DNA damage-inducible and RAD52-independent repair of DNA double-strand breaks in Saccharomyces cerevisiae

Bioavailability of Nutritional Resources From Cells Killed by Oxidation Supports Expansion of Survivors in Ustilago maydis Populations

After heavy exposure of Ustilago maydis cells to clastogens, a great increase in viability was observed if the treated cells were kept under starvation conditions. This restitution of viability is based on cell multiplication at the expense of the intracellular compounds freed from the damaged cells. Analysis of the effect of the leaked material on the growth of undamaged cells revealed opposing biological activity, indicating that U. maydis must possess cellular mechanisms involved not only in reabsorption of the released compounds from external environment but also in contending with their treatment-induced toxicity. From a screen for mutants defective in the restitution of viability, we identified four genes (adr1, did4, kel1, and tbp1) that contribute to the process. The mutants in did4, kel1, and tbp1 exhibited sensitivity to different genotoxic agents implying that the gene products are in some overlapping fashion involved in the protection of genome integrity. The genetic determinants identified by our analysis have already been known to play roles in growth regulation, protein turnover, cytoskeleton structure, and transcription. We discuss ecological and evolutionary implications of these results.

Loss of a 20S proteasome activator in Saccharomyces cerevisiae downregulates genes important for genomic integrity, increases DNA damage, and selectively sensitizes cells to agents with diverse mechanisms of action

Cytoprotective functions of a 20S proteasome activator were investigated. Saccharomyces cerevisiae Blm10 and human 20S proteasome activator 200 (PA200) are homologs. Comparative genome-wide analyses of untreated diploid cells lacking Blm10 and growing at steady state at defined growth rates revealed downregulation of numerous genes required for accurate chromosome structure, assembly and repair, and upregulation of a specific subset of genes encoding protein-folding chaperones. Blm10 loss or truncation of the Ubp3/Blm3 deubiquitinating enzyme caused massive chromosomal damage and cell death in homozygous diploids after phleomycin treatments, indicating that Blm10 and Ubp3/Blm3 function to stabilize the genome and protect against cell death. Diploids lacking Blm10 also were sensitized to doxorubicin, hydroxyurea, 5-fluorouracil, rapamycin, hydrogen peroxide, methyl methanesulfonate, and calcofluor. Fluorescently tagged Blm10 localized in nuclei, with enhanced fluorescence after DNA replication. After DNA damage that caused a classic G2/M arrest, fluorescence remained diffuse, with evidence of nuclear fragmentation in some cells. Protective functions of Blm10 did not require the carboxyl-terminal region that makes close contact with 20S proteasomes, indicating that protection does not require this contact or the truncated Blm10 can interact with the proteasome apart from this region. Without its carboxyl-terminus, Blm10((-339aa)) localized to nuclei in untreated, nonproliferating (G(0)) cells, but not during G(1) S, G(2), and M. The results indicate Blm10 functions in protective mechanisms that include the machinery that assures proper assembly of chromosomes. These essential guardian functions have implications for ubiquitin-independent targeting in anticancer therapy. Targeting Blm10/PA200 together with one or more of the upregulated chaperones or a conventional treatment could be efficacious.

Live cell microscopy of DNA damage response in Saccharomyces cerevisiae

Fluorescence microscopy of the DNA damage response in living cells stands out from many other DNA repair assays by its ability to monitor the response to individual DNA lesions in single cells. This is particularly true in yeast, where the frequency of spontaneous DNA lesions is relatively low compared to organisms with much larger genomes such as mammalian cells. Single cell analysis of individual DNA lesions allows specific events in the DNA damage response to be correlated with cell morphology, cell cycle phase, and other specific characteristics of a particular cell. Moreover, fluorescence live cell imaging allows for multiple cellular markers to be monitored over several hours. This chapter reviews useful fluorescent markers and genotoxic agents for studying the DNA damage response in living cells and provides protocols for live cell imaging, time-lapse microscopy, and for induction of site-specific DNA lesions.

Extract of Lillium candidum L. can modulate the genotoxicity of the antibiotic zeocin

Lilium candidum L. extract (LE) is well known in folk medicine for the treatment of burns, ulcers, inflammations and for healing wounds. This work aims to clarify whether the genotoxic potential of the radiomimetic antibiotic zeocin (Zeo) could be modulated by LE. Our results indicate that LE exerts no cytotoxic, DNA-damaging and clastogenic activity in in Chlamydomonas reinhardtii, Pisum sativum L. and Hordeum vulgare L. test systems over a broad concentration range. Weak but statistically significant clastogenic effects due to the induction of micronuclei and chromosome aberrations have been observed in H. vulgare L. after treatment with 200 and 300 μg/mL LE. To discriminate protective from adverse action of LE different experimental designs have been used. Our results demonstrate that the treatment with mixtures of LE and Zeo causes an increase in the level of DNA damage, micronuclei and "metaphases with chromatid aberrations" (MwA). Clear evidence has been also obtained indicating that pretreatment with LE given 4 h before the treatment with Zeo accelerates the rejoining kinetics of Zeo-induced DNA damage in P. sativum L. and C. reinhardtii, and can decrease clastogenic effect of Zeo measured as frequencies of micronuclei and MwA in H. vulgare L. Here, we show for the first time that LE can modulate the genotoxic effects of zeocin. The molecular mode of action strongly depends on the experimental design and varies from synergistic to protective effect (adaptive response-AR). Our results also revealed that LE-induced AR to zeocin involves up-regulation of DSB rejoining in C. reinhardtii and P. sativum L. cells.

Cell biology of homologous recombination in yeast

Homologous recombination is an important pathway for error-free repair of DNA lesions, such as single- and double-strand breaks, and for rescue of collapsed replication forks. Here, we describe protocols for live cell imaging of single-lesion recombination events in the yeast Saccharomyces cerevisiae using fluorescence microscopy.

Does single-dose cell resistance to the radio-mimetic zeocin correlate with a zeocin-induced adaptive response in Chlamydomonas reinhardtii strains?

This study aimed to test whether a correlation exists between single-dose resistance to zeocin and the ability to develop a zeocin-induced adaptive response (AR) in Chlamydomonas reinhardtii strains. Three genotypes were used: wild type (WT) strain 137C and two strains (H-3 and AK-9-9), which are highly resistant to radiation based on survival studies. Based on a micro-colony assay, the strains could be arranged according to their single-dose resistance to zeocin as follows: AK-9-9 > H-3 > 137C. However, zeocin induced a similar level of DSB in strains AK-9-9, H-3 and 137C. The radio- and zeocin-resistant strains AK-9-9 and H-3 showed higher DSB rejoining capacity than the WT strain 137C, suggesting that DSB rejoining can at least partly account for different cell survival. Both WT and radio-resistant strains develop zeocin-induced AR involving increased DSB rejoining. The radio- and zeocin-resistant strains AK-9-9 and H-3 again showed higher DSB rejoining capacity than the WT strain 137C. The higher resistance of strains H-3 and AK-9-9 did not abrogate their ability to adapt, albeit with a smaller magnitude as compared to the WT strain. The obtained results characterize new radio-resistant C. reinhardtii strains, which enrich the collection of resistant C. reinhardtii strains.

JAK2 stimulates homologous recombination and genetic instability: potential implication in the heterogeneity of myeloproliferative disorders

The JAK2V617F mutation is frequently observed in classical myeloproliferative disorders, and disease progression is associated with a biallelic acquisition of the mutation occurring by mitotic recombination. In this study, we examined whether JAK2 activation could lead to increased homologous recombination (HR) and genetic instability. In a Ba/F3 cell line expressing the erythropoietin (EPO) receptor, mutant JAK2V617F and, to a lesser extent, wild-type (wt) JAK2 induced an increase in HR activity in the presence of EPO without modifying nonhomologous end-joining efficiency. Moreover, a marked augmentation in HR activity was found in CD34+-derived cells isolated from patients with polycythemia vera or primitive myelofibrosis compared with control samples. This increase was associated with a spontaneous RAD51 foci formation. As a result, sister chromatid exchange was 50% augmented in JAK2V617F Ba/F3 cells compared with JAK2wt cells. Moreover, JAK2 activation increased centrosome and ploidy abnormalities. Finally, in JAK2V617F Ba/F3 cells, we found a 100-fold and 10-fold increase in mutagenesis at the HPRT and Na/K ATPase loci, respectively. Together, this work highlights a new molecular mechanism for HR regulation mediated by JAK2 and more efficiently by JAK2V617F. Our study might provide some keys to understand how a single mutation can give rise to different pathologies.

Xrs2 facilitates crossovers during DNA double-strand gap repair in yeast

Xrs2 is a member of the MRX complex (Mre11/Rad50/Xrs2) in Saccharomyces cerevisiae. In this study we demonstrate the important role of the MRX complex and in more detail of Xrs2 for the repair of radiation-induced chromosomal double-strand breaks by pulsed field gel electrophoresis. By using a newly designed in vivo plasmid-chromosome recombination system, we could show that gap repair efficiency and the association with crossovers were reduced in the MRX null mutants, but repair accuracy was unaffected. For these processes, an intact Mre11-binding domain of Xrs2 is crucial, whereas the FHA- and BRCT-domains as well as the Tel1-binding domain of Xrs2 are dispensable. Obviously, the Mre11-binding domain of the Xrs2 protein is crucial for the analysed functions and our results suggest a new role of the MRX complex for the formation of crossovers. Analysis of double mutants showed that the phenotype of the Deltaxrs2 null mutant concerning the crossover frequency is dominant over the phenotypes of Deltasrs2 and Deltasgs1 null mutants. Thus, the complex seems to be involved in early steps of double-strand break and gap repair, and we propose that it has a regulatory role for the selection of homologous recombination pathways.

Assessment of DNA strand breaks induced by bleomycin in barley by the comet assay

Comet assay was applied to study induction and repair of DNA damage produced by bleomycin in barley genome. Experimental conditions were adapted to achieve efficient detection of both DNA single- and double-strand breaks. Substantial increase of the parameter "% of DNA in tail" was observed coupled with almost linear dependence from bleomycin concentration, more pronounced for the induction of DNA double-strand breaks. Data obtained at different recovery periods displayed rapid restoration of breakage, revealing that efficient mechanisms for repair of strand discontinuities induced by bleomycin are functional in barley DNA loop domains.

Induction of DNA double-strand breaks by zeocin in Chlamydomonas reinhardtii and the role of increased DNA double-strand breaks rejoining in the formation of an adaptive response

This study aimed to test the potential of the radiomimetic chemical zeocin to induce DNA double-strand breaks (DSB) and "adaptive response" (AR) in Chlamydomonas reinhardtii strain CW15 as a model system. The AR was measured as cell survival using a micro-colony assay, and by changes in rejoining of DSB DNA. The level of induced DSB was measured by constant field gel electrophoresis based on incorporation of cells into agarose blocks before cell lysis. This avoids the risk of accidental induction of DSB during the manipulation procedures. Our results showed that zeocin could induce DSB in C. reinhardtii strain CW15 in a linear dose-response fashion up to 100 microg ml(-1) which marked the beginning of a plateau. The level of DSB induced by 100 microg ml(-1) zeocin was similar to that induced by 250 Gy of gamma-ray irradiation. It was also found that, similar to gamma rays, zeocin could induce AR measured as DSB in C. reinhardtii CW15 and this AR involved acceleration of the rate of DSB rejoining, too. To our knowledge, this is the first demonstration that zeocin could induce AR in some low eukaryotes such as C. reinhardtii.

Roles of Saccharomyces cerevisiae RAD17 and CHK1 checkpoint genes in the repair of double-strand breaks in cycling cells

Checkpoints are components of signalling pathways involved in genome stability. We analysed the putative dual functions of Rad17 and Chk1 as checkpoints and in DNA repair using mutant strains of Saccharomyces cerevisiae. Logarithmic populations of the diploid checkpoint-deficient mutants, chk1Delta/chk1Delta and rad17Delta/rad17Delta, and an isogenic wild-type strain were exposed to the radiomimetic agent bleomycin (BLM). DNA double-strand breaks (DSBs) determined by pulsed-field electrophoresis, surviving fractions, and proliferation kinetics were measured immediately after treatments or after incubation in nutrient medium in the presence or absence of cycloheximide (CHX). The DSBs induced by BLM were reduced in the wild-type strain as a function of incubation time after treatment, with chromosomal repair inhibited by CHX. rad17Delta/rad17Delta cells exposed to low BLM concentrations showed no DSB repair, low survival, and CHX had no effect. Conversely, rad17Delta/rad17Delta cells exposed to high BLM concentrations showed DSB repair inhibited by CHX. chk1Delta/chk1Delta cells showed DSB repair, and CHX had no effect; these cells displayed the lowest survival following high BLM concentrations. Present results indicate that Rad17 is essential for inducible DSB repair after low BLM-concentrations (low levels of oxidative damage). The observations in the chk1Delta/chk1Delta mutant strain suggest that constitutive nonhomologous end-joining is involved in the repair of BLM-induced DSBs. The differential expression of DNA repair and survival in checkpoint mutants as compared to wild-type cells suggests the presence of a regulatory switch-network that controls and channels DSB repair to alternative pathways, depending on the magnitude of the DNA damage and genetic background.

Homologous recombination and the yKu70/80 complex exert opposite roles in resistance against the killer toxin from Pichia acaciae

The linear plasmid (pPac1-2) encoded killer toxin (PaT) of the yeast Pichia acaciae arrests sensitive Saccharomyces cerevisiae cells in the S-phase of the cell cycle and induces mutations. Here we provide evidence for opposite effects in PaT resistance of homologous recombination (HR) and non-homologous end joining (NHEJ), the two alternative repair mechanisms acting on DNA double strand breaks (DSB). As mutants defective in genes of the RAD52 epistasis group react hypersensitive and cells lacking YKU70 or YKU80 are partially resistant, the yKu70/80 complex facilitates PaT toxicity, whereas HR is antagonistic. In contrast to yku70 and yku80, lif1 mutants, the latter being defective in the ligation step of NHEJ, are PaT sensitive, confining toxicity promoting effects of NHEJ to the DSB end binding Ku proteins. Since rad52 yku80 double mutants display strong hypersensitivity, yku80 mediated resistance depends on HR. Opposite effects of the yKu70/80 complex and HR are consistent with the occurrence of replication dependent (one sided) DSBs in PaT treated cells. Concordantly, two cellular markers signaling DSBs are induced during PaT mediated S-phase arrest, i.e. histone H2A phosphorylation and formation of subnuclear repair foci by GFP tagged recombination protein Rad52. As only moderate chromosome fragmentation could be detected by PFGE, transient occurrence and efficient in vivo repair of PaT induced DSBs is assumed. Consistent with replication dependent DSB formation induced by PaT, we demonstrate a protective function of the RecQ helicase Sgs1 and the structure specific endonuclease Mus81, both of which are considered to be involved in processing and restart of stalled replication forks.

The Smc5-Smc6 complex and SUMO modification of Rad52 regulates recombinational repair at the ribosomal gene locus

Homologous recombination (HR) is crucial for maintaining genome integrity by repairing DNA double-strand breaks (DSBs) and rescuing collapsed replication forks. In contrast, uncontrolled HR can lead to chromosome translocations, loss of heterozygosity, and deletion of repetitive sequences. Controlled HR is particularly important for the preservation of repetitive sequences of the ribosomal gene (rDNA) cluster. Here we show that recombinational repair of a DSB in rDNA in Saccharomyces cerevisiae involves the transient relocalization of the lesion to associate with the recombination machinery at an extranucleolar site. The nucleolar exclusion of Rad52 recombination foci entails Mre11 and Smc5-Smc6 complexes and depends on Rad52 SUMO (small ubiquitin-related modifier) modification. Remarkably, mutations that abrogate these activities result in the formation of Rad52 foci within the nucleolus and cause rDNA hyperrecombination and the excision of extrachromosomal rDNA circles. Our study also suggests a key role of sumoylation for nucleolar dynamics, perhaps in the compartmentalization of nuclear activities.

Relative contribution of homologous recombination and non-homologous end-joining to DNA double-strand break repair after oxidative stress in Saccharomyces cerevisiae

Oxidative damage to DNA seems to be an important factor in developing many human diseases including cancer. It involves base and sugar damage, base-free sites, DNA-protein cross-links and DNA single-strand (SSB) and double-strand (DSB) breaks. Oxidative DSB can be formed in various ways such as their direct induction by the drug or their generation either through attempted and aborted repair of primary DNA lesions or through DNA replication-dependent conversion of SSB. In general, two main pathways are responsible for repairing DSB, homologous recombination (HR) and non-homologous end-joining (NHEJ), with both of them being potential candidates for the repair of oxidative DSB. We have examined relative contribution of HR and NHEJ to cellular response after oxidative stress in Saccharomyces cerevisiae. Therefore, cell survival, mutagenesis and DSB induction and repair in the rad52, yku70 and rad52 yku70 mutants after hydrogen peroxide (H(2)O(2)), menadione (MD) or bleomycin (BLM) exposure were compared to those obtained for the corresponding wild type. We show that MD exposure does not lead to observable DSB induction in yeast, suggesting that the toxic effects of this agent are mediated by other types of DNA damage. Although H(2)O(2) treatment generates some DSB, their yield is relatively low and hence DSB may only partially be responsible for toxicity of H(2)O(2), particularly at high doses of the agent. On the other hand, the basis of the BLM toxicity resides primarily in DSB induction. Both HR and NHEJ act on BLM-induced DSB, although their relative participation in the process is not equal. Based on our results we suggest that the complexity and/or the quality of the BLM-induced DSB might represent an obstacle for the NHEJ pathway.

DNA strand breaks signal the induction of DNA double-strand break repair in Saccharomyces cerevisiae

Genotoxic stress induces a checkpoint signaling cascade to generate a stress response. Saccharomyces cerevisiae shows an altered radiation response under different type of stress. Although the induction of repair has been implicated in enhanced survival after exposure to the challenging stress, the nature of the signal remains poorly understood. This study demonstrates that low doses of gamma radiation and bleomycin induce RAD52-dependent recombination repair pathway in the wild-type strain D-261. Prior exposure of cells to DNA-damaging agents (gamma radiation or bleomycin) equips them better for the subsequent damage caused by challenging doses. However, exposure to UV light, which does not cause strand breaks, was ineffective. This was confirmed by PFGE studies. This indicates that the strand breaks probably serve as the signal for induction of the recombination repair pathway while pyrimidine dimers do not. The nature of the induced repair was investigated by mutation scoring in special strain D-7, which showed that the induced repair is essentially error free.

RAD6 gene is involved in heat shock induction of bleomycin resistance in Saccharomyces cerevisiae

Cells react to environmental and endogenous challenges such as high temperature, reactive oxygen species, DNA damage, and nutrient starvation by activating several defense mechanisms known as stress responses. An important feature is the overlap between different stress responses that contributes at least in part to the phenomenon of cross-protection. We previously demonstrated that pretreatment with a heat shock (HS) induces resistance to the lethal and mutagenic effects of the antineoplastic drug Bleomycin (BLM) in wild-type Saccharomyces cerevisiae. At the DNA level, the HS resulted in more efficient repair of BLM-induced DNA damage. In the present study, we have investigated the mechanisms involved in this HS-induced BLM resistance. Since the RAD6 gene is involved in the ubiquitin system and DNA repair, we analyzed the effects of HS on the lethality of BLM in a rad6Delta (ubc2) mutant strain of S. cerevisiae. The rad6Delta mutant was more sensitive to the lethal effects of BLM than wild-type yeast and HS had no effect on the lethality of BLM in the mutant. Analysis of cell proliferation kinetics indicated that the HS-induced cell cycle delay observed in the wild-type yeast was absent in the rad6Delta mutant strain. BLM treatment impaired mutant cell proliferation, and HS had no effect on the delayed cell kinetics of the mutant. In addition, pulsed-field electrophoresis of chromosomes damaged by BLM indicated that there was very little recovery from damage in the mutant after 24 hr of incubation in BLM-free nutrient medium, and that HS had little effect on the recovery. These data indicate that the RAD6 gene is involved in the HS-induced BLM resistance observed in the isogenic wild-type strain.

Cell cycle progression in G1 and S phases is CCR4 dependent following ionizing radiation or replication stress in Saccharomyces cerevisiae

To identify new nonessential genes that affect genome integrity, we completed a screening for diploid mutant Saccharomyces cerevisiae strains that are sensitive to ionizing radiation (IR) and found 62 new genes that confer resistance. Along with those previously reported (Bennett et al., Nat. Genet. 29:426-434, 2001), these genes bring to 169 the total number of new IR resistance genes identified. Through the use of existing genetic and proteomic databases, many of these genes were found to interact in a damage response network with the transcription factor Ccr4, a core component of the CCR4-NOT and RNA polymerase-associated factor 1 (PAF1)-CDC73 transcription complexes. Deletions of individual members of these two complexes render cells sensitive to the lethal effects of IR as diploids, but not as haploids, indicating that the diploid G1 cell population is radiosensitive. Consistent with a role in G1, diploid ccr4Delta cells irradiated in G1 show enhanced lethality compared to cells exposed as a synchronous G2 population. In addition, a prolonged RAD9-dependent G1 arrest occurred following IR of ccr4Delta cells and CCR4 is a member of the RAD9 epistasis group, thus confirming a role for CCR4 in checkpoint control. Moreover, ccr4Delta cells that transit S phase in the presence of the replication inhibitor hydroxyurea (HU) undergo prolonged cell cycle arrest at G2 followed by cellular lysis. This S-phase replication defect is separate from that seen for rad52 mutants, since rad52Delta ccr4Delta cells show increased sensitivity to HU compared to rad52Delta or ccr4Delta mutants alone. These results indicate that cell cycle transition through G1 and S phases is CCR4 dependent following radiation or replication stress.

Fungal cell wall septation and cytokinesis are inhibited by bleomycins

When the essential and distinctive cell walls of either pathogenic or nonpathogenic fungi break, cytoplasmic membranes rupture and fungi die. This fungicidal activity was discovered previously on nonproliferating Saccharomyces cerevisiae cells treated briefly with the oxidative tool and anticancer drug family of bleomycins. The present studies investigated effects of bleomycin on growing fungal organisms. These included the medically important Aspergillus fumigatus and Cryptococcus neoformans, as well as the emerging human pathogen and fungal model, S. cerevisiae. Bleomycin had its highest potency against A. fumigatus. Scanning electron microscopy and thin-section transmission electron microscopy were used to study morphological growth characteristics. Killing and growth inhibition were also measured. Long, thin, and segmented hyphae were observed when A. fumigatus was grown without bleomycin but were never observed when the mold was grown with the drug. Bleomycin arrested conidial germination, hyphal development, and the progression and completion of cell wall septation. Similarly, the drug inhibited the construction of yeast cell wall septa, preventing cytokinesis and progression in the cell division cycle of S. cerevisiae. Even when cytoplasms of mother and daughter cells separated, septation and cell division did not necessarily occur. Bizarre cell configurations, abnormally thickened cell walls at mother-daughter necks, abnormal polarized growth, large undivided cells, fragmented cells, and empty cell ghosts were also produced. This is the first report of a fungicidal agent that arrests fungal growth and development, septum formation, and cytokinesis and that also preferentially localizes to cell walls and alters isolated cell walls as well as intact cell walls on nongrowing cells.

Multiple roles for Saccharomyces cerevisiae histone H2A in telomere position effect, Spt phenotypes and double-strand-break repair

Telomere position effects on transcription (TPE, or telomeric silencing) are nucleated by association of nonhistone silencing factors with the telomere and propagated in subtelomeric regions through association of silencing factors with the specifically modified histones H3 and H4. However, the function of histone H2A in TPE is unknown. We found that deletion of either the amino or the carboxyltails of H2A substantially reduces TPE. We identified four H2A modification sites necessary for wild-type efficiency of TPE. These "hta1tpe" alleles also act as suppressors of a delta insertion allele of LYS2, suggesting shared elements of chromatin structure at both loci. Interestingly, we observed combinatorial effects of allele pairs, suggesting both interdependent acetylation and deacetylation events in the amino-terminal tail and a regulatory circuit between multiple phosphorylated residues in the carboxyl-terminal tail. Decreases in silencing and viability are observed in most hta1tpe alleles after treatment with low and high concentrations, respectively, of bleomycin, which forms double-strand breaks (DSBs). In the absence of the DSB and telomere-binding protein yKu70, the bleomycin sensitivity of hta1tpe alleles is further enhanced. We also provide data suggesting the presence of a yKu-dependent histone H2A function in TPE. These data indicate that the amino- and carboxyl-terminal tails of H2A are essential for wild-type levels of yKu-mediated TPE and DSB repair.

Impaired mitochondrial function protects against free radical-mediated cell death

Free radical damage can have fatal consequences. Mitochondria carry out essential cellular functions and produce high levels of reactive oxygen species (ROS). Many agents also generate ROS. Using the yeast Saccharomyces cerevisiae as a eukaryotic model, the role of functional mitochondria in surviving free radical damage was investigated. Respiratory-deficient cells lacking mitochondrial DNA (rho(0)) were up to 100-fold more resistant than isogenic rho(+) cells to killing by ROS generated by the bleomycin-phleomycin family of oxidative agents. Up to approximately 90% of the survivors of high oxidative stress lost mitochondrial function and became "petites." The selective advantage of respiratory deficiency was studied in several strains, including DNA repair-deficient rad52/rad52 and blm5/blm5 diploid strains. These mutant strains are hypersensitive to lethal effects of free radicals and accumulate more DNA damage than related wild-type strains. Losses in mitochondrial function were dose-dependent, and mutational alteration of the RAD52 or BLM5 gene did not affect the resistance of surviving cells lacking mitochondrial function. The results indicate that inactivation of mitochondrial function protects cells against lethal effects of oxygen free radicals.

Phenotypic analysis and virulence of Candida albicans LIG4 mutants

In previous studies, we reported the isolation and preliminary characterization of a DNA ligase-encoding gene of Candida albicans. This gene (LIG4) is the structural and functional homologue of both yeast and human ligase IV, which is involved in nonhomologous end joining (NHEJ) of DNA double-strand breaks. In the present study, we have shown that there are no other LIG4 homologues in C. albicans. In order to study the function of LIG4 in morphogenesis and virulence, we constructed gene deletions. LIG4 transcript levels were reduced in the heterozygote and were completely absent in null strains. Concomitantly, the heterozygote showed a pronounced defect in myceliation, which was slightly greater in the null strain. This was true with several solid and liquid media, such as Spider medium, medium 199, and 2% glucose-1% yeast extract-2% Bacto Peptone, at several pHs. Reintroduction of the wild-type allele into the null mutant partially restored the ability of cells to form hyphae. In agreement with the positive role of LIG4 in morphogenesis, we detected a significant rise in mRNA levels during the morphological transition. LIG4 is not essential for DNA replication or for the repair of DNA damage induced by ionizing radiation or UV light, indicating that these lesions are repaired primarily by homologous recombination. However, our data show that the NHEJ apparatus of C. albicans may control morphogenesis in this diploid organism. In addition, deletion of one or both copies of LIG4 resulted in attenuation of virulence in a murine model of candidiasis.