Purification and properties of mitochondrial deoxyribonucleic acid dependent ribonucleic acid polymerase from ovaries of Xenopus laevis

Germ-line mitochondria exhibit suppressed respiratory activity to support their accurate transmission to the next generation

Mitochondria are accurately transmitted to the next generation through a female germ cell in most animals. Mitochondria produce most ATP, accompanied by the generation of reactive oxygen species (ROS). A specialized mechanism should be necessary for inherited mitochondria to escape from impairments of mtDNA by ROS. Inherited mitochondria are named germ-line mitochondria, in contrast with somatic ones. We hypothesized that germ-line mitochondria are distinct from somatic ones. The protein profiles of germ-line and somatic mitochondria were compared, using oocytes at two different stages in Xenopus laevis. Some subunits of ATP synthase were at a low level in germ-line mitochondria, which was confirmed immunologically. Ultrastructural histochemistry using 3,3'-diaminobenzidine (DAB) showed that cytochrome c oxidase (COX) activity of germ-line mitochondria was also at a low level. Mitochondria in one oocyte were segregated into germ-line mitochondria and somatic mitochondria, during growth from stage I to VI oocytes. Respiratory activity represented by ATP synthase expression and COX activity was shown to be low during most of the long gametogenetic period. We propose that germ-line mitochondria that exhibit suppressed respiration alleviate production of ROS and enable transmission of accurate mtDNA from generation to generation.

Two forms of mitochondrial DNA ligase III are produced in Xenopus laevis oocytes

Full-length cDNAs for DNA ligase IV and the alpha and beta isoforms of DNA ligase III were cloned from Xenopus laevis to permit study of the genes encoding mitochondrial DNA ligase. DNA ligase III alpha and III beta share a common NH(2) terminus that encodes a mitochondrial localization signal capable of targeting green fluorescent protein to mitochondria while the NH(2) terminus of DNA ligase IV does not. Reverse transcriptase-polymerase chain reaction analyses with adult frog tissues demonstrate that while DNA ligase III alpha and DNA ligase IV are ubiquitously expressed, DNA ligase III beta expression is restricted to testis and ovary. Mitochondrial lysates from X. laevis oocytes contain both DNA ligase III alpha and III beta but no detectable DNA ligase IV. Gel filtration, sedimentation, native gel electrophoresis, and in vitro cross-linking experiments demonstrate that mtDNA ligase III alpha exists as a high molecular weight complex. We discuss the possibility that DNA ligase III alpha exists in mitochondria in association with novel mitochondrial protein partners or as a homodimer.

Expression of cloned cDNA for the human mitochondrial RNA polymerase in Escherichia coli and purification

A full-length cDNA for the mature, mitochondrial form of human mitochondrial RNA polymerase was cloned and expressed under the control of T5 or tac promoter in Escherichia coli. The cDNA was efficiently expressed at 37 degrees C, but virtually all the polymerase produced was insoluble, and renaturation of the inclusion bodies was unsuccessful. When the cells were grown at 25 degrees C, however, a portion of approximately 10% was soluble and active. The protein was purified 100-fold from the soluble lysates to homogeneity by two-step chromatography using Ni-nitrilotriacetic acid-Sepharose and heparin-agarose columns, as an N-terminal histidine tag attached and as the tag cleaved away. The purified polymerases with and without the histidine tag were both active in RNA polymerization in vitro as measured with poly(dA-dT) template, and specific activity was 140,000 units/mg. The purified enzyme has the same biochemical properties as the polymerase fraction partially purified from the human mitochondria, except for the promoter-specific activity that was not observed with the purified polymerase in the presence of mitochondrial transcription factor A. Additional factor(s) and/or mammalian-specific or regulatory modification(s) of the polymerase should be necessary for promoter-specific transcription.

Distinct roles for two purified factors in transcription of Xenopus mitochondrial DNA

Transcription of Xenopus laevis mitochondrial DNA (xl-mtDNA) by the mitochondrial RNA polymerase requires a dissociable factor. This factor was purified to near homogeneity and identified as a 40-kDa protein. A second protein implicated in the transcription of mtDNA, the Xenopus homolog of the HMG box protein mtTFA, was also purified to homogeneity and partially sequenced. The sequence of a cDNA clone encoding xl-mtTFA revealed a high degree of sequence similarity to human and Saccharomyces cerevisiae mtTFA. xl-mtTFA was not required for basal transcription from a minimal mtDNA promoter, and this HMG box factor could not substitute for the basal factor, which is therefore designated xl-mtTFB. An antibody directed against the N terminus of xl-mtTFA did not cross-react with xl-mtTFB. xl-mtTFA is an abundant protein that appears to have at least two functions in mitochondria. First, it plays a major role in packaging mtDNA within the organelle. Second, DNase I footprinting experiments identified preferred binding sites for xl-mtTFA within the control region of mtDNA next to major mitochondrial promoters. We show that binding of xl-mtTFA to a site separating the two clusters of bidirectional promoters selectively stimulates specific transcription in vitro by the basal transcription machinery, comprising mitochondrial RNA polymerase and xl-mtTFB.

Purification of Xenopus laevis mitochondrial RNA polymerase and identification of a dissociable factor required for specific transcription

The Xenopus laevis mitochondrial RNA (mtRNA) polymerase was purified to near homogeneity with an overall yield approaching 50%. The major polypeptides in the final fraction were a doublet of proteins of approximately 140 kilodaltons that copurified with the mtRNA polymerase activity. It appeared likely that the smaller polypeptide is a breakdown product of the larger one. The highly purified polymerase was active in nonspecific transcription but required a dissociable factor for specific transcription of X. laevis mtDNA. The factor could be resolved from mtRNA polymerase by hydrophobic chromatography and had a sedimentation coefficient of 3.0 S. The transcription factor eluted from both the hydrophobic column and a Mono Q anion-exchange column as a single symmetrical peak. The mtRNA polymerase and this factor together are necessary and sufficient for active transcription from four promoters located in a noncoding region of the mtDNA genome between the gene for tRNA(Phe) and the displacement loop.

Purification of Xenopus laevis mitochondrial RNA polymerase and identification of a dissociable factor required for specific transcription

The Xenopus laevis mitochondrial RNA (mtRNA) polymerase was purified to near homogeneity with an overall yield approaching 50%. The major polypeptides in the final fraction were a doublet of proteins of approximately 140 kilodaltons that copurified with the mtRNA polymerase activity. It appeared likely that the smaller polypeptide is a breakdown product of the larger one. The highly purified polymerase was active in nonspecific transcription but required a dissociable factor for specific transcription of X. laevis mtDNA. The factor could be resolved from mtRNA polymerase by hydrophobic chromatography and had a sedimentation coefficient of 3.0 S. The transcription factor eluted from both the hydrophobic column and a Mono Q anion-exchange column as a single symmetrical peak. The mtRNA polymerase and this factor together are necessary and sufficient for active transcription from four promoters located in a noncoding region of the mtDNA genome between the gene for tRNA(Phe) and the displacement loop.

Accurate in vitro transcription of Xenopus laevis mitochondrial DNA from two bidirectional promoters

The mitochondrial RNA polymerase from Xenopus laevis oocytes was partially purified by heparin-Sepharose chromatography and phosphocellulose chromatography. This RNA polymerase preparation specifically initiated the transcription of X. laevis mitochondrial DNA (mtDNA) from two bidirectional promoters contained within a 123-base-pair segment of the mtDNA between the heavy-strand replication origin and the rRNA cistrons. Transcription in vitro initiated from precisely the same start sites previously mapped as initiation sites for transcription in vivo. At each of the four sites, initiation occurred within a conserved nucleotide sequence, ACPuTTATA. This consensus sequence is not related to promoters for transcription of human mtDNA.

Accurate in vitro transcription of Xenopus laevis mitochondrial DNA from two bidirectional promoters

The mitochondrial RNA polymerase from Xenopus laevis oocytes was partially purified by heparin-Sepharose chromatography and phosphocellulose chromatography. This RNA polymerase preparation specifically initiated the transcription of X. laevis mitochondrial DNA (mtDNA) from two bidirectional promoters contained within a 123-base-pair segment of the mtDNA between the heavy-strand replication origin and the rRNA cistrons. Transcription in vitro initiated from precisely the same start sites previously mapped as initiation sites for transcription in vivo. At each of the four sites, initiation occurred within a conserved nucleotide sequence, ACPuTTATA. This consensus sequence is not related to promoters for transcription of human mtDNA.

Characterization of phage-Xp10-coded RNA polymerase

A bacteriophage-coded RNA polymerase was isolated from bacteriophage-Xp10-infected Xanthomonas campestris pv. oryzae. The enzyme was purified to homogeneity through precipitation by poly(ethylene glycol) and chromatography on DEAE-cellulose, heparin--Sepharose 4B and blue-dextran--Sepharose 4B. It is composed of a single polypeptide of Mr96,000. The enzyme preferred denatured Xp10 DNA, calf thymus DNA, host bacterium DNA and poly[d(A-T)] as templates. The optimal concentration of MgCl2 is 16 mM. The optimal temperature and pH are 37 degrees C and 8.0, respectively. The Km of ATP is 26 microM. DNA, MgCl2 and four ribonucleotides were required for enzyme activity. If ATP alone was present, half of the Xp10 RNA polymerase activity was retained. The enzyme activity was inhibited by KCl, spermidine, actinomycin D, heparin, blue dextran and ethidium bromide; it was resistant to rifampicin and streptovaricin. N-Ethylmaleimide did not affect the enzyme activity. The transcription site and product of Xp10 RNA polymerase upon Xp10 DNA were analyzed by DNA/RNA hybridization and polyacrylamide-agarose composite gel electrophoresis. The enzyme could specifically transcribe the late region of Xp10 genome and produce two RNA bands.

Mitochondrial DNA-binding proteins that bind preferentially to supercoiled molecules containing the D-loop region of Xenopus laevis mtDNA

From the bulk of the Xenopus laevis mitochondrial proteins insoluble in 1% Triton X-100 + 1M NaCl, we have isolated, by DNA-cellulose chromatography, a protein fraction enriched in DNA-binding proteins. This fraction contains proteins showing a specific affinity for supercoiled DNA molecules containing the mitochondrial DNA displacement-loop region, as measured by filter binding and competition assays.

Transcription of mitochondrial DNA

While mitochondrial DNA (mtDNA) is the simplest DNA in nature, coding for rRNAs and tRNAs, results of DNA sequence, and transcript analysis have demonstrated that both the synthesis and processing of mitochondrial RNAs involve remarkably intricate events. At one extreme, genes in animal mtDNAs are tightly packed, both DNA strands are completely transcribed (symmetric transcription), and the appearance of specific mRNAs is entirely dependent on processing at sites signalled by the sequences of the tRNAs, which abut virtually every gene. At the other extreme, gene organization in yeast (Saccharomyces) is anything but compact, with long stretches of AT-rich DNA interspaced between coding sequences and no obvious logic to the order of genes. Transcription is asymmetric and several RNAs are initiated de novo. Nevertheless, extensive RNA processing occurs due largely to the presence of split genes. RNA splicing is complex, is controlled by both mitochondrial and nuclear genes, and in some cases is accompanied by the formation of RNAs that behave as covalently closed circles. The present article reviews current knowledge of mitochondrial transcription and RNA processing in relation to possible mechanisms for the regulation of mitochondrial gene expression.

A new RNA polymerase and in vitro transcription of the origin of replication from rat mitochondrial DNA

A new RNA polymerase was found in a rat mitochondrial extract. This enzyme showed strong template preference in vitro for the supercoiled recombinant plasmid consisting of pBR322 and the D-loop region of rat mtDNA carrying the origin of heavy-strand replication. The main products synthesized by the D-loop region were two RNAs of different sizes. Both of these products were light-strand products transcribed from the region upstream from the origin of replication. This specific transcription is discussed in relation to initiation of primer RNA synthesis for heavy-strand replication of rat mtDNA.

DNA primase activity associated with DNA polymerase alpha from Xenopus laevis ovaries

One of the two forms of DNA polymerase alpha from ovaries of the frog Xenopus laevis catalyzed ribonucleoside triphosphate-dependent DNA synthesis on single-stranded circular fd phage DNA templates. DNA synthesis was dependent on ATP and added template. CTP, GTP, and UTP stimulated DNA synthesis but were not required and could not substitute for ATP. DNA synthesis was not inhibited by alpha-amanitin. Neither poly(dT) nor double-stranded DNA served as template. Analysis of [32P]-dTMP-labeled product by neutral and alkaline agarose gel electrophoresis showed that 0.1- to 1-kilobase DNA fragments (average size of approximately equal to 0.25 kilobase) were synthesized. The fragments were not covalently linked to the template. Either [alpha-32P]NMP, [gamma-32P]ATP, or [gamma-32P]GTP were incorporated also into the product. Analysis of the product after hydrolysis by KOH, alkaline phosphatase, or bacteriophage T4 3' leads to 5' exonuclease showed the presence of a small oligoribonucleotide primer at the 5' end of the newly synthesized DNA. NTP-dependent DNA-synthesizing activity copurified on six columns and cosedimented during glycerol gradient centrifugation with one form of DNA polymerase alpha activity but not with the other form. These results suggest that DNA primase activity is associated with one of the two forms of X. laevis DNA polymerase alpha.

Some properties of rat liver mitochondrial RNA polymerase

A rapid method suitable for purifying large amounts of mitochondria from rat liver using isopycnic zonal centrifugation is described. The RNA polymerase isolated from the purified mitochrondria was found associated with one peak when resolved by DEAE Sephadex chromatography. The enzyme was next fractionated on a phosphocellulose column followed by glycerol gradient centrifugation. A 600-fold purification was achieved when the enzyme was finally filtered through agarose gel. This final enzyme fraction consisted of one polypeptide chain as shown by polyacrylamide gel electrophoresis profiles. The enzyme has a greater preference for poly [d(A-T)] templates than for rat liver mitochondrial DNA. Inhibition of the enzyme activity required high concentrations of the inhibitors. The resistance of the enzyme to alpha-amanitin indicated that there was no contamination from nuclear RNA polymerase II. The conclusion is drawn that the mitochondrial RAN polymerase activity is associated with a single polypeptide.

The isolation and properties of a DNA-directed RNA polymerase from yeast mitochondria

A method is described for the rapid isolation of yeast mitochondrial DNA-directed RNA polymerase. The enzyme obtained had a specific activity of 1.56 nmol UMP incorporated per mg protein in 20 min at 37 degrees C, and is some 95% pure. This purified enzyme upon polyacrylamide gel elctrophoresis consists of a single polypeptide of 68 000 mol. wt. However, the enzyme forms aggregates easily which are affected by ionic strength, an increase decreasing the apparent molecular weight of the aggregates. This property also explains the presence of two peaks of activity upon DEAE-cellulose chromatography. The purified enzyme is still sensitive to rifamycin and to a number of rifamycin derivatives. The enzyme's sensitivity to rifamycin and rifamycin derivatives was compared with Escherichia coli and yeast nuclear RNA polymerases.

Contemporaneous isolation of deoxyribonucleic acid-dependent ribonucleic acid polymerase and poly(A) polymerase from rat liver mitochondria

1. Poly(A) polymerase and DNA-dependent RNA polymerase from rat liver mitochondria can be completely separated by using two different chromatographic procedures. 2. Poly(A) polymerase can only incorporate ATP into acid-insoluble material and strongly depends on the addition of an endogenous factor (probably containing a mixture of oligoribonucleotides), but it is not stimulated by DNA. 3. RNA polymerase is fully DNA-dependent and rifampicin-sensitive, but was not stimulated by the endogenous factor mentioned above. 4. The chromatographic behaviour of the two enzymes, together with the properties described, suggest that they represent two different protein molecules.

In vivo and in vitro effects of rifampicin and streptolydigin on transcription of Kluyveromyces lactis in the presence of nystatin

Rifampicin and streptolydigin, if used in conjunction with nystatin, depress the growth of Kluyveromyces lactis. The incorporation of labeled leucine into protein is inhibited by nystatin whereas the incorporation of labeled uracil into RNA is inhibited by rifampicin in nystatin-treated cells. In order to study the mechanism of inhibition of RNA synthesis we purified by DEAE-Sephadex column chromatography four forms of RNA polymerase from K.lactis cells. The general properties of these enyzmes are similar to those of Saccharomyces cerevisiae and of other eukaryotic RNA polymerases. In particular, enzymes IA, IB and III are more active with poly[d(A-T)] template and Mn-2+ than with native or denatured calf thymus DNA. Enzyme II shows optimal activity with denatured calf thymus DNA and Mn2+. When challenged with native calf thymus DNA all enzymes prefer Mg-2+ as a divalent cation whereas with denatured calf thymus DNA all enzymes are more active with Mn-2+. Enzyme II is inhibited by lambda-amanitin but no enzyme is sensitive to rifampicin and streptolydigin. The inhibition of growth and uracil uptake observed when rifampicin is added to nystatin treated cells is probably not caused by a specific inhibition of transcription.

The polypeptide subunit structure of the DNA-dependent RNA polymerase of Zea mays chloroplasts

Analysis of the maize chloroplast DNA-dependent RNA polymerase by electrophoresis on polyacrylamide gels that contain sodium dodecyl sulfate shows that the enzyme is multimeric. It contains at least two polypeptides of 180,000 and 140,000 daltons. Polypeptides of lower molecular mass that are associated with the enzyme at several stages of purification may also be subunits of the enzyme. Electrophoresis of a mixture of purified maize nuclear DNA-dependent RNA polymerase IIA and chloroplast polymerase on sodium dodecyl sulfate-polyacrylamide gels resolves the 160,000 dalton polypeptide of IIA from the 140,000 dalton polypeptide of the chloroplast enzyme, but does not resolve the 180,000 dalton components found in both enzymes. The polypeptides of less than 40,000 daltons in the highly purified maize nuclear IIA preparations are absent from preparations of purified maize chloroplast RNA polymerase.