Creation of ARS activity in yeast through iteration of non-functional sequences

DNA structure directs positioning of the mitochondrial genome packaging protein Abf2p

The mitochondrial genome (mtDNA) is assembled into nucleo-protein structures termed nucleoids and maintained differently compared to nuclear DNA, the involved molecular basis remaining poorly understood. In yeast (Saccharomyces cerevisiae), mtDNA is a ∼80 kbp linear molecule and Abf2p, a double HMG-box protein, packages and maintains it. The protein binds DNA in a non-sequence-specific manner, but displays a distinct 'phased-binding' at specific DNA sequences containing poly-adenine tracts (A-tracts). We present here two crystal structures of Abf2p in complex with mtDNA-derived fragments bearing A-tracts. Each HMG-box of Abf2p induces a 90° bend in the contacted DNA, causing an overall U-turn. Together with previous data, this suggests that U-turn formation is the universal mechanism underlying mtDNA compaction induced by HMG-box proteins. Combining this structural information with mutational, biophysical and computational analyses, we reveal a unique DNA binding mechanism for Abf2p where a characteristic N-terminal flag and helix are crucial for mtDNA maintenance. Additionally, we provide the molecular basis for A-tract mediated exclusion of Abf2p binding. Due to high prevalence of A-tracts in yeast mtDNA, this has critical relevance for nucleoid architecture. Therefore, an unprecedented A-tract mediated protein positioning mechanism regulates DNA packaging proteins in the mitochondria, and in combination with DNA-bending and U-turn formation, governs mtDNA compaction.

Replication of bovine papillomavirus type 1 (BPV-1) DNA in Saccharomyces cerevisiae following infection with BPV-1 virions

Saccharomyces cerevisiae protoplasts exposed to bovine papillomavirus type 1 (BPV-1) virions demonstrated uptake of virions on electron microscopy. S. cerevisiae cells looked larger after exposure to BPV-1 virions, and cell wall regeneration was delayed. Southern blot hybridization of Hirt DNA from cells exposed to BPV-1 virions demonstrated BPV-1 DNA, which could be detected over 80 days of culture and at least 13 rounds of division. Two-dimensional gel analysis of Hirt DNA showed replicative intermediates, confirming that the BPV-1 genome was replicating within S. cerevisiae. Nicked circle, linear, and supercoiled BPV-1 DNA species were observed in Hirt DNA preparations from S. cerevisiae cells infected for over 50 days, and restriction digestion showed fragments hybridizing to BPV-1 in accord with the predicted restriction map for circular BPV-1 episomes. These data suggest that BPV-1 can infect S. cerevisiae and that BPV-1 episomes can replicate in the infected S. cerevisiae cells.

Long range cooperative interactions regulate the initiation of replication in the Tetrahymena thermophila rDNA minichromosome

The Tetrahymena thermophila rDNA exists as a 21 kb palindromic minichromosome with two initiation sites for replication in each half palindrome. These sites localize to the imperfect, repeated 430 bp segments that include the nucleosome-free domains 1 and 2 (D1 and D2). To determine if the D1 and D2 segments act independently or in concert to control initiation, stable DNA transformation assays were performed. Single domain derivatives of the plasmid prD1 failed to support autonomous replication in Tetrahymena. Instead, such constructs propagated exclusively by integration into endogenous rDNA minichromosomes and displayed weak origin activity as detected by 2D gel electrophoresis. D1/D1 and D2/D2 derivatives also transformed Tetrahymena poorly, showing similar replication defects. Hence, the D1 and D2 segments are functionally non-redundant and cooperate rather than compete to control initiation. The observed replication defect was greatly reduced in a plasmid derivative that undergoes palindrome formation in Tetrahymena, suggesting that a compensatory mechanism overcomes this replication block. Finally, using a transient replication assay, we present evidence that phylogenetically-conserved type I elements directly regulate DNA replication. Taken together, our data support a model in which cooperative interactions between dispersed elements coordinately control the initiation of DNA replication.

Short DNA fragments without sequence similarity are initiation sites for replication in the chromosome of the yeast Yarrowia lipolytica

We have previously shown that both a centromere (CEN) and a replication origin are necessary for plasmid maintenance in the yeast Yarrowia lipolytica (). Because of this requirement, only a small number of centromere-proximal replication origins have been isolated from Yarrowia. We used a CEN-based plasmid to obtain noncentromeric origins, and several new fragments, some unique and some repetitive sequences, were isolated. Some of them were analyzed by two-dimensional gel electrophoresis and correspond to actual sites of initiation (ORI) on the chromosome. We observed that a 125-bp fragment is sufficient for a functional ORI on plasmid, and that chromosomal origins moved to ectopic sites on the chromosome continue to act as initiation sites. These Yarrowia origins share an 8-bp motif, which is not essential for origin function on plasmids. The Yarrowia origins do not display any obvious common structural features, like bent DNA or DNA unwinding elements, generally present at or near eukaryotic replication origins. Y. lipolytica origins thus share features of those in the unicellular Saccharomyces cerevisiae and in multicellular eukaryotes: they are discrete and short genetic elements without sequence similarity.

Conserved cis- and trans-acting determinants for replication initiation and regulation of replication fork movement in tetrahymenid species

The rDNA minichromosomes of Tetrahymena thermophila and Tetrahymena pyriformis share a high degree of sequence similarity and structural organization. The T.thermophila 5' non-transcribed spacer (5' NTS) is sufficient for replication and contains three repeated sequence elements that are conserved in T.pyriformis , including type I elements, the only known determinant for replication control. To assess the role of conserved sequences in replication control, structural and functional studies were performed on T.pyriformis rDNA. Similar to T.thermophila , replication initiates exclusively in the 5' NTS, localizing to a 900 bp segment. Elongating replication forks arrest transiently at one site which bears strong similarity to a tripartite sequence element present at fork arrest sites in T.thermophila rDNA. An in vitro type I element binding activity indistinguishable from the T.thermophila protein, ssA-TIBF, was detected in T.pyriformis extracts. The respective TIBF proteins bind with comparable affinity to type I elements from both species, suggesting that in vivo recognition could cross species boundaries. Despite these similarities, the T.pyriformis 5' NTS failed to support replication in transformed T.thermophila cells, suggesting a more complex genetic organization than previously realized.

A high-copy-number ADE2-bearing plasmid for transformation of Candida glabrata

The Candida glabrata ADE2 gene encoding aminoimidazole ribonucleotide (AIR) carboxylase (EC 4.1.1.21) was isolated by complementation of the ade2-1 mutation in Saccharomyces cerevisiae. The predicted amino acid (aa) sequence is 75% identical to that of S. cerevisiae. Integrative transformation was used to produce a C. glabrata strain bearing a deletion of ADE2 coding sequences. A high-copy-number shuttle vector bearing the ADE2 gene was constructed and contains a fragment of S. cerevisiae mitochondrial (mt) DNA that confers the ability to replicate autonomously in C. glabrata.

Identification of autonomously replicating sequence (ARS) elements in eukaryotic cells

Autonomously replicating sequence (ARS) elements were first identified in the budding yeast Saccharomyces cerevisiae as chromosomal DNA fragments that promoted high frequency of transformation and extrachromosomal maintenance of plasmid DNA. These specific sequence elements were subsequently shown to function as origins of DNA replication. Detailed analysis of the structure and function of ARS elements has been limited largely to S. cerevisiae and more recently the fission yeast Schizosaccharomyces pombe. Characterization of ARS activity in other eukaryotes is far less complete. Here we describe the ARS assay developed in yeast and its application to the study of origin function in other eukaryotes. Other available methods for detecting autonomous replication in these systems are also presented.

Tandem repeats of the 5' non-transcribed spacer of Tetrahymena rDNA function as high copy number autonomous replicons in the macronucleus but do not prevent rRNA gene dosage regulation

The rRNA genes in the somatic macronucleus of Tetrahymena thermophila are normally on 21 kb linear palindromic molecules (rDNA). We examined the effect on rRNA gene dosage of transforming T.thermophila macronuclei with plasmid constructs containing a pair of tandemly repeated rDNA replication origin regions unlinked to the rRNA gene. A significant proportion of the plasmid sequences were maintained as high copy circular molecules, eventually consisting solely of tandem arrays of origin regions. As reported previously for cells transformed by a construct in which the same tandem rDNA origins were linked to the rRNA gene [Yu, G.-L. and Blackburn, E. H. (1990) Mol. Cell. Biol., 10, 2070-2080], origin sequences recombined to form linear molecules bearing several tandem repeats of the origin region, as well as rRNA genes. The total number of rDNA origin sequences eventually exceeded rRNA gene copies by approximately 20- to 40-fold and the number of circular replicons carrying only rDNA origin sequences exceeded rRNA gene copies by 2- to 3-fold. However, the rRNA gene dosage was unchanged. Hence, simply monitoring the total number of rDNA origin regions is not sufficient to regulate rRNA gene copy number.

Kluyveromyces marxianus small DNA fragments contain both autonomous replicative and centromeric elements that also function in Kluyveromyces lactis

Two fragments containing both an autonomous replicating sequence (ARS) and a centromere have been isolated and sequenced from the yeast Kluyveromyces marxianus. The ARS and centromeric core sequences are only 500 bp apart, but ARS activity could be separated from the centromeric sequences. Centromeric sequences are organized in a similar way to those of budding yeasts: two well-conserved elements: CDEI (5' TCACGTG 3') and CDEIII (5' TNTTCCGAAAGTWAAA 3'), are separated by a 165 bp AT-rich (+/- 90%) CDEII element whose length is twice that of Saccharomyces cerevisiae CDEII but almost identical to that of K. lactis. The ARS-core consensus sequence (5' TTTATTGTT 3') is also similar to that of K. lactis. Both ARS and centromeric elements function in this strain, albeit inefficiently, but not in S. cerevisiae. A third ARS-containing fragment with a different organization has been isolated and sequenced.

Mutational analysis of centromeric DNA elements of Kluyveromyces lactis and their role in determining the species specificity of the highly homologous centromeres from K. lactis and Saccharomyces cerevisiae

The centromere of Kluyveromyces lactis was delimited to a region of approximately 280 bp, encompassing KlCDEI, II, and III. Removal of 6 bp from the right side of KlCDEIII plus flanking sequences abolished centromere function, and removal of 5 bp of KlCDEI and flanking sequences resulted in strongly reduced centromere function. Deletions of 20-80 bp from KlCDEII resulted in a decrease in plasmid stability, indicating that KlCDEII must have a certain length for proper centromere function. Centromeres of K. lactis do not function in Saccharomyces cerevisiae and vice versa. Adapting the length of KlCDEII to that of ScCDEII did not improve KlCEN function in S. cerevisiae, while doubling the ScCDEII length did not improve ScCEN function in K. lactis. Thus the difference in CDEII length is not in itself responsible for the species specificity of the centromeres from each of the two species of budding yeast. A chimeric K. lactis centromere with ScCDEIII instead of KlCDEIII was no longer functional in K. lactis, but did improve plasmid stability in S. cerevisiae, although to a much lower level than a wild-type ScCEN. This indicates that the exact CDEIII sequence is important, and suggests that the flanking AT-rich CDEII has to conform to specific sequence requirements.

Autonomous replication of bacterial DNA plasmid oligomers in Leishmania

Extrachromosomal amplicons are frequently observed in drug-resistant Leishmania. A dominant selectable marker, the neomycin phosphotransferase gene, was introduced by gene targeting in a circular amplicon derived from the H locus of Leishmania in a mutant cell. This recombinant amplicon was isolated and transfected in a wild-type cell. The amplicon was kept in the wild-type cells, provided the selective pressure was maintained, suggesting that it was capable of autonomous replication. Novel Leishmania expression vectors suited for stable transfections were made to isolate, by a high transformation assay, the putative origin of replication in the amplicons. However, these plasmids, which did not contain a single Leishmania nucleotide, were found as extrachromosomal circular oligomers in Leishmania transfectants. Their relative stability, in addition to changes in their methylation pattern, indicated that these plasmids were most likely replicating. No specific sequences seem to be required for replication (and expression) in Leishmania, therefore precluding the isolation of origins of replication by genetic transformation.

Intergenic DNA and the sequence requirements for replication initiation in eukaryotes

Replication in eukaryotes initiates at many origins per chromosome. The locations of most of these origins appear to be restricted to intergenic spacers. In this review, I propose that the sequence dependence of initiation seen in lower eukaryotes may be a by-product of the small size of intergenic sequences and may not reflect a general requirement of the mechanisms that control the initiation of replication.

Developmentally regulated processing and replication of the Tetrahymena rDNA minichromosome

The ribosomal DNA locus of Tetrahymena thermophila undergoes a dramatic series of developmentally regulated processing events to generate the amplified rDNA minichromosome during formation of the somatic macronucleus. DNA transformation and classical genetic approaches have identified cis-acting elements that regulate rDNA processing in the developing macronucleus and subsequent vegetative rDNA maintenance.

Colocalization of centromeric and replicative functions on autonomously replicating sequences isolated from the yeast Yarrowia lipolytica

Two sequences (ARS18 and ARS68) displaying autonomous replication activity were previously cloned in the yeast Yarrowia lipolytica. The smallest fragment (1-1.3 kb) required for extrachromosomal replication of a plasmid is significantly larger in Y. lipolytica than in Saccharomyces cerevisiae. Neither autonomously replicating sequence (ARS) is homologous with known ARS or centromere (CEN) consensus sequences. They share short regions of sequence similarity with each other. These ARS fragments also contain Y. lipolytica centromeres: (i) integration of marker genes at the ARS loci results in a CEN-linked segregation of the markers, (ii) an ARS on a plasmid largely maintains sister chromatid attachment in meiosis I, and (iii) integration of these sequences at the LEU2 locus leads to chromosome breakage. Deletions performed on ARS18 show that CEN and ARS functions can be physically separated, but both are needed to establish a replicating plasmid.

Autonomous replication in human cells of multimers of specific human and bacterial DNA sequences

Using modules of a specific 2,712-bp human DNA sequence and a specific 2,557-bp Escherichia coli DNA sequence, we created plasmids containing between 1 and 12 modules of single or chimeric sequence composition and tested them in human cells for their autonomous replication ability. We found that replication efficiency per generation increased with successive addition of human modules, to essentially 100% by six copies. Although a single copy of the bacterial module had negligible replication ability, the replication efficiency per generation of 12 bacterial modules was 66%. Chimeras composed of human and bacterial modules displayed intermediate replication levels. We also used two-dimensional gel electrophoresis to physically map where replication initiated on a half human-half E. coli plasmid. Our results suggest that autonomous replication in human cells is stimulated by simple sequence features which occur frequently in human DNA but are more rare in bacterial DNA.

Autonomous replication in human cells of multimers of specific human and bacterial DNA sequences

Using modules of a specific 2,712-bp human DNA sequence and a specific 2,557-bp Escherichia coli DNA sequence, we created plasmids containing between 1 and 12 modules of single or chimeric sequence composition and tested them in human cells for their autonomous replication ability. We found that replication efficiency per generation increased with successive addition of human modules, to essentially 100% by six copies. Although a single copy of the bacterial module had negligible replication ability, the replication efficiency per generation of 12 bacterial modules was 66%. Chimeras composed of human and bacterial modules displayed intermediate replication levels. We also used two-dimensional gel electrophoresis to physically map where replication initiated on a half human-half E. coli plasmid. Our results suggest that autonomous replication in human cells is stimulated by simple sequence features which occur frequently in human DNA but are more rare in bacterial DNA.

Titration of replication activity by increasing ARS dosage in yeast plasmids

The rep1 region of the yeast mitochondrial genome, a putative replication origin, contains a weak autonomously replicating sequence (ARS). Nucleotide-sequence and deletion analyses have identified two 11-base pair ARS consensus sequences, numerous near matches to the ARS core, and a region of curvature that may contribute to ARS function. Based on the amplified nature of petite-derivative mitochondrial DNA encompassing this locus, we have constructed plasmids containing an increasing dosage of ARS elements. The rep1 ARS element can have an additive effect on plasmid stability when present either as a tandem dimer or as an unlinked pair. However, the presence of a third ARS copy does not further enhance plasmid stability. These results indicate that measurable dosage effects can be defined only in circumstances where weak ARS elements are employed, and that plasmid maintenance within yeast cells is saturable and varies among the different sequences promoting replication.

A synthetic silencer mediates SIR-dependent functions in Saccharomyces cerevisiae

Copies of the mating-type genes are present at three loci on chromosome III of the yeast Saccharomyces cerevisiae. The genes at the MAT locus are transcribed, whereas the identical genes at the silent loci, HML and HMR, are not transcribed. Several genes, including the four SIR genes, and two sites, HMR-E and HMR-I, are required for repression of transcription at the HMR locus. Three elements have been implicated in the function of the HMR-E silencer: a binding site for the RAP1 protein, a binding site for the ABF1 protein, and an 11-bp consensus sequence common to nearly all autonomously replicating sequence (ARS) elements (putative origins of DNA replication). RAP1 and ABF1 binding sites of different sequence than those found at HMR-E were joined with an 11-bp ARS consensus sequence to form a synthetic silencer. The synthetic silencer was able to repress transcription of the HMRa1 gene, confirming that binding sites for RAP1 and ABF1 and the 11-bp ARS consensus sequence were the functional components of the silencer in vivo. Mutations in the ABF1 binding site or in the ARS consensus sequence of the synthetic silencer caused nearly complete derepression of transcription at HMR. The ARS consensus sequence mutation also eliminated the ARS activity of the synthetic silencer. These data suggested that replication initiation at the HMR-E silencer was required for establishment of the repressed state at the HMR locus.

A synthetic silencer mediates SIR-dependent functions in Saccharomyces cerevisiae

Copies of the mating-type genes are present at three loci on chromosome III of the yeast Saccharomyces cerevisiae. The genes at the MAT locus are transcribed, whereas the identical genes at the silent loci, HML and HMR, are not transcribed. Several genes, including the four SIR genes, and two sites, HMR-E and HMR-I, are required for repression of transcription at the HMR locus. Three elements have been implicated in the function of the HMR-E silencer: a binding site for the RAP1 protein, a binding site for the ABF1 protein, and an 11-bp consensus sequence common to nearly all autonomously replicating sequence (ARS) elements (putative origins of DNA replication). RAP1 and ABF1 binding sites of different sequence than those found at HMR-E were joined with an 11-bp ARS consensus sequence to form a synthetic silencer. The synthetic silencer was able to repress transcription of the HMRa1 gene, confirming that binding sites for RAP1 and ABF1 and the 11-bp ARS consensus sequence were the functional components of the silencer in vivo. Mutations in the ABF1 binding site or in the ARS consensus sequence of the synthetic silencer caused nearly complete derepression of transcription at HMR. The ARS consensus sequence mutation also eliminated the ARS activity of the synthetic silencer. These data suggested that replication initiation at the HMR-E silencer was required for establishment of the repressed state at the HMR locus.

A close relative of the nuclear, chromosomal high-mobility group protein HMG1 in yeast mitochondria

ABF2 (ARS-binding factor 2), a small, basic DNA-binding protein that binds specifically to the autonomously replicating sequence ARS1, is located primarily in the mitochondria of the yeast Saccharomyces cerevisiae. The abundance of ABF2 and the phenotype of abf2- null mutants argue that this protein plays a key role in the structure, maintenance, and expression of the yeast mitochondrial genome. The predicted amino acid sequence of ABF2 is closely related to the high-mobility group proteins HMG1 and HMG2 from vertebrate cell nuclei and to several other DNA-binding proteins. Additionally, ABF2 and the other HMG-related proteins are related to a globular domain from the heat shock protein hsp70 family. ABF2 interacts with DNA both nonspecifically and in a specific manner within regulatory regions, suggesting a mechanism whereby it may aid in compacting the mitochondrial genome without interfering with expression.

Mapping of the ARS-like activity and transcription initiation sites in the non-canonical yeast mitochondrial ori 6 region

The insert-containing, non-canonical ori 6 region of yeast mitochondrial DNA of Saccharomyces cerevisiae was dissected into 15 different segments that were ligated to the integrative yeast vector YIp5. Six recombinant plasmids exhibited replicative ability in yeast and carried consensus sequences similar to the previously described 11 bp motifs active as autonomous replication sequences (ARS). In addition, all active constructions carry one or more of the characteristic GC-rich domains A, B or C present in the ori 6 region, thus confirming and expanding the study of Blanc (Gene 30 (1984) 47-61) with the canonical ori 5. Also a new transcriptional origin is activated in the ori 6 region, apparently circumventing a disruption by insertion of a GC-rich sequence that, in this ori, removes the mitochondrial promoter usually present next to the C element. The ARS-positive constructions correspond to the retained segments of spontaneous well-characterized suppressive or neutral petite genomes that contain segments of the ori sequence.

The DNA unwinding element in a yeast replication origin functions independently of easily unwound sequences present elsewhere on a plasmid

We have previously identified a DNA unwinding element (DUE) in autonomously replicating sequences (ARSs) and demonstrated a correlation between single-strand-specific nuclease hypersensitivity of the DUE and ARS-mediated plasmid replication in yeast. The DUE in the H4 ARS is the most easily unwound sequence in a supercoiled DNA molecule, in the context of the Ylp5 plasmid. To determine whether sequences which are more readily unwound than the ARS can influence replication activity, we have inserted such sequences, called 'torsional sinks', into the plasmids at a site distal to the ARS. We show that the torsional sink sequences effect reduction or elimination of the nuclease hypersensitivity of a variety of H4 ARS derivatives. However, we detect no difference in the in vivo replication activity of an individual ARS plasmid with or without a torsional sink. Thus, the function of the DUE in a yeast replication origin is unaffected by easily unwound sequences present elsewhere on the same plasmid.

Mutational analysis of the consensus sequence of a replication origin from yeast chromosome III

Yeast autonomously replicating sequence (ARS) elements contain an 11-base-pair core consensus sequence (5'-[A/T]TTTAT[A/G]TTT[A/T]-3') that is required for function. The contribution of each position within this sequence to ARS activity was tested by creating all possible single-base mutations within the core consensus sequence of ARS307 (formerly called the C2G1 ARS) and testing their effects on high-frequency transformation and on plasmid stability. Of the 33 mutations, 22 abolished ARS function as measured by high-frequency transformation, 7 caused more than twofold reductions in plasmid stability, and 4 had no effect on plasmid stability. Mutations that reduced or abolished ARS activity occurred at each position in the consensus sequence, demonstrating that each position of this sequence contributes to ARS function. Of the four mutations that had no effect on ARS activity, three created alternative perfect matches to the core consensus sequence, demonstrating that the alternate bases allowed by the consensus sequence are, indeed, interchangeable. In addition, a change from T to C at position 6 did not perturb wild-type efficiency. To test whether the essential region extends beyond the 11-base-pair consensus sequence, the effects on plasmid stability of point mutations one base 3' to the T-rich strand of the core consensus sequence (position 12) and deletion mutations that altered bases 5' to the T-rich strand of the core consensus sequence were examined. An A at position 12 or the removal of three T residues 5' to the core consensus sequence severely diminished ARS efficiency, showing that the region required for full ARS efficiency extends beyond the core consensus sequence in both directions.

Mutational analysis of the consensus sequence of a replication origin from yeast chromosome III

Yeast autonomously replicating sequence (ARS) elements contain an 11-base-pair core consensus sequence (5'-[A/T]TTTAT[A/G]TTT[A/T]-3') that is required for function. The contribution of each position within this sequence to ARS activity was tested by creating all possible single-base mutations within the core consensus sequence of ARS307 (formerly called the C2G1 ARS) and testing their effects on high-frequency transformation and on plasmid stability. Of the 33 mutations, 22 abolished ARS function as measured by high-frequency transformation, 7 caused more than twofold reductions in plasmid stability, and 4 had no effect on plasmid stability. Mutations that reduced or abolished ARS activity occurred at each position in the consensus sequence, demonstrating that each position of this sequence contributes to ARS function. Of the four mutations that had no effect on ARS activity, three created alternative perfect matches to the core consensus sequence, demonstrating that the alternate bases allowed by the consensus sequence are, indeed, interchangeable. In addition, a change from T to C at position 6 did not perturb wild-type efficiency. To test whether the essential region extends beyond the 11-base-pair consensus sequence, the effects on plasmid stability of point mutations one base 3' to the T-rich strand of the core consensus sequence (position 12) and deletion mutations that altered bases 5' to the T-rich strand of the core consensus sequence were examined. An A at position 12 or the removal of three T residues 5' to the core consensus sequence severely diminished ARS efficiency, showing that the region required for full ARS efficiency extends beyond the core consensus sequence in both directions.