatin-induced intrastrand cross-link between two guanines. Pol eta preferentially incorporated dAMP and dGMP opposite AP, and dCMP opposite AAF-G and cisplatin-GG, but other nucleotides were also incorporated opposite these lesions. However, after incorporating an incorrect nucleotide opposite a lesion, pol eta could not continue chain elongation. In contrast, after incorporating the correct nucleotide opposite a lesion, pol eta could continue chain elongation, whereas pol alpha could not. Thus, the fidelity of translesion synthesis by human pol eta relies not only on the ability of this enzyme to incorporate the correct nucleotide opposite a lesion, but also on its ability to elongate only DNA chains that have a correctly incorporated nucleotide opposite a lesion.     

Elevated mutation rates in the germline of Polkappa mutant male mice

Mutation rates at two expanded simple tandem repeat (ESTR) loci were studied in the germline of DNA polymerase kappa (Polkappa(-/-)) deficient mice. The spontaneous mutation rate in homozygous Polkappa(-/-) males was significantly higher than in isogenic wild-type mice (Polkappa(+/+)), but the ESTR mutation spectrum in Polkappa(-/-) animals did not differ from that in Polkappa(+/+) males. We suggest that compromised translesion synthesis in Polkappa(-/-) mice may result in replication fork pausing which, in turn, may affect ESTR mutation rate.     

Efficiency of extension of mismatched primer termini across from cisplatin and oxaliplatin adducts by human DNA polymerases beta and eta in vitro

DNA polymerases beta and eta are among the few eukaryotic polymerases known to efficiently bypass cisplatin and oxaliplatin adducts in vitro. Our laboratory has previously established that both polymerases misincorporated dTTP with high frequency across from cisplatin- and oxaliplatin-GG adducts. This decrease in polymerase fidelity on platinum-damaged DNA could lead to in vivo mutations, if this base substitution were efficiently elongated. In this study, we performed a steady-state kinetic analysis of the steps required for fixation of dTTP misinsertion during translesion synthesis past cisplatin- and oxaliplatin-GG adducts by pol beta and pol eta. The efficiency of translesion synthesis by pol eta past Pt-GG adducts was very similar to that observed for this polymerase when the template contains thymine-thymine dimers. This finding suggested that pol eta could play a role in translesion synthesis past platinum-GG adducts in vivo. On the other hand, translesion synthesis past platinum-GG adducts by pol beta was much less efficient. Translesion synthesis by pol eta is likely to be predominantly error-free, since the probability of correct insertion and extension by pol eta was 1000-2000-fold greater than the probability of incorrect insertion and extension. Our results also indicated that for pol eta the frequency of misincorporation is the same across from the 3'G and the 5'G of the platinum-GG adducts for both cisplatin and oxaliplatin adducts. On the other hand, pol beta is more likely to misinsert at the 3'G of the adducts and misinsertion occurs at higher frequency for oxaliplatin-GG than for cisplatin-GG adducts.     

DNA excision-repair processes in human cells can eliminate the cytotoxic and mutagenic consequences of ultraviolet irradiation

The ability of DNA excision-repair processes in diploid human fibroblasts to eliminate potentially cytotoxic and mutagenic lesions induced by UV radiation (254 nm) was demonstrated in two ways: (1) Cells with normal rates of excision were compared with cells with an intermediate rate of excision (XP2BE) and cells with an excision rate less than or equal to 1% that of normal (XP12BE) for sensitivity to the killing and mutagenic action of UV radiation. The normal cells proved resistant to doses of UV which reduced the survival of the XP cells to 14% and 0.7%, respectively, and increased the frequency of mutations to 8-azaguanine resistance in the XP cells 5- to 10-fold over background. (2) Cells in confluence were irradiated with cytotoxic and mutagenic doses of UV and allowed to carry out excision repair. After various lengths of time they were replated at lower densities to allow for expression of mutations to 6-thioguanine resistance and/or at cloning densities to assay survival. Normal cells and XP cells with reduced rates of excision repair (from complementation groups C and D) exhibited a gradual increase in survival from an initial level of 15--20% to 100% if held approximately 20 h in confluence. In contrast, XP12BE cells showed no increase from an initial survival of 20% even when held for 7 days. Normal cells irradiated in confluence but prevented from replicating for 7 days exhibited background mutation frequencies, whereas the mutation frequency in XP12BE cells did not change with the time in confluence.     

Mutagenic and recombinagenic responses to defective DNA polymerase delta are facilitated by the Rev1 protein in pol3-t mutants of Saccharomyces cerevisiae

Defective DNA replication can result in substantial increases in the level of genome instability. In the yeast Saccharomyces cerevisiae, the pol3-t allele confers a defect in the catalytic subunit of replicative DNA polymerase delta that results in increased rates of mutagenesis, recombination, and chromosome loss, perhaps by increasing the rate of replicative polymerase failure. The translesion polymerases Pol eta, Pol zeta, and Rev1 are part of a suite of factors in yeast that can act at sites of replicative polymerase failure. While mutants defective in the translesion polymerases alone displayed few defects, loss of Rev1 was found to suppress the increased rates of spontaneous mutation, recombination, and chromosome loss observed in pol3-t mutants. These results suggest that Rev1 may be involved in facilitating mutagenic and recombinagenic responses to the failure of Pol delta. Genome stability, therefore, may reflect a dynamic relationship between primary and auxiliary DNA polymerases.     

The Werner syndrome protein limits the error-prone 8-oxo-dG lesion bypass activity of human DNA polymerase kappa

Human DNA polymerase kappa (hpol κ) is the only Y-family member to preferentially insert dAMP opposite 7,8-dihydro-8-oxo-2′-deoxyguanosine (8-oxo-dG) during translesion DNA synthesis. We have studied the mechanism of action by which hpol κ activity is modulated by the Werner syndrome protein (WRN), a RecQ helicase known to influence repair of 8-oxo-dG. Here we show that WRN stimulates the 8-oxo-dG bypass activity of hpol κ in vitro by enhancing the correct base insertion opposite the lesion, as well as extension from dC:8-oxo-dG base pairs. Steady-state kinetic analysis reveals that WRN improves hpol κ-catalyzed dCMP insertion opposite 8-oxo-dG ∼10-fold and extension from dC:8-oxo-dG by 2.4-fold. Stimulation is primarily due to an increase in the rate constant for polymerization (k_pol), as assessed by pre-steady-state kinetics, and it requires the RecQ C-terminal (RQC) domain. In support of the functional data, recombinant WRN and hpol κ were found to physically interact through the exo and RQC domains of WRN, and co-localization of WRN and hpol κ was observed in human cells treated with hydrogen peroxide. Thus, WRN limits the error-prone bypass of 8-oxo-dG by hpol κ, which could influence the sensitivity to oxidative damage that has previously been observed for Werner''s syndrome cells.     

Development of a liquid-holding technique for the study of DNA-repair in human diploid fibroblasts.

Liquid-holding conditions can be obtained for human diploid skin fibroblasts by keeping confluent cultures stationary over periods of 7 days or longer by means of conditioned medium. Under this condition recovery of radiation damage induced by ultraviolet light or X-rays is observed as an increase in cloning efficiency. The amount of recovery when expressed in a dose-modifying-factor appears higher than in bacteria and yeast. The repair-deficient human cell strains XP25Ro and XP7Be (xeroderma pigmentosum from complementation groups A and D respectively) exhibit less but still discernible recovery after UV-irradiation and the same was observed for AT5Bi (ataxia telangiectasia) after X-irradiation. Experiments on mutation induction indicated that the repair which takes place during liquid holding of UV-irradiated XP7Be cells reduces the mutant frequency considerably while after liquid holding of UV-irradiated wild-type cells the same or lower mutant frequencies were found for the lower exposures and the same or higher mutant frequencies for the higher exposures.