C. elegans RAD-5/CLK-2 defines a new DNA damage checkpoint protein

Background: In response to genotoxic stress, cells activate checkpoint pathways that lead to a transient cell cycle arrest that allows for DNA repair or to apoptosis, which triggers the demise of genetically damaged cells.Results: During positional cloning of the C. elegans rad-5 DNA damage checkpoint gene, we found, surprisingly, that rad-5(mn159) is allelic with clk-2(qm37), a mutant previously implicated in regulation of biological rhythms and life span. However, clk-2(qm37) is the only C. elegans clock mutant that is defective for the DNA damage checkpoint. We show that rad-5/clk-2 acts in a pathway that partially overlaps with the conserved C. elegans mrt-2/S. cerevisiae RAD17/S. pombe rad1(+) checkpoint pathway. In addition, rad-5/clk-2 also regulates the S phase replication checkpoint in C. elegans. Positional cloning reveals that the RAD-5/CLK-2 DNA damage checkpoint protein is homologous to S. cerevisiae Tel2p, an essential DNA binding protein that regulates telomere length in yeast. However, the partial loss-of-function C. elegans rad-5(mn159) and clk-2(qm37) checkpoint mutations have little effect on telomere length, and analysis of the partial loss-of-function of S. cerevisiae tel2-1 mutant failed to reveal typical DNA damage checkpoint defects.Conclusions: Using C. elegans genetics we define the novel DNA damage checkpoint protein RAD-5/CLK-2, which may play a role in oncogenesis. Given that Tel2p has been shown to bind to a variety of nucleic acid structures in vitro, we speculate that the RAD-5/CLK-2 checkpoint protein may act at sites of DNA damage, either as a sensor of DNA damage or to aid in the repair of damaged DNA.     

The C. elegans maternal-effect gene clk-2 is essential for embryonic development, encodes a protein homologous to yeast Tel2p and affects telomere length

The Caenorhabditis elegans maternal-effect clk genes are involved in the temporal control of development and behavior. We report the genetic and molecular characterization of clk-2. A temperature-sensitive mutation in the gene clk-2 affects embryonic and post-embryonic development, reproduction, and rhythmic behaviors. Yet, virtually all phenotypes are fully maternally rescued. Embryonic development strictly requires the activity of maternal clk-2 during a narrow time window between oocyte maturation and the two- to four-cell embryonic stage. Positional cloning of clk-2 reveals that it encodes a protein homologous to S. cerevisiae Tel2p. In yeast, the gene TEL2 regulates telomere length and participates in gene silencing at subtelomeric regions. In C. elegans, clk-2 mutants have elongated telomeres, and clk-2 overexpression can lead to telomere shortening. Tel2p has been reported to bind to telomeric DNA repeats in vitro. However, we find that a functional CLK-2::GFP fusion protein is cytoplasmic in worms. We discuss how the phenotype of clk-2 mutants could be the result of altered patterns of gene expression.     

Functional Dissection of Caenorhabditis elegans CLK-2/TEL2 Cell Cycle Defects during Embryogenesis and Germline Development

CLK-2/TEL2 is essential for viability from yeasts to vertebrates, but its essential functions remain ill defined. CLK-2/TEL2 was initially implicated in telomere length regulation in budding yeast, but work in Caenorhabditis elegans has uncovered a function in DNA damage response signalling. Subsequently, DNA damage signalling defects associated with CLK-2/TEL2 have been confirmed in yeast and human cells. The CLK-2/TEL2 interaction with the ATM and ATR DNA damage sensor kinases and its requirement for their stability led to the proposal that CLK-2/TEL2 mutants might phenocopy ATM and/or ATR depletion. We use C. elegans to dissect developmental and cell cycle related roles of CLK-2. Temperature sensitive (ts) clk-2 mutants accumulate genomic instability and show a delay of embryonic cell cycle timing. This delay partially depends on the worm p53 homolog CEP-1 and is rescued by co-depletion of the DNA replication checkpoint proteins ATL-1 (C. elegans ATR) and CHK-1. In addition, clk-2 ts mutants show a spindle orientation defect in the eight cell stages that lead to major cell fate transitions. clk-2 deletion worms progress through embryogenesis and larval development by maternal rescue but become sterile and halt germ cell cycle progression. Unlike ATL-1 depleted germ cells, clk-2–null germ cells do not accumulate DNA double-strand breaks. Rather, clk-2 mutant germ cells arrest with duplicated centrosomes but without mitotic spindles in an early prophase like stage. This germ cell cycle arrest does not depend on cep-1, the DNA replication, or the spindle checkpoint. Our analysis shows that CLK-2 depletion does not phenocopy PIKK kinase depletion. Rather, we implicate CLK-2 in multiple developmental and cell cycle related processes and show that CLK-2 and ATR have antagonising functions during early C. elegans embryonic development.     

Distinct modes of ATR activation after replication stress and DNA double-strand breaks in Caenorhabditis elegans

ATM and ATR are key components of the DNA damage checkpoint. ATR primarily responds to UV damage and replication stress, yet may also function with ATM in the checkpoint response to DNA double-strand breaks (DSBs), although this is less clear. Here, we show that atl-1 (Caenorhabditis elegans ATR) and rad-5/clk-2 prevent mitotic catastrophe, function in the S-phase checkpoint and also cooperate with atm-1 in the checkpoint response to DSBs after ionizing radiation (IR) to induce cell cycle arrest or apoptosis via the cep-1(p53)/egl-1 pathway. ATL-1 is recruited to stalled replication forks by RPA-1 and functions upstream of rad-5/clk-2 in the S-phase checkpoint. In contrast, mre-11 and atm-1 are dispensable for ATL-1 recruitment to stalled replication forks. However, mre-11 is required for RPA-1 association and ATL-1 recruitment to DSBs. Thus, DNA processing controlled by mre-11 is important for ATL-1 activation at DSBs but not following replication fork stalling. We propose that atl-1 and rad-5/clk-2 respond to single-stranded DNA generated by replication stress and function with atm-1 following DSB resection.     

Human CLK2 links cell cycle progression,apoptosis, and telomere length regulation

Mutations in the clk-2 gene of the nematode Caenorhabditis elegans affect organismal features such as development, behavior, reproduction, and aging as well as cellular features such as the cell cycle, apoptosis, the DNA replication checkpoint, and telomere length. clk-2 encodes a novel protein (CLK-2) with a unique homologue in each of the sequenced eukaryotic genomes. We have studied the human homologue of CLK-2 (hCLK2) to determine whether it affects the same set of cellular features as CLK-2. We find that overexpression of hCLK2 decreases cell cycle length and that inhibition of hCLK2 expression arrests the cell cycle reversibly. Overexpression of hCLK2, however, renders the cell hypersensitive to apoptosis triggered by oxidative stress or DNA replication block and gradually increases telomere length. The evolutionary conservation of the pattern of cellular functions affected by CLK-2 suggests that the function of hCLK2 in humans might also affect the same organismal features as in worms, including life span. Surprisingly, we find that hCLK2 is present in all cellular compartments and exists as a membrane-associated as well as a soluble form.     

Abrogation of the CLK-2 checkpoint leads to tolerance to base-excision repair intermediates

Incorporation of uracil during DNA synthesis is among the most common types of endogenously generated DNA damage. Depletion of Caenorhabditis elegans dUTPase by RNA interference allowed us to study the role of DNA damage response (DDR) pathways when responding to high levels of uracil in DNA. dUTPase depletion compromised development, caused embryonic lethality and led to activation of cell-cycle arrest and apoptosis. These phenotypes manifested as a result of processing misincorporated uracil by the uracil-DNA glycosylase UNG-1. Strikingly, abrogation of the clk-2 checkpoint gene rescued lethality and developmental defects, and eliminated cell-cycle arrest and apoptosis after dUTPase depletion. These data show a genetic interaction between UNG-1 and activation of the CLK-2 DDR pathway after uracil incorporation into DNA. Our results indicate that persistent repair intermediates and/or single-stranded DNA formed during repair of misincorporated uracil are tolerated in the absence of the CLK-2 checkpoint in C. elegans.     

Roles for Caenorhabditis elegans rad-51 in meiosis and in resistance to ionizing radiation during development

We have investigated the role of Caenorhabditis elegans RAD-51 during meiotic prophase and embryogenesis, making use of the silencing effect of RNA interference (RNAi). rad-51 RNAi leads to severe defects in chromosome morphology in diakinesis oocytes. We have explored the effect of rad-51 RNAi in mutants lacking fundamental components of the recombination machinery. If double-strand breaks are prevented by spo-11 mutation, rad-51 RNAi does not affect chromosome appearance. This is consistent with a role for RAD-51 downstream of the initiation of recombination. In the absence of MRE-11, as in the absence of SPO-11, RAD-51 depletion has no effect on the chromosomes, which appear intact, thus indicating a role for MRE-11 in DSB induction. Intriguingly, rad-51 silencing in oocytes that lack MSH-5 leads to chromosome fragmentation, a novel trait that is distinct from that seen in msh-5 mutants and in rad-51 RNAi oocytes, suggesting new potential roles for the msh-5 gene. Silencing of the rad-51 gene also causes a reduction in fecundity, which is suppressed by mutation in the DNA damage checkpoint gene rad-5, but not in the cell death effector gene ced-3. Finally, RAD-51 depletion is also seen to affect the soma, resulting in hypersensitivity to ionizing radiation in late embryogenesis.     

Orthologues of the Caenorhabditis elegans Longevity Gene clk-1 in Mouse and Human

The clk-1 gene was isolated from the long-lived mutant of Caenorhabditis elegans and was suggested to play a biological role in longevity (Ewbank et al., 1997, Science 275: 980-983). The primary structure of CLK-1 showed a significant homology to Saccharomyces cerevisiae Coq7p/Cat5p, which is required for the biosynthesis of ubiquinone and the derepression of gluconeogenic genes. In the present study, we isolated and characterized human and mouse orthologues of the COQ7/CLK-1 gene. Sequence analysis of both the human and the mouse COQ7 cDNAs showed an open reading frame composed of 217 amino acids with calculated molecular mass of 24,309 and 24,044 Da, respectively. Homology search revealed that human COQ7 showed 85% identity to mouse COQ7, 89% identity to rat COQ7, 53% identity to C. elegans CLK-1, and 37% identity to S. cerevisiae Coq7p/Cat5p. Zoo blot analysis implied that the COQ7 gene was well conserved among mammal, bird, and reptile genomes. Tissue blot analysis showed that human COQ7 is dominantly transcribed in heart and skeletal muscle. Genomic analyses revealed that the human COQ7 gene is composed of six exons spanning 11 kb of human genome as a single-copy gene. Radiation hybrid mapping assigned the COQ7 gene to human chromosome 16p12.3-p13.11.     

Tel2 is required for activation of the Mrc1-mediated replication checkpoint

Proteins belonging to the Tel2/Rad-5/Clk-2 family are conserved among eukaryotes and are involved in various cellular processes, such as cell proliferation, telomere maintenance, the biological clock, and the DNA damage checkpoint. However, the molecular mechanisms underlying the functions of these molecules remain largely unclear. Here we report that in the fission yeast, Schizosaccharomyces pombe, Tel2 is required for efficient phosphorylation of Mrc1, a mediator of DNA replication checkpoint signaling, and for activation of Cds1, a replication checkpoint kinase, when DNA replication is blocked by hydroxyurea. In fact, Tel2 is required for survival of replication fork arrest and for the replication checkpoint in cells lacking Chk1, another checkpoint kinase the role of which overlaps that of Cds1 in cell cycle arrest by replication block. In addition, Tel2 plays important roles in entry into S phase and in genome stability. Tel2 is essential for vegetative cell growth, and the tel2Delta strain accumulated cells with 1C DNA content after germination. In the absence of hydroxyurea, Tel2 is vital in the mutant lacking Swi1, a component of the replication fork protection complex, and multiple Rad22 DNA repair foci were frequently observed in Tel2-repressed swi1Delta cells especially at S phase. In contrast, the cds1Deltaswi1Delta mutant did not show such lethality. These results indicate that S. pombe Tel2 plays important roles in the Mrc1-mediated replication checkpoint as well as in the Cds1-independent regulation of genome integrity.     

CLK-1/Coq7p is a DMQ mono-oxygenase and a new member of the di-iron carboxylate protein family.

Strains of Caenorhabditis elegans mutant for clk-1 exhibit a 20-40% increase in mean lifespan. clk-1 encodes a mitochondrial protein thought to be either an enzyme or regulatory molecule acting within the ubiquinone biosynthesis pathway. Here CLK-1 is shown to be related to the ubiquinol oxidase, alternative oxidase, and belong to the functionally diverse di-iron-carboxylate protein family which includes bacterioferritin and methane mono-oxygenase. Construction and analysis of a homology model indicates CLK-1 is a 2-polyprenyl-3-methyl-6-methoxy-1,4-benzoquinone mono-oxygenase as originally predicted. Analysis of known CLK-1/Coq7p mutations also supports this notion. These findings raise the possibility of developing CLK-1-specific inhibitors to test for lifespan extension in higher organisms.     

RAD-51-dependent and -independent roles of a Caenorhabditis elegans BRCA2-related protein during DNA double-strand break repair

The BRCA2 tumor suppressor is implicated in DNA double-strand break (DSB) repair by homologous recombination (HR), where it regulates the RAD51 recombinase. We describe a BRCA2-related protein of Caenorhabditis elegans (CeBRC-2) that interacts directly with RAD-51 via a single BRC motif and that binds preferentially to single-stranded DNA through an oligonucleotide-oligosaccharide binding fold. Cebrc-2 mutants fail to repair meiotic or radiation-induced DSBs by HR due to inefficient RAD-51 nuclear localization and a failure to target RAD-51 to sites of DSBs. Genetic and cytological comparisons of Cebrc-2 and rad-51 mutants revealed fundamental phenotypic differences that suggest a role for Cebrc-2 in promoting the use of an alternative repair pathway in the absence of rad-51 and independent of nonhomologous end joining (NHEJ). Unlike rad-51 mutants, Cebrc-2 mutants also accumulate RPA-1 at DSBs, and abnormal chromosome aggregates that arise during the meiotic prophase can be rescued by blocking the NHEJ pathway. CeBRC-2 also forms foci in response to DNA damage and can do so independently of rad-51. Thus, CeBRC-2 not only regulates RAD-51 during HR but can also function independently of rad-51 in DSB repair processes.     

coq7/clk-1 regulates mitochondrial respiration and the generation of reactive oxygen species via coenzyme Q

coq7/clk-1 was isolated from a long-lived mutant of Caenorhabditis elegans, and shows sluggish behaviours and an extended lifespan. In C. elegans and Saccharomyces cerevisiae, coq7/clk-1 is required for the biosynthesis of coenzyme Q (CoQ), an essential co-factor in mitochondrial respiration. The clk-1 mutant contains dietary CoQ(8) from Escherichia coli and demethoxyubiquinone 9 (DMQ9) instead of CoQ(9). In a previous study, we generated COQ7-deficient mice by targeted disruption of the coq7 gene and reported that mouse coq7/clk-1 is also essential for CoQ synthesis, maintenance of mitochondrial integrity and neurogenesis. In the present study, we rescued COQ7-deficient mice from embryonic lethality and established a mouse model with decreased CoQ level by transgene expression of COQ7/CLK-1. A biochemical analysis showed a concomitant decrease in CoQ(9), mitochondrial respiratory enzyme activity and the generation of reactive oxygen species (ROS) in the mitochondria of CoQ-insufficient mice. This implied that the depressed activity of respiratory enzymes and the depressed production of ROS may play a physiological role in the control of lifespan in mammalian species and of C. elegans.     

Reproductive fitness and quinone content of Caenorhabditis elegans clk-1 mutants fed coenzyme Q isoforms of varying length

Caenorhabditis elegans clk-1 mutants lack coenzyme Q9 and accumulate the biosynthetic intermediate demethoxy-Q9. A dietary source of ubiquinone (Q) is required for larval growth and development of the gonad and germ cells. We considered that uptake of the shorter Q8 isoform present in the Escherichia coli food may contribute to the Clk phenotypes of slowed development and reduced brood size observed when the animals are fed Q-replete E. coli. To test the effect of isoprene tail length, N2 and clk-1 animals were fed E. coli engineered to produce Q7, Q8, Q9, or Q10. Wild-type nematodes showed no change in reproductive fitness regardless of the Qn isoform fed. clk-1(e2519) fed the Q9 diet showed increased egg production; however, this diet did not improve reproductive fitness of the clk-1(qm30) animals. Furthermore, animals with the more severe clk-1(qm30) allele become sterile and their progeny inviable when fed Q7-containing bacteria. The content of Q7 in the mitochondria of clk-1 animals was decreased relative to Q8, suggesting less effective transport of Q7 to the mitochondria, impaired retention, or decreased stability. Additionally, regardless of E. coli diet, clk-1(qm30) animals contain a dysfunctional dense form of mitochondria. The gonads of clk-1(qm30) worms fed Q7-containing food were severely shrunken and disordered. The differential fertility of clk-1 mutant nematodes fed Q isoforms may result from changes in Q localization, altered recognition by Q-binding proteins, and/or potential defects in mitochondrial function resulting from the mutant CLK-1 polypeptide itself.     

Molecular mechanism of maternal rescue in the clk-1 mutants of Caenorhabditis elegans

The clk-1 mutants of Caenorhabditis elegans display an average slowing down of physiological rates, including those of development, various behaviors, and aging. clk-1 encodes a hydroxylase involved in the biosynthesis of the redox-active lipid ubiquinone (co-enzyme Q), and in clk-1 mutants, ubiquinone is replaced by its biosynthetic precursor demethoxyubiquinone. Surprisingly, homozygous clk-1 mutants display a wild-type phenotype when issued from a heterozygous mother. Here, we show that this maternal effect is the result of the persistence of small amounts of maternally derived CLK-1 protein and that maternal CLK-1 is sufficient for the synthesis of considerable amounts of ubiquinone during development. However, gradual depletion of CLK-1 and ubiquinone, and expression of the mutant phenotype, can be produced experimentally by developmental arrest. We also show that the very long lifespan observed in daf-2 clk-1 double mutants is not abolished by the maternal effect. This suggests that, like developmental arrest, the increased lifespan conferred by daf-2 allows for depletion of maternal CLK-1, resulting in the expression of the synergism between clk-1 and daf-2. Thus, increased adult longevity can be uncoupled from the early mutant phenotypes, indicating that it is possible to obtain an increased adult lifespan from the late inactivation of processes required for normal development and reproduction.     

A conserved checkpoint pathway mediates DNA damage--induced apoptosis and cell cycle arrest in C. elegans

To maintain genomic stability following DNA damage, multicellular organisms activate checkpoints that induce cell cycle arrest or apoptosis. Here we show that genotoxic stress blocks cell proliferation and induces apoptosis of germ cells in the nematode C. elegans. Accumulation of recombination intermediates similarly leads to the demise of affected cells. Checkpoint-induced apoptosis is mediated by the core apoptotic machinery (CED-9/CED-4/CED-3) but is genetically distinct from somatic cell death and physiological germ cell death. Mutations in three genes--mrt-2, which encodes the C. elegans homolog of the S. pombe rad1 checkpoint gene, rad-5, and him-7-block both DNA damage-induced apoptosis and cell proliferation arrest. Our results implicate rad1 homologs in DNA damage-induced apoptosis in animals.     

Mutator phenotype of Caenorhabditis elegans DNA damage checkpoint mutants

DNA damage response proteins identify sites of DNA damage and signal to downstream effectors that orchestrate either apoptosis or arrest of the cell cycle and DNA repair. The C. elegans DNA damage response mutants mrt-2, hus-1, and clk-2(mn159) displayed 8- to 15-fold increases in the frequency of spontaneous mutation in their germlines. Many of these mutations were small- to medium-sized deletions, some of which had unusual sequences at their breakpoints such as purine-rich tracts or direct or inverted repeats. Although DNA-damage-induced apoptosis is abrogated in the mrt-2, hus-1, and clk-2 mutant backgrounds, lack of the apoptotic branch of the DNA damage response pathway in cep-1/p53, ced-3, and ced-4 mutants did not result in a Mutator phenotype. Thus, DNA damage checkpoint proteins suppress the frequency of mutation by ensuring that spontaneous DNA damage is accurately repaired in C. elegans germ cells. Although DNA damage response defects that predispose humans to cancer are known to result in large-scale chromosome aberrations, our results suggest that small- to medium-sized deletions may also play roles in the development of cancer.     

Increased genome instability and telomere length in the elg1-deficient Saccharomyces cerevisiae mutant are regulated by S-phase checkpoints

Gross chromosomal rearrangements (GCRs) are frequently observed in cancer cells. Abnormalities in different DNA metabolism including DNA replication, cell cycle checkpoints, chromatin remodeling, telomere maintenance, and DNA recombination and repair cause GCRs in Saccharomyces cerevisiae. Recently, we used genome-wide screening to identify several genes the deletion of which increases GCRs in S. cerevisiae. Elg1, which was discovered during this screening, functions in DNA replication by participating in an alternative replication factor complex. Here we further characterize the GCR suppression mechanisms observed in the elg1Delta mutant strain in conjunction with the telomere maintenance role of Elg1. The elg1Delta mutation enhanced spontaneous DNA damage and resulted in GCR formation. However, DNA damage due to inactivation of Elg1 activates the intra-S checkpoints, which suppress further GCR formation. The intra-S checkpoints activated by the elg1Delta mutation also suppress GCR formation in strains defective in the DNA replication checkpoint. Lastly, the elg1Delta mutation increases telomere size independently of other previously known telomere maintenance proteins such as the telomerase inhibitor Pif1 or the telomere size regulator Rif1. The increase in telomere length caused by the elg1Delta mutation was suppressed by a defect in the DNA replication checkpoint, which suggests that DNA replication surveillance by Dpb11-Mec1/Tel1-Dun1 also has an important role in telomere length regulation.     

CeBRC-2 stimulates D-loop formation by RAD-51 and promotes DNA single-strand annealing

The BRCA2 tumour suppressor regulates the RAD-51 recombinase during double-strand break (DSB) repair by homologous recombination (HR) but how BRCA2 executes its functions is not well understood. We previously described a functional homologue of BRCA2 in Caenorhabditis elegans (CeBRC-2) that binds preferentially to single-stranded DNA via an OB-fold domain and associates directly with RAD-51 via a single BRC domain. Consistent with a direct role in HR, Cebrc-2 mutants are defective for repair of meiotic and radiation-induced DSBs due to an inability to regulate RAD-51. Here, we explore the function of CeBRC-2 in HR processes using purified proteins. We show that CeBRC-2 stimulates RAD-51-mediated D-loop formation and reduces the rate of ATP hydrolysis catalysed by RAD-51. These functions of CeBRC-2 are dependent upon direct association with RAD-51 via its BRC motif and on its DNA-binding activity, as point mutations in the BRC domain that abolish RAD-51 binding or the BRC domain of CeBRC-2 alone, lacking the DNA-binding domain, fail to stimulate RAD-51-mediated D-loop formation and do not reduce the rate of ATP hydrolysis by RAD-51. Phenotypic comparison of Cebrc-2 and rad-51 mutants also revealed a role for CeBRC-2 in an error-prone DSB repair pathway independent of rad-51 and non-homologous end joining, raising the possibility that CeBRC-2 may have replaced the role of vertebrate Rad52 in DNA single-strand annealing (SSA), which is missing from C. elegans. Indeed, we show here that CeBRC-2 mediates SSA of RPA-oligonucleotide complexes similar to Rad52. These results reveal RAD-51-dependent and -independent functions of CeBRC-2 that provide an explanation for the difference in DNA repair defects observed in Cebrc-2 and rad-51 mutants, and define mechanistic roles for CeBRC-2 in HR and in the SSA pathway for DSB repair.     

Altered quinone biosynthesis in the long-lived clk-1 mutants of Caenorhabditis elegans

Mutations in the clk-1 gene of Caenorhabditis elegans result in an extended life span and an average slowing down of developmental and behavioral rates. However, it has not been possible to identify biochemical changes that might underlie the extension of life span observed in clk-1 mutants, and therefore the function of CLK-1 in C. elegans remains unknown. In this report, we analyzed the effect of clk-1 mutation on ubiquinone (UQ(9)) biosynthesis and show that clk-1 mutants mitochondria do not contain detectable levels of UQ(9). Instead, the UQ(9) biosynthesis intermediate, demethoxyubiquinone (DMQ(9)), is present at high levels. This result demonstrates that CLK-1 is absolutely required for the biosynthesis of UQ(9) in C. elegans. Interestingly, the activity levels of NADH-cytochrome c reductase and succinate-cytochrome c reductase in mutant mitochondria are very similar to those in the wild-type, suggesting that DMQ(9) can function as an electron carrier in the respiratory chain. To test this possibility, the short side chain derivative DMQ(2) was chemically synthesized. We find that DMQ(2) can act as an electron acceptor for both complex I and complex II in clk-1 mutant mitochondria, while another ubiquinone biosynthesis precursor, 3-hydroxy-UQ(2), cannot. The accumulation of DMQ(9) and its use in mutant mitochondria indicate, for the first time in any organism, a link between the alteration in the quinone species used in respiration and life span.