The Mitochondrial Nucleoid Protein, Mgm101p, of Saccharomyces cerevisiae Is Involved in the Maintenance of ⁺ and ori/rep-Devoid Petite Genomes but Is Not Required for Hypersuppressive ^- mtDNA

Corresponding author: Xin Jie Chen, Research School of Biological Sciences, The Australian National University, GPO Box 475, Canberra City, ACT 2601, Australia., chen{at}rsbs.anu.edu.au (E-mail)

Communicating editor: L. PILLUS

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

The Saccharomyces cerevisiae MGM101 gene encodes a DNA-binding protein targeted to mitochondrial nucleoids. MGM101 is essential for maintenance of a functional ⁺ genome because meiotic segregants, with a disrupted mgm101 allele, cannot undergo more than 10 divisions on glycerol medium. Quantitative analysis of mtDNA copy number in a ⁺ strain carrying a temperature-sensitive allele, mgm101-1, revealed that the amount of mtDNA is halved each cell division upon a shift to the restrictive temperature. These data suggest that mtDNA replication is rapidly blocked in cells lacking MGM101. However, a small proportion of meiotic segregants, disrupted in MGM101, have ^- genomes that are stably maintained. Interestingly, all surviving ^- mtDNAs contain an ori/rep sequence. Disruption of MGM101 in hypersuppressive (HS) strains does not have a significant effect on the propagation of HS ^- mtDNA. However, in petites lacking an ori/rep, disruption of MGM101 leads to either a complete loss or a dramatically decreased stability of mtDNA. This discriminatory effect of MGM101 suggests that replication of ⁺ and ori/rep-devoid ^- mtDNAs is carried out by the same process. By contrast, the persistence of ori/rep-containing mtDNA in HS petites lacking MGM101 identifies a distinct replication pathway. The alternative mtDNA replication mechanism provided by ori/rep is independent of mitochondrial RNA polymerase encoded by RPO41 as a HS ^- genome is stably maintained in a mgm101, rpo41 double mutant.

THE mitochondrial genome of eukaryotic cells encodes integral components of the energy-producing oxidative phosphorylation pathway. Central to the cell's energy metabolism is the need to maintain the organelle DNA (mtDNA). However, in the budding yeast Saccharomyces cerevisiae, a complete loss of mtDNA is not lethal provided that fermentable carbon sources such as glucose are supplied. Consequently, S. cerevisiae provides an excellent experimental vehicle for the identification of genes involved in mtDNA replication and transmission. Several proteins participating in mtDNA replication have been identified (for recent reviews, see HERMANN and SHAW 1998 ; SHADEL 1999 ; CONTAMINE and PICARD 2000 ; LECRENIER and FOURY 2000 ). The pol- DNA polymerase, Mip1p (FOURY 1989 ), the single-stranded DNA (ssDNA)-binding protein, Rim1p (VAN DYCK et al. 1992 ), and a DNA ligase activity encoded by the CDC9 gene (WILLER et al. 1999 ) directly participate in mtDNA replication. Deleterious mutations in genes encoding these proteins lead to a complete loss of the mitochondrial genome (^o).

MGM101 is another S. cerevisiae gene whose disruption leads to the loss of a functional mitochondrial genome. The MGM101 locus was initially identified in a genetic screen for mutations affecting the maintenance of mtDNA (CHEN et al. 1993 ). It encodes a polypeptide of 269 residues rich in positively charged amino acids. A homolog of MGM101 has been identified in the petite-negative yeast Kluyveromyces lactis by functional complementation of a S. cerevisiae mgm101 mutant (CLARK-WALKER and CHEN 1996 ). K. lactis cells carrying a disrupted mgm101 allele form nonviable microcolonies due to loss of mtDNA. Subsequently, it has been found that the Mgm101 protein has a high affinity for DNA and is a component of mitochondrial nucleoids (MEEUSEN et al. 1999 ). Mgm101p can be specifically crosslinked to mtDNA in vivo (KAUFMAN et al. 2000 ). Functionally, it has been proposed that Mgm101p might be involved in the repair of oxidatively damaged mtDNA (MEEUSEN et al. 1999 ).

Yeast mitochondria contain a number of proteins required for the maintenance of the mitochondrial genome by their involvement in recombination and repair. The Pif1 helicase (FOURY and KOLODYNSKI 1983 ), the cruciform-cutting endonuclease Cce1p/Mgt1p (KLEFF et al. 1992 ; LOCKSHON et al. 1995 ; WHITE and LILLEY 1996 ), the mitochondrial high-mobility group protein Abf2p (DIFFLEY and STILLMAN 1992 ; KAO et al. 1993 ; MACALPINE et al. 1998 ; ZELENAYA-TROITSKAYA et al. 1998 ), and a putative recombinase defined by the MHR1 locus (LING et al. 1995 ) are involved in mtDNA recombination. The Msh1 protein, homologous to bacterial MutS, is needed for mismatch repair (REENAN and KOLODNER 1992 ; CHI and KOLODNER 1994 ). In addition to their role in DNA recombination, the Pif1 helicase and the putative Mhr1 recombinase are also suggested to be involved in the repair of UV-damaged and spontaneously occurring DNA lesions (FOURY and KOLODYNSKI 1983 ; LING et al. 2000 ).

S. cerevisiae has a mitochondrial genome of 85.8 kb (FOURY et al. 1998 ). Respiratory-competent yeast cells contain 50–100 copies of mtDNA that are organized into punctate protein-DNA complexes called mitochondrial nucleoids (mt-nucleoids) with a diameter of 20–50 nm (WILLIAMSON and FENNELL 1979 ; MIYAKAWA et al. 1987 , MIYAKAWA et al. 1995 ). Each individual nucleoid, estimated to contain 3–4 copies of mtDNA, has been proposed to be the unit of mtDNA inheritance (LOCKSHON et al. 1995 ). If this is the case, it could resolve the discrepancy between the number of segregating units for mtDNA being smaller than the number of mtDNA molecules (DUJON 1981 ; GINGOLD 1988 ). A subset of proteins that are enriched in mt-nucleoids purified by sedimentation has been identified (MIYAKAWA et al. 1987 ; NEWMAN et al. 1996 ). In organello formaldehyde crosslinking studies have shown that up to 20 proteins are crosslinked to mtDNA (KAUFMAN et al. 2000 ). Among them are Abf2p, Rim1p, and Mgm101p, which are involved in mtDNA maintenance.

Despite the increasing number of genes identified as participating in mtDNA metabolism and the emergence of a better picture for the in vivo organization of the mitochondrial genome, initiation of mtDNA replication is not fully understood. It has been proposed that mtDNA replication in S. cerevisiae is a transcription-primed process similar to the mechanism observed in vertebrates (SCHINKEL and TABAK 1989 ; SCHMITT and CLAYTON 1993 ). The mitochondrial genome is supposed to contain four active ori/rep sequences that are composed of three GC-rich clusters and a mitochondrial promoter. It is believed that transcription from the promoter by the Rpo41 RNA polymerase primes synthesis of the leading strand of DNA replication. This notion has gained support from biochemical experiments showing that DNA replication initiates from the ori/rep promoters and that the formation of transcription-dependent RNA/DNA hybrids is detected in these sequences (BALDACCI et al. 1984 ; XU and CLAYTON 1995 ; GRAVES et al. 1998 ; VAN DYCK and CLAYTON 1998 ). Consistent with this notion is genetic evidence showing that S. cerevisiae mutants lacking either the mitochondrial RNA polymerase Rpo41p or the mitochondrial transcription initiation factor Mtf1p are unable to maintain a functional mitochondrial genome (GREENLEAF et al. 1986 ; LISOWSKY and MICHAELIS 1988 ). Furthermore, it is known that a subset of mtDNA deletion mutants (called petite or ^- mutants), containing an ori/rep sequence amplified into tandem repeats, has a "replicative advantage" over the wild-type (⁺) genome. These particular ^- mutants, called hypersuppressive (HS) mutants, can give rise to diploid progeny composed almost exclusively of petite mutants in a cross against a strain harboring a ⁺ DNA, whereas ^- genomes lacking ori-rep (or neutral petites) do not share the replicative advantage in similar genetic crosses.

However, several studies seem to contradict the transcription-dependent mtDNA replication model. First, an ori/rep-devoid ^- genome of 35 bp, composed entirely of A·T base pairs, can be stably maintained (FANGMAN et al. 1989 ). Second, ^- genomes can also be stably propagated in the absence of mtRNA polymerase (FANGMAN et al. 1990 ). Finally, it has been shown that transcription from the ori/rep sequences is not essential for the biased inheritance of HS genomes in HS ^- x neutral ^- crosses (LORIMER et al. 1995 ). These studies suggest that mtDNA replication is independent of ori/rep sequences and mtRNA polymerase-driven initiation. An alternative mode may exist for the replication of the ^- genome devoid of ori/rep sequences. A recombination-mediated DNA replication process that converts a recombinational intermediate into a replication fork, as in the bacteriophage T₄, has been proposed to occur in the replication of S. cerevisiae mtDNA (FANGMAN et al. 1990 ; PREISER et al. 1996 ; MACALPINE et al. 1998 ). Support for such a replication mechanism comes from studies showing the presence of single- or double-stranded lariats and large concatemers (MALESZKA et al. 1991 ; BENDICH 1996 ) and the correlation between the level of recombinational intermediates and the mtDNA copy number and stability (MACALPINE et al. 1998 ; ZELENAYA-TROITSKAYA et al. 1998 ).

In this study, we demonstrate that the mitochondrial genome maintenance gene, MGM101, is required for the propagation of ⁺ and petite genomes lacking an ori/rep sequence but is dispensable for HS mtDNA. The results suggest that the replication of ^- genomes devoid of ori/rep needs the participation of a distinct set of proteins, which can be spared by the presence of a highly amplified ori/rep sequence in HS strains. As the maintenance of a wild-type mitochondrial genome is also MGM101 dependent (CHEN et al. 1993 ; MEEUSEN et al. 1999 ), its replication is likely to be mediated by the proposed ori/rep-independent mtDNA replication mechanism.

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Media and strains:
Complete medium for growth of yeast (GYP) contains 0.5% Bacto yeast extract, 1% Bacto peptone, and 2% glucose. Glycerol medium (GlyYP) contains 2% glycerol instead of glucose. Minimal medium (GMM) contains 0.17% Difco yeast nitrogen base without amino acids, 0.5% ammonium sulfate, and 2% glucose. Nutrients essential for auxotrophic strains were added to GMM at 25 µg/ml for bases and 50 µg/ml for amino acids. GYP containing 5 and 10% glucose, respectively, was used for mating and presporulation medium. Sporulation medium consists of 1% potassium acetate, 0.1% Bacto yeast extract, and 0.05% glucose. For solid medium, 2% Bacto agar was added. G418 medium is GYP supplemented with the drug at 200 µg/ml.

Yeast strains used in this study are listed in Table 1. CS6 is derived from CS4 (CHEN et al. 1993 ) by disrupting MGM101 using the mgm101::URA3 cassette. N1-U2 and E3-U are derived from DS400/N1 and E3 with the introduction of the ura3 mutation by selecting for 5-fluoroorotic acid (5-FOA)-resistant colonies on GMM supplemented with 5-FOA (Toronto Research Biochemicals) at 500 µg/ml and uracil at 10 µg/ml.

Manipulation of S. cerevisiae:
Standard methods were used for cell culture, mating, and sporulation (KAISER et al. 1994 ). Tetrad dissection was undertaken with a Singer MSM System (series 200, Singer Instruments, Somerset, UK) following brief treatment with Zymolyase (Seikaguku, Tokyo). Transformation of yeast cells was performed by the lithium acetate-dimethyl sulfoxide method (GIETZ et al. 1995 ). Yeast colony hybridization and total DNA extraction were carried out according to standard procedures (KAISER et al. 1994 ). Yeast colony PCR was conducted as previously described (BROWN and TUITE 1998 ).

Southern blot analysis and preparation of mtDNA probes:
Yeast total DNA was electrophoresed on a 1% agarose gel and Southern blot analysis was performed as described (SAMBROOK et al. 1989 ). For the preparation of mtDNA probes, the mitochondrial genomes were purified on a cesium chloride/bis-benzimide gradient. The ⁺ mtDNA was purified from the strain W303-1B and the 0.43-kb DS400/N1 mtDNA was from DS400/N1. The probe specific for ori3 was prepared from the PstI-digested pSCM107 plasmid DNA containing the ori3 sequence. mtDNAs were labeled with [³²P]dATP using the random-priming GigaPrime DNA labeling kit (Bresatec, Thebarton, Australia). Quantitation of hybridization signals was undertaken by using a phosphorimage analyzer (Molecular Dynamics, Sunnyvale, CA) with ImageQuant software (version 5.1).

Plasmid construction and gene disruption:
The mgm101::kan cassette was isolated from the plasmid pScmgm101::kan/1 by digestion with SphI and SacI. The 2.4-kb cassette contains MGM101 with the kan module (GULDENER et al. 1996 ) inserted at codon 218. Yeast strains were transformed with the disruption cassette and selected for G418^R. Correct disruption of the gene was confirmed by Southern blot analysis of genomic DNA from stable G418^R transformants, after digestion with SphI and SacI, by using a ³²P-labeled 714-bp HindIII-SacI fragment derived from MGM101 as a probe. Disruptions of ABF2, CCE1, MHR1, and RPO41 were conducted by using a PCR-based gene disruption method (GULDENER et al. 1996 ; BROWN and TUITE 1998 ). The oligonucleotide primers, listed in Table 2, contain 35–36 nucleotides on the 5'-end matching sequences in the target genes and 20–21 nucleotides complementary to the flanking region of the kan module in pFA6a-kanMX4 (WACH et al. 1994 ). The primers used for gene disruptions are as follows: ScABF2P4 and ScABF2P5 for ABF2; ScCCE1P1 and ScCCE1P2 for CCE1; ScMHR1P1 and ScMHR1P2 for MHR1; and ScRPO41P1 and ScRPO41P2 for RPO41. For disruption of MGM101 in HS^-15/rpo41, the primers ScMGM101HIS3P1 and ScMGM101HIS3P2 were used to amplify the mgm101::HIS3 cassette with S. cerevisiae genomic DNA as template. The PCR DNA was used to transform HS^-15/rpo41 by selecting for stable His⁺ colonies, resulting in the rpo41::kan mgm101::HIS3 double mutants. The correct disruption of MGM101, ABF2, RPO41, MHR1, and CCE1 in the yeast strains was confirmed either by Southern blot analysis of genomic DNA or by PCR using appropriate primers.

To introduce the mgm101-1^ts allele into the E3 background, the plasmid pURA-mgm101.1, carrying the mgm101-1^ts allele, was integrated into the StuI site of the URA3 locus in E3-U by selecting for Ura⁺ transformants. The wild-type MGM101 allele was then disrupted by using the mgm101::kan cassette. As confirmed by Southern blot analysis, three independent transformants, named E3m25, E3m30, and E3m36, were found to have the MGM101 locus correctly disrupted while retaining the mgm101-1^ts allele.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

MGM101 is essential for the maintenance of a functional ⁺ genome:
Several mitochondrial nucleoid proteins, including Abf2p, Ilv5p, Hsp60p, Kgd2p, and Rim1p, have been reported to be involved in the maintenance of mtDNA stability in S. cerevisiae (KAUFMAN et al. 2000 ). In the case of Abf2p and Ilv5p, although null mutants of the genes lose their ⁺ mtDNA when grown in complete glucose medium, a functional mitochondrial genome can be maintained when cells are grown on a nonfermentable carbon source such as glycerol (DIFFLEY and STILLMAN 1992 ; MEGRAW and CHAE 1993 ; ZELENAYA-TROITSKAYA et al. 1995 ). Consequently, these proteins are not directly involved in the essential steps of mtDNA replication or are functionally redundant with unidentified proteins. On the other hand, disruption of the RIM1 gene, which encodes a mitochondrial single-stranded DNA-binding protein directly involved in mtDNA synthesis, results in cells that are unable to maintain mtDNA even on glycerol medium (VAN DYCK et al. 1992 ). Previous studies have shown that inactivation of MGM101 converts cells into respiratory-deficient mutants on glucose medium (CHEN et al. 1993 ; MEEUSEN et al. 1999 ). To know whether MGM101 participates in an essential step of mtDNA replication/maintenance, we dissected the strain CS6 (mgm101::URA3/+, ura3/ura3) on a glycerol plate. It was found that in addition to two large Ura^- segregants, each tetrad gave rise to two spores that form microcolonies of 300–1000 cells. These spores are all Ura⁺ when tested on GMM minus uracil medium but cannot be further propagated when transferred to a fresh GlyYP plate. This observation suggests that cells are unable to maintain a functional ⁺ genome upon inactivation of MGM101 even in the presence of a strong selective pressure against loss of mtDNA.

mgm101 mutants are likely to be blocked in mtDNA replication:
The rapid cell growth arrest of mgm101 mutants on glycerol medium favors the proposal that the primary role of Mgm101p might be a direct participation in mtDNA replication. To examine this possibility, we analyzed changes to the ⁺ mitochondrial genome in a conditional mgm101 mutant upon a shift to restrictive temperature. Fig 1A shows that the mgm101 mutant has a decreasing mtDNA level under the restrictive condition of 37°. The mtDNA level relative to the nuclear large ribosomal subunit gene (n-LRS), which cross-hybridizes to mtDNA LRS (our unpublished observation), was estimated and plotted against cell doubling (Fig 1B). It was revealed that at the first doubling time, mtDNA decreased by 48%. At the second doubling time, only 19% of mtDNA was retained. The rapid decrease of total mtDNA in the cell population, which is reciprocally related to cell doubling, suggests that an efficient block in mtDNA synthesis occurs in cells inactivated in Mgm101p.

View larger version (37K): In this window In a new window Download PPT slide   Figure 1. Time course analysis showing the loss of the + and the E3 - genomes from mgm101-1ts cells upon a shift to the restrictive temperature. (A) A representative example of three independent Southern blot analyses of yeast total DNA showing the loss of the + genome in M2915-7C (mgm101-1ts). Yeast strains were grown at 25° overnight to a late exponential phase and diluted into fresh GYP to an OD600 of 0.5. The cultures were shifted to 37° and aliquots were removed at different time points before OD600 reached 2.0. The harvested cell fractions were therefore all at the exponential phase within two cell divisions (not shown). Total DNA was extracted, digested with CfoI, and analyzed by Southern blots using 32P-labeled + mtDNA as a probe. The level of mtDNA in the strains relative to the n-LRS, which cross-hybridizes to mtDNA LRS (our unpublished observation), was quantified by phosphorimaging. ts, M2915-7C (mgm101-1ts); wt, M2915-6A; o, M2915-6A/o. (B) The levels of the + and the E3 - genomes in M2915-7C (mgm101-1ts) and E3/m25 (mgm101-1ts) are plotted against the number of doubling times (generations) to show the kinetics of mtDNA decrease from growing mgm101-1 cells under restrictive conditions. Three independent experiments were carried out for each strain and the standard errors for each time point are indicated.

From Fig 1A, it appears that inactivation of MGM101 does not change the mtDNA profile other than causing a decrease in the amount of mtDNA. When undigested DNA was subjected to Southern blot analysis, the hybridization signals from the mgm101-1 mutants exposed to restrictive temperature were found mainly in a high-molecular-weight position as in a wild-type strain (not illustrated). No prominent bands were detected that could be interpreted as products of specific mtDNA deletion or degradation. In addition, it was found that the effect of Mgm101p inactivation is not manifested under a nongrowth condition. After the mgm101-1 mutant, M2915-7C, was incubated in water at 37° for up to 24 hr, no significant increase in petite frequency was observed when cells were returned to the permissive temperature (data not shown). This finding suggests that the loss of Mgm101p function is reversible and that the mitochondrial genome remains intact under the nongrowth condition.

Escape of ^- genomes from MGM101 disruption:
In Southern blot analysis of mgm101-disrupted cells, we detected the presence of mtDNA in some mutant strains (not illustrated). This raised the possibility that upon disruption of MGM101, a subpopulation of cells could have rearrangements in mtDNA giving rise to a ^- genome that can be subsequently maintained. To examine this notion, we dissected CS6 (mgm101::URA3/+) on GYP plates (Fig 2A) followed by testing for the presence of mtDNA by colony hybridization (Fig 2B). It was found that after a long exposure for 7 days, 25% of mgm101::URA3 segregants had detectable signals when hybridized to a total mtDNA probe. These segregants are all respiratory deficient as they do not grow on GlyYP medium. The presence of mtDNA in the mgm101::URA3 strains was further confirmed by Southern blot analysis. A total of 10 mgm101::URA3 segregants were chosen at random and 4 of them were found to contain ^- mtDNA hybridizing to a total mtDNA probe (data not shown).

View larger version (74K): In this window In a new window Download PPT slide   Figure 2. Genome deletion/rearrangement leads to the escape of - genomes from the requirement for MGM101 in meiotic segregants. (A) Dissection of asci from CS6 (+/mgm101::URA3)on GYP medium. (B) CS6 tetrads were analyzed for the presence of mtDNA by colony hybridization using a total mtDNA probe. Circled are the mgm101::URA3 segregants giving rise to a positive signal on hybridization. (C) Southern blot analysis showing that the - genomes that escaped from MGM101 disruption contain an ori/rep sequence. Total DNA was extracted from six mtDNA-containing mgm101::URA3 segregants identified by the colony hybridization and digested with DraI. After electrophoresis on a 1% agarose gel, DNA was transferred to nylon membrane and hybridized to a 32P-labeled ori3 probe. As controls, M2915-6A/o (o), M2915-6A (+), and HS416 (ori3) were included in the analysis.

To know whether the detection of the hybridization signals in mgm101::URA3 segregants is due to a delayed loss of mtDNA, the strains were subcloned on GYP plates for three rounds, equivalent to 60 generations. Individual colonies were subsequently examined for the presence of mtDNA by hybridization. It was found that all colonies gave rise to positive signals indicative of the presence of mtDNA (data not shown), suggesting that the detected ^- genomes in the mgm101::URA3 strains can be stably maintained.

The mtDNA-containing mgm101::URA3 segregants were examined further by Southern blot analysis. It was found that all the retained ^- genomes contained an ori/rep sequence (Fig 2C). The six clones, chosen at random, all contained sequences that hybridized to an ori3 probe. These results suggest that an amplification of an ori/rep sequence could be required to escape the need for Mgm101 function.

Mgm101p is dispensable for the maintenance of HS ^- genomes:
The above observations prompted us to investigate the role of MGM101 in the maintenance of genetically well-defined HS petite genomes that contain an amplified ori/rep sequence. MGM101 was disrupted in the strains HS^-15, HS416, and HS^-20, which contain an amplified ori2, -3, and -5 of 1.9, 0.7, and 2.1 kb, respectively. The resulting strains, HS^-15/mgm101, HS416/mgm101, and HS^-20/mgm101, were examined by Southern blot analysis (Fig 3). It was found that disruption of MGM101 does not significantly affect the mtDNA copy number in the cell populations. The hybridization pattern of mtDNA in the mgm101-disrupted strains is almost identical to their parental strains with a wild-type MGM101. These data indicate that loss of Mgm101 function does not have a significant effect on maintenance of the three HS genomes.

View larger version (89K): In this window In a new window Download PPT slide   Figure 3. Southern blot analysis showing that disruption of MGM101 does not affect the maintenance of hypersuppressive - genomes. Total DNA was extracted from yeast strains, digested with DraI, and probed with 32P-labeled total mtDNA. The strains examined include M2915-6A (+), M2915-6A/o (o), HS-20 (MGM101, ori5), HS416 (MGM101, ori3), HS-15 (MGM101, ori2), HS-20/mgm101 (mgm101::kan, ori5), HS416/mgm101 (mgm101::kan, ori3), and HS-15/mgm101 (mgm101::kan, ori2).

Involvement of other genes in maintenance of HS ^- mtDNA:
The lack of a requirement for MGM101 in maintenance of HS ^- mtDNA prompted us to examine whether other genes, especially ones involved in recombination, are similarly dispensable. We chose for this analysis MHR1, encoding a putative recombinase, and CCE1, encoding a cruciform-cutting endonuclease (KLEFF et al. 1992 ; LING et al. 1995 , LING et al. 2000 ; WHITE and LILLEY 1996 ). For comparison, as possible controls, we included the ABF2 and RPO41 genes that are required for maintenance of ⁺ mtDNA, but are not needed for ^- genomes (FANGMAN et al. 1990 ; ZELENAYA-TROITSKAYA et al. 1998 ; SEDMAN et al. 2000 ). Consequently, each of the above genes was disrupted in HS^-15 and retention of the ori/rep-containing mtDNA was examined by Southern blot analysis before (Fig 4A) and after (Fig 4B) restriction enzyme digestion. Although all disrupted strains retain HS^-15 mtDNA, the one lacking CCE1 shows the greatest decline, retaining 14% of the mtDNA present in the parent (Fig 4C). In Fig 4A, showing unrestricted mtDNA, the presence of the ori/rep genome in the CCE1 disrupted strain is barely detectable. The apparent lack of mtDNA in the unrestricted mutant sample could be due to the presence of networks, accumulating in the absence of the cruciform-cutting endonuclease, that prevent DNA from entering the gel.

View larger version (40K): In this window In a new window Download PPT slide   Figure 4. Effect of gene disruptions on the abundance of the hypersuppressive HS-15 mtDNA. Total DNA of equal absorbance at 260 nm was analyzed by Southern blots before (A) and after (B) digestion with DraI. The DNA samples are from HS-15 (WT) and its abf2, mhr1, cce1, rpo41, and mgm101 disruption derivatives. o and + controls are M2915-6A/o and M2915-6A, respectively. The ori3 sequence was used as a probe that identifies the 0.6- and 1.4-kb fragments released from the digestion of the HS-15 - mtDNA by DraI. (C) Histogram for a comparison of the total amount of mtDNA in the strains disrupted in various genes, relative to their parent HS-15 (WT).

MGM101 is required for the propagation of ^- genomes devoid of ori/rep:
To know whether the formation of a ^- genome by the amplification of any mtDNA segment is sufficient to escape the requirement for Mgm101p, we examined the effect of mgm101 mutation on ^- genomes that do not contain an ori/rep sequence. The petite mutants E3, DS400/A3, and DS400/N1 harbor ^- genomes of 1.6, 1.8, and 0.43 kb, respectively. MGM101 was disrupted in the three mutants and mtDNA in the resulting strains was examined by Southern blot analysis. Determination of 15 independent mgm101 isolates, each derived from E3 or DS400/A3, revealed that all clones have completely lost their mtDNA. An example is given in Fig 5A, which shows that the E3 and DS400/A3 ^- genomes are no longer detectable in E3/mgm101 and A3/mgm101. These results indicate that MGM101 is essential for the maintenance of these two ^- genomes.

View larger version (80K): In this window In a new window Download PPT slide   Figure 5. Southern blot analysis showing that MGM101 is required for the propagation of - mtDNA lacking an ori/rep sequence. (A) Equal amounts of total DNA from E3/mgm101 (E3, mgm101), E3 (E3, MGM101), DS400/A3/mgm101 (A3, mgm101), and DS400/A3 (A3, MGM101) were analyzed using a S. cerevisiae total mtDNA probe. (B) Total DNA was extracted from DS400/N1 (MGM101) and N1-U2/mgm101 (mgm101) and equal amounts of DNA, as estimated by OD260, were analyzed before (left) and after (right) digestion with Sau3AI. The probe used was 32P-labeled DS400/N1 - mtDNA purified from a cesium chloride gradient. The 0.43-kb DS400/N1 repeat and the 0.86-kb recombination intermediate are indicated. n.s., a nonspecific band resulting from cross-hybridization between nuclear DNA and the DS400/N1 repeat.

The requirement for Mgm101p in the maintenance of an ori/rep-devoid ^- genome was further confirmed by examining the loss of the E3 mtDNA in the mgm101-1^ts background. The strains E3m25, E3m30, and E3m36 were grown at the restrictive temperature of 37° and loss of the E3 mtDNA in the cell population was monitored by Southern blot analysis (data not shown). It was found that at the first doubling time, the mtDNA level was decreased by only 8% (Fig 1B) while 60% of mtDNA was retained at the second doubling time. However, a complete loss of the E3 ^- genome was observed after growth for approximately five doubling times under the restrictive conditions (data not shown). From these data, it can be concluded that Mgm101p is essential for the stable maintenance of the E3 ^- genome. In contrast to the ⁺ mtDNA, the highly repeated E3 ^- genome has a delayed mtDNA loss in mgm101 cells.

A different situation was observed with the DS400/N1 ^- genome. As shown in Fig 5B (left), hybridization of unrestricted total DNA from DS400/N1 (MGM101) with a probe specific for the ^- DS400/N1 repeat shows the presence of two species of mtDNA molecules: those in the form of circular oligomers and those in a distinct band of higher molecular weight that is most likely concatemers formed by DS400/N1 repeats. We found that upon the disruption of MGM101, cells still retain a measurable level of the DS400/N1 ^- mtDNA. When 12 independent mgm101 isolates were examined by Southern blotting, the presence of a markedly reduced level of mtDNA, hybridizable to the DS400/N1 ^- genome, was detected in all clones (not illustrated). Quantitation by phosphorimaging showed that the total amount of mtDNA in N1/mgm101, as exemplified in Fig 5B (left), is reduced to 8–21% of a strain with a wild-type MGM101, depending on different isolates. The amount of mtDNA molecules represented in the circular oligomer and concatemer forms appears to be proportionally reduced. When total DNA was analyzed after digestion with Sau3AI, which has a single site in the DS400/N1 sequence, repeats were released as a band of 0.43 kb (Fig 5B, right). Phosphorimager analysis estimates a reduction of these repeats in N1/mgm101 (mgm101::kan) to 15% of the level in DS400/N1 (MGM101), which is comparable to the amount found from unrestricted DNA.

In the analysis of retained mtDNA in N1-U2/mgm101 digested with Sau3AI (Fig 5B, right), a distinct band of 0.86 kb, which is double the size of the DS/400/N1 repeat, can be reproducibly detected. This band is inferred to be molecules comprised of two DS/400/N1 repeats linked by a recombination junction, as suggested by others (LOCKSHON et al. 1995 ; MACALPINE et al. 1998 ). It was found that although total mtDNA is decreased by over sixfold in N1-U2/mgm101 (mgm101::kan) compared with DS/400/N1 (MGM101), both strains have the 0.86-kb recombination intermediate at 11% of the total mtDNA. From the observation that disruption of MGM101 does not change the proportion between the circular oligomers and the linear concatemers, it can be concluded that MGM101 does not noticeably affect the level of homologous recombination of the DS/400/N1 genome.

However, although the analysis of Sau3AI-restricted DNAs revealed the presence of the 0.43-kb DS/400/N1 repeats in N1/mgm101 (mgm101::kan), which has an apparent size similar to ones in a wild-type MGM101 culture, the mtDNAs in the two strains may not be identical. When the autoradiograph involving the analysis of unrestricted DNA was overexposed (not illustrated), the oligomers from N1/mgm101 did not appear to have the same gel mobility compared with those from DS400/N1. The DS400/N1 mtDNA in the mgm101 mutant may have been subjected to some minor changes that alter the conformation of the oligomers.

The above observation raised the possibility that inactivation of MGM101 may destabilize the DS400/N1 genome, leading to a rearranged or mutated version that is no longer dependent on MGM101 for its replication. Indeed, when we subcultured N1-U2/mgm101 on GYP medium and examined 24 clones by Southern blotting, it was found that mtDNA could not be detected in 16 clones (66.7%) and that the eight remaining strains (33.3%) displayed variable levels of mtDNA (data not illustrated). The mtDNA also showed variable banding patterns as revealed by hybridization analysis of undigested DNA. It therefore appears that MGM101 is required for maintenance of DS400/N1 mtDNA but possible mutations in this ^- genome can generate molecules that are stably maintained in the absence of Mgm101p.

Involvement of other genes in maintenance of ori/rep-devoid ^- genomes:
The data described above indicate that MGM101 is dispensable for the maintenance of ori/rep-containing ^- genomes but is essential for the propagation of ori/rep-devoid ^- mtDNA. To know whether this feature is unique to MGM101 among the mtDNA maintenance genes, the effects of disrupting ABF2, MHR1, CCE1, and RPO41 on the three nonsuppressive mtDNAs in DS400/N1, E3, and DS400/A3 were examined. As an example, the effects of the disruptions on the DS400/N1 ^- genome are illustrated in Fig 6A and Fig B. As summarized in Fig 6C, a substantial fraction of the three ^- mtDNAs is present in cells disrupted in ABF2, MHR1, and RPO41. As in the case of the HS^-15 genome (Fig 4), strains disrupted in CCE1 show a strong decline in mtDNA. These observations are in sharp contrast to the mgm101 mutants in which the DS400/A3 and E3 ^- genomes are hardly measurable, while a low level of mutated mtDNA derived from the DS400/N1 mtDNA can still be detected (see above). It can therefore be concluded that MGM101 is distinct in that it is dispensable for the replication of a HS ^- mtDNA but is essential for the propagation of the three ori/rep-devoid ^- genomes examined in this study.

View larger version (46K): In this window In a new window Download PPT slide   Figure 6. Effect of gene disruptions on the abundance of the DS400/N1, DS400/A3, and E3 - mtDNAs. (A and B) A representative Southern blot analysis showing the effect of gene disruptions on the DS400/N1 - genome. Total DNA of equal absorbance at 260 nm was analyzed by Southern blot analysis before (A) and after (B) digestion with Sau3AI. The DNA samples are from DS400/N1 (WT) and its abf2, mhr1, cce1, rpo41, and mgm101 disruption derivatives. o and + controls are M2915-6A/o and M2915-6A, respectively. The probe, specific for the DS400/N1 genome, identifies the 0.43-kb repeat unit and the 0.86-kb recombination intermediate after digestion with Sau3AI. (C) Histogram for a comparison of the amount of mtDNA in the strains disrupted in various genes relative to that in the parental strains DS400/N1, DS/400/A3, and E3.

mgm101 and rpo41 double mutants can stably maintain a HS ^- genome:
In light of the results described above, it is evident that Mgm101p participates in a mtDNA replication mechanism essential for the propagation of ori/rep-devoid ^- mtDNA. However, the HS ^- genomes, containing an amplified ori/rep, could be replicated by a distinct mechanism involving transcription by the Rpo41 RNA polymerase. As individual disruption of MGM101 and RPO41 has little effect on the propagation of the HS ^- genomes, the possibility was considered that replication of HS ^- mtDNA may proceed either by an MGM101-based mechanism in the absence of RPO41 or by a RPO41-mediated process in strains lacking MGM101. To test this possibility, we disrupted both MGM101 and RPO41 in HS^-15-carrying ori2. Southern blot analysis of four independent double mutants showed that such strains have a level of HS^-15 ^- mtDNA comparable to that of mgm101 or rpo41 single mutants (not illustrated). Thus, a combination of the two mutations does not have a synergistic effect in destabilizing the HS ^- genome.

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Initiation of mtDNA replication in yeast is still a matter of controversy (SHADEL 1999 ; LECRENIER and FOURY 2000 ). Central to the issue are the roles of ori/rep sequences and the mitochondrial RNA synthesis machinery. Studies have shown that synthesis of a RNA primer from the ori/rep sequences does not play a significant role in the replication of ^- mtDNA (FANGMAN et al. 1990 ). An alternative model, implicating recombination, has been proposed (FANGMAN et al. 1990 ; PREISER et al. 1996 ; MACALPINE et al. 1998 ). However, because the HS ^- petites still retain their hypersuppressiveness in the absence of the mitochondrial RNA polymerase gene RPO41 (LORIMER et al. 1995 ), the possibility was raised that the highly amplified ori/rep sequences might initiate DNA replication by a mechanism distinct from the one used by ⁺ mtDNA and ^- mtDNA devoid of ori/rep. This mechanism would not need transcriptional priming by the Rpo41 RNA polymerase.

In this study we have found that the mitochondrial genome maintenance gene, MGM101, is required for the replication of ⁺ and ori/rep-devoid ^- genomes but is not needed for HS ^- mtDNA. The effect on the maintenance of ⁺ and ori/rep-devoid ^- genomes is likely to be through a direct role of the Mgm101 protein in mtDNA replication. Genes required for the maintenance of ⁺ but not for ^- mtDNA have previously been reported and include ABF2, RPO41, and HMI1, as well as ones involved in mitochondrial protein synthesis (WEISLOGEL and BUTOW 1970 ; MYERS et al. 1985 ). These genes, as described by other investigators and supported by this study, are not essential for the propagation of either HS ^- or ^- mtDNA devoid of ori/rep (FANGMAN et al. 1990 ; VAN DYCK and CLAYTON 1998 ; ZELENAYA-TROITSKAYA et al. 1998 ; SEDMAN et al. 2000 ). As mtDNA maintenance genes that discriminate between these two types of ^- genomes have not been reported, MGM101 is therefore the first gene of this category.

An indication that Mgm101p has a role in replication of mtDNA arises from analysis of the ⁺ genome in the temperature-sensitive mgm101-1 mutant. Upon inactivation of the mutant Mgm101p at the restrictive temperature, it was found that the total amount of mtDNA is almost halved at each cell division. This rapid diminution suggests that mtDNA replication is immediately blocked on the loss of Mgm101 function. In a recent report, it was shown clearly that Mgm101p does not play any appreciable role in mtDNA packaging and segregation/partitioning (MEEUSEN et al. 1999 ). However, by using an in vivo detection technique involving indirect immunofluorescence, these investigators found the incorporation of 5-bromodeoxyuridine into mitochondria when the mgm101-2 mutant was transferred to the restrictive temperature, suggesting that mtDNA synthesis continues in the absence of a functional Mgm101p. It has been suggested that the observed phenomenon may reflect rolling circle replication that is independent of initiation (MEEUSEN et al. 1999 ). However, in view of the present results, the detection of nucleotide incorporation might have arisen from replication of ori/rep-containing ^- mtDNA derived from the ⁺ genome, since replication of such genomes is no longer dependent on MGM101. In support of this notion is the observation that ori/rep-containing ^- mtDNAs occur in 25% of meiotic mgm101 segregants.

The mtDNA replication mechanism, which involves Mgm101p, could be a rolling circle process initiated by recombination (FANGMAN et al. 1990 ; MALESZKA et al. 1991 ; PREISER et al. 1996 ; MACALPINE et al. 1998 ). Recombination-dependent DNA replication has been well documented in Escherichia coli and the bacteriophage T₄ (ASAI et al. 1994 ; for a recent review, see KOWALCZYKOWSKI 2000 ). Following the initiation of recombination, DNA strand invasion by the 3' end of ssDNA allows it to serve as a primer for DNA replication. A recombination-dependent mtDNA replication mechanism in yeast has drawn strong support from the correlation between the level of recombinational intermediates and mtDNA copy number and stability (MACALPINE et al. 1998 ; ZELENAYA-TROITSKAYA et al. 1998 ). However, the adoption of such a mechanism leaves many intriguing questions to be answered. For example, how is a recombination intermediate converted into a replication fork? Is there any specific type of recombination intermediate required for replication initiation or is the one created by homologous recombination sufficient for the loading of the DNA replication machinery? If the Mgm101 protein is involved in mtDNA replication, does it intervene in the formation of a replication-competent recombination intermediate?

Because HS ^- genomes can be replicated in a MGM101-independent manner, it is likely that ori/rep sequences participate in an alternative replication initiation mechanism. The ori/rep sequences are known to contain three GC-rich clusters designated A, B, and C and it has been well documented that GC clusters are highly recombinogenic (DIECKMANN and GANDY 1987 ; ZINN et al. 1988 ; CLARK-WALKER 1989 ). Double-stranded DNA breaks can be readily detected in GC-rich sequences (ZINN et al. 1988 ; VAN DYCK and CLAYTON 1998 ). Thus, one explanation for the initiation of mtDNA replication could be that the ori/rep sequences form structures that prime DNA synthesis. Establishment of these structures would not require Mgm101p or Rpo41p because HS genomes can be stably maintained in rpo41 mgm101 double mutants. Alternatively, the ori/rep sequences may utilize a primase to initiate replication. A mitochondrial primase activity has been previously detected (DESAI et al. 1989 ) but the identity of the encoding gene(s) remains to be established (reviewed by LECRENIER and FOURY 2000 ).

Three ori/rep-devoid ^- genomes have been examined for the effect of MGM101 disruption. It is interesting to find that loss of the E3 ^- genome occurs in MGM101-inactivated cells at a slower rate compared with the ⁺ mtDNA. This observation could reflect the presence of preinitiated replicative circles derived from the highly repetitive ^- genome that causes a lag in mtDNA loss upon MGM101 inactivation. It is also interesting to find, in contrast to the E3 and DS400/A3 ^- mtDNAs where the mitochondrial genomes are rapidly lost upon the inactivation of MGM101, that the DS400/N1 ^- genome can be maintained in some mgm101 cells. Replication of a portion of mtDNA is apparently no longer dependent on Mgm101p, suggesting a possible change has occurred. Although it is known that the DS400/N1 sequence does not contain any recognizable GC clusters (NOBREGA and TZAGOLOFF 1980 ), it is possible that a putative mutation in the ^- genome has created a structure functionally resembling ori/rep that can initiate mtDNA replication in a Mgm101p-independent manner.

In summary, this study suggests that two distinct mtDNA replication initiation mechanisms operate in S. cerevisiae. The synthesis of ⁺ and ori/rep-devoid ^- genomes is dependent on Mgm101p whereas replication of ori/rep-containing ^- mtDNA proceeds in its absence. In this context, the presence of three or four ori/rep sequences at low density in the ⁺ genome may not play any significant role in mtDNA replication. Support for this notion comes from studies of another budding yeast, K. lactis, where mtDNA maintenance is dependent on MGM101 but ori/rep sequences are lacking (CLARK-WALKER and CHEN 1996 ).

	ACKNOWLEDGMENTS

X. M. Zuo is a recipient of an Overseas Postgraduate Research Scholarship from The Australian National University.

Manuscript received March 2, 2001; Accepted for publication January 21, 2002.

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