Expression of the mitochondrial genome in HeLa cells. XIV. The relative positions of the 4 S RNA genes and of the ribosomal RNA genes in mitochondrial DNA

Transcriptional origin of Euglena chloroplast tRNAs

tRNA.DNA hybridization studies indicate that Euglena chloroplast tRNAs are transcriptional products of the chloroplast genome, which contains approximately 26 tRNA cistrons. Hybridization with purified chloroplast tRNA(Phe) and tRNA(Asp) shows that the chloroplast genome contains one cistron for each of these two species. No hybridization of chloroplast tRNA with nuclear DNA was observed. tRNAs from Euglena cytoplasm, Escherichia coli, and Agmenellum quadraduplicatum do not compete with chloroplast tRNA for hybridization with chloroplast DNA. Evidence is presented that photoinduction of chloroplast tRNAs is at the level of transcription rather than maturation of tRNA precursor molecules.

Assignment of the chloramphenicol resistance gene to mitochondrial deoxyribonucleic acid and analysis of its expression in cultured human cells

The mitochondrial deoxyribonucleic acids (mtDNA's) from human HeLa and HT1080 cells differed in their restriction endonuclease cleavage patterns for HaeII, HaeIII, and HhaI. HaeII digestion yielded a 9-kilobase fragment in HT1080, which was replaced by 4.5-, 2.4-, and 2.1-kilobase fragments in HeLa. HaeIII and HhaI yielded distinctive 1.35- and 0.68-kilobase HeLa fragments. These restriction endonuclease polymorphisms were used as mtDNA markers in HeLa-HT1080 cybrid and hybrid crosses involving the cytoplasmic chloramphenicol resistance mutation was used. mtDNA's were purified and digested with the restriction endonucleases, the fragments were separated on agarose gels, and the bands were detected by ethidium bromide staining and Southern transfer analysis. Three cybrids and four hybrids (four expressing HeLa and three expressing HT1080 chloramphenicol resistance) contained 2- to 10-fold excesses of the mtDNA of the chloramphenicol-resistant parent. One cybrid, which was permitted to segregate chloramphenicol resistance and was then rechallenged with chloramphenicol, had approximately equal proportions of the two mtDNA's. Only one hybrid was discordant. These results indicated that chloramphenicol resistance is encoded in mtDNA and that expression of chloramphenicol resistance is related to the ratio of chloramphenicol-resistant and -sensitive genomes within cells.

Discrete poly(A)- RNA species from rat liver mitochondria are fragments of 16S mitochondrial rRNA carrying its 5'-termini

During electrophoresis in polyacrylamide gels containing 7M urea the major discrete components of preparations of rat liver mitochondrial poly(A)+ and poly(A)- RNA species have similar mobilities. Poly(A)- RNA components hybridize to the 16S rRNA gene of mtDNA. Analysis of 5'-terminal sequences of these components revealed their identity to the 5'-terminal sequence of 16S rRNA. These results show that poly(A)- RNA components are fragmentation products of 16S rRNA. Fragmentation occurs nonrandomly from the 3'-end of the original rRNA molecules and lead to formation of products with electrophoretic mobilities similar to those of poly(A)+ RNA components.

Organization of rRNA structural genes in the archaebacterium Thermoplasma acidophilum

In the archaebacterium Thermoplasma acidophilum, each of the structural genes for 5S, 16S and 23S rRNA occur once per genome. In contrast to those of eubacteria and eukaryotes, they appear unlinked. The distance between the 16S and the 23S rDNA is at least 7.5 Kb, that between 23S and 5S rDNA at least 6 Kb and that between 16S and 5S rDNA at least 1.5 Kb. No linkage between those genes has been found by the analysis of recombinant plasmids carrying Bam HI and Hind III rDNA fragments as by hybridizing those plasmids to fragments of Thermoplasma DNA generated by 6 individual restriction endonucleases, recognizing hexanucleotide sequences.

Nucleotide sequences of small ribosomal RNA and adjacent transfer RNA genes in rat mitochondrial DNA

The nucleotide sequences of genes for small 12 S ribosomal RNA and two transfer RNAs (tRNAPhe and tRNAVal) and their common upstream region in cloned rat mitochondrial DNA were determined. This DNA sequence of 1709 bp extends between the HindII and HindIII cleavage sites in the EcoRI-A and EcoRI-D fragments, respectively. There is no long reading frame for protein synthesis starting from the HindIII site and extending all the way to the HindII site, or vice versa. The heavy strand was found to be the template for these RNA transcripts. The 5' and 3' ends of the 12 S ribosomal RNA gene are contiguous with the tRNAPhe and tRNAVal genes, respectively. The structural features of the two tRNA genes are significantly different from the standard prokaryotic tRNA pattern. Since similar contiguous natures of these genes in human (Crews and Attardi, Cell 19 (1980) 775; Eperon et al., Nature 286 (1980) 460) and mouse (Van Etten et al., Cell 22 (1980) 157) mitochondrial DNA have been reported, comparative studies on the structures of these genes were performed by computer analysis and the characteristic features are discussed.

Assignment of the chloramphenicol resistance gene to mitochondrial deoxyribonucleic acid and analysis of its expression in cultured human cells

The mitochondrial deoxyribonucleic acids (mtDNA's) from human HeLa and HT1080 cells differed in their restriction endonuclease cleavage patterns for HaeII, HaeIII, and HhaI. HaeII digestion yielded a 9-kilobase fragment in HT1080, which was replaced by 4.5-, 2.4-, and 2.1-kilobase fragments in HeLa. HaeIII and HhaI yielded distinctive 1.35- and 0.68-kilobase HeLa fragments. These restriction endonuclease polymorphisms were used as mtDNA markers in HeLa-HT1080 cybrid and hybrid crosses involving the cytoplasmic chloramphenicol resistance mutation was used. mtDNA's were purified and digested with the restriction endonucleases, the fragments were separated on agarose gels, and the bands were detected by ethidium bromide staining and Southern transfer analysis. Three cybrids and four hybrids (four expressing HeLa and three expressing HT1080 chloramphenicol resistance) contained 2- to 10-fold excesses of the mtDNA of the chloramphenicol-resistant parent. One cybrid, which was permitted to segregate chloramphenicol resistance and was then rechallenged with chloramphenicol, had approximately equal proportions of the two mtDNA's. Only one hybrid was discordant. These results indicated that chloramphenicol resistance is encoded in mtDNA and that expression of chloramphenicol resistance is related to the ratio of chloramphenicol-resistant and -sensitive genomes within cells.

DNA electron microscopy

In recent years DNA electron microscopy has become a tool of increasing interest in the fields of molecular genetics and molecular and cell biology. Together with the development of in vitro recombination and DNA cloning, new electron microscope techniques have been developed with the aim of studying the structural and functional organization of genetic material. The most important methods are based on nucleic acid hybridizations: DNA-DNA hybridization (heteroduplex, D-loop), RNA-DNA hybridization (R-loop), or combinations of both (R-hybrid). They allow both qualitative and quantitative analysis of gene organization, position and extension of homology regions, and characterization of transcription. The reproducibility and resolution of these methods make it possible to map a specific DNA region within 50 to 100 nucleotides. Therefore they have become a prerequisite for determining regions of interest for subsequent nucleotide sequencing. Special methods have been developed also for the analysis of protein-DNA interaction: e.g., direct visualization of specific protein-DNA complexes (enzymes, regulatory proteins), and analysis of structures with higher complexity (chromatin, transcription complexes).

Precise localization and nucleotide sequence of the two mouse mitochondrial rRNA genes and three immediately adjacent novel tRNA genes

The complete DNA sequence of the ribosomal RNA region of mouse L cell mitochondrial DNA has been determined. Genes for the small (12S) and large (16S) rRNAs have been precisely located by direct sequencing of the termini of the two mature rRNAs. A comparison of the lengths (956 and 1582 nucleotides) and terminal sequences of the mature rRNAs with the DNA coding sequences indicates that mouse mt rRNAs are not spliced. Computer analysis of the complete DNA sequence has identified three potential transfer RNA genes. A gene for phenylalanine tRNA is located immediately adjacent to the 5' end of the 12S rRNA gene, a valine tRNA gene occupies the entire region between the 12S and 16S rRNA genes and a leucine tRNA gene is located immediately adjacent to the 3' end of the 16S gene. Hybridization of 32P-labeled, tRNA-sized mtRNA to selected DNA restriction endonuclease fragments from the rRNA region confirms the existence of small, abundant mtRNAs transcribed from these DNA sequences. All three tRNA genes and both rRNA genes are transcribed from the heavy strand of mtDNA. The mt rRNA sequences exhibit notable homologies to other rRNAs and, in particular, to those of E. coli. Within the 3' terminal 50 nucleotides, the mouse mt 12S rRNA contains a potential 10 bp hairpin structure and a sequence of 15 consecutive nucleotides common to the RNA of the small ribosomal subunit in all systems, but does not contain the mRNA binding site (ACCUCC) found in E. coli and corn chloroplast rRNAs. The mt tRNA genes do not have the 3' terminal CCA sequence encoded in the DNA, nor do they contain any intervening sequences. Two of the three tRNSa would lack many features which are known to be strictly conserved in all other nonorganelle tRNAs which have been sequenced. The fact that all the genes in this region are directly contiguous with at most one intervening nucleotide suggests that the entire region is transcribed into a polycistronic precursor RNA which is processed by endonucleolytic cleavages. The organization of the genes of the rRNA operon of mouse mtDNA, when compared to the organization of rRNA and tRNA genes in bacterial or eucaryotic nuclear genomes, provides evidence for the endosymbiotic hypothesis of the biogenesis of mammalian mitochondria.

Uniqueness of sperm mtDNA as compared to somatic mtDNA in ram

Sperm mtDNA exhibits unique physicochemical properties compared with mtDNA from somatic tissues in the ram. The buoyant density of sperm mtDNA was 1,6983 g/cm3 while the brain, heart and liver mtDNAs was about 1.7005 g/cm3. The Tm of the liver and sperm mtDNA was 71.0 degrees C and 69.5 degrees C, respectively. The G + C content of sperm mtDNA was 3.0 moles % lower than the liver mtDNA. The contour length of the circular sperm mtDNA was 5.01 micrometers compared with the liver mtDNA with contour length of 5.33 micrometers.

Nucleotide sequence of a cloned fragment of rat mitochondrial DNA containing the replication origin

The nucleotide sequence was determined for the 717 bp HapII subfragment (HapEcoA5) of the EcoRI-A fragment of rat mitochondrial DNA, which contains the heavy-strand replication origin. Analysis of the heavy-strand initiation segments released from the D-loop molecules has revealed that some 5'-ends of these initiation segments are linked to ribonucleotide(s) and are heterogeneous. Sequence analysis of the 5'-end portion of the initiation segment indicated that one of the start points of the deoxyribonucleotide polymerization corresponds to the 425th bp on the HapEcoA5. Two-fold rotational symmetry and palindrome structures, and a G-cluster sequence around the start point have been discussed in connection with unidirectional replication.

Mapping of nascent light and heavy strand transcripts on the physical map of HeLa cell mitochondrial DNA

The sequences complementary to the nascent RNA molecules isolated from transcription complexes of HeLa cell mtDNA have been mapped on the H and L strands of mtDNA by the S1 protection technique. The distribution of these sequences among different Hpa II restriction fragments was found to reflect the position of these fragments in the Hpa II map of mtDNA. Thus, the S1-resistant hybrids formed with the L strand corresponded almost exclusively to the right half of the genome past the origin of replication in the direction of L strand transcription, and were especially concentrated in the region immediately adjacent to the origin. By contrast, the hybrid duplexes involving the H strand appeared to be localized in the left half of the genome, and in particular in the quadrant of the map adjacent to the origin in the direction of H strand transcription. These results strongly suggest that the region of mtDNA around the origin of replication contains an initiation site for L strand transcription and an initiation site for H strand transcription.

Expression of the mitochondrial genome in Xenopus laevis: a map of transcripts

The twelve most abundant transcripts in X. laevis mitochondria were characterized, and their coding regions were mapped on the mitochondrial DNA (mtDNA) by R loop mapping in the electron microscope and by RNA gel transfer hybridization. The transcripts map in nonoverlapping positions with one exception, and they account for about 80% of the coding capacity of mtDNA. Ten of the twelve RNA molecules contain poly(A): the two poly(A)-lacking transcripts are the rRNAs. Analysis withe single-strand-specific nucleases clearly demonstrated the absence of intervening sequences from the coding regions for seven RNAs. For two RNAs, uninterrupted coding sequences are strongly suggested and one RNA could not be analyzed. Eight transcripts of low abundance and high molecular weight were characterized, and their coding regions were mapped approximately. They overlap the coding regions of the abundant mitochondrial RNAs and could be precursors of these RNAs. Most of the RNA molecules characterized were shown to be transcribed from the heavy strand of mtDNA. No abundant discrete light-strand transcript was found.

Fractionation and identification of spinach chloroplast transfer RNAs and mapping of their genes on the restriction map of chloroplast DNA

Spinach chloroplast 4S RNAs has been separated by two-dimensional polyacrylamide gel electrophoresis into about 35 species. After extraction from the gel, 27 of these RNA species were identified by aminoacylation as tRNAs specific for 16 amino acids. Individual tRNAs were labeled in vitro with 125I and hybridized to DNA fragments obtained by digestion of spinach chloroplast DNA with KpnI, PstI, SalI and XmaI restriction endonucleases. A minimum of 21 genes corresponding to tRNAs for 14 different amino acids have been localized on the restriction endonuclease cleavage site map of the DNA molecule. Of these, 15 genes corresponding to tRNAs for 12 amino acids are located in the larger of the two single-copy regions which separate the two inverted copies of the repeat region. Each copy of this repeat region contains a set of genes for the ribosomal RNAs and a gene for tRNA2Ile in the "spacer" sequence between the 16S and 23S ribosomal RNAs. The genes for tRNA1Ile, tRNA2Leu and tRNA3Leu also map in the repeat region, but outside the ribosomal DNA unit. At present, two more chloroplast tRNAs (for Pro and Lys) have been identified, but not mapped, while 4 unidentified 4S RNAs have been mapped in the large single-copy region of the DNA molecule. Evidence is presented that isoaccepting tRNA species can be transcripts from different loci.

The organization of ribosomal RNA genes in the mitochondrial DNA of Tetrahymena pyriformis strain ST

1. We have constructed a physical map of the mtDNA of Tetrahymena pyriformis strain ST using the restriction endonucleases EcoRI, PstI, SacI, HindIII and HhaI. 2. Hybridization of mitochondrial 21 S and 14 S ribosomal RNA to restriction fragments of strain ST mtDNA shows that this DNA contains two 21-S and only one 14-S ribosomal RNA genes. By S1 nuclease treatment of briefly renatured single-stranded DNA the terminal duplication-inversion previously detected in this DNA (Arnberg et al. (1975) Biochim. Biophys. Acta 383, 359--369) has been isolated and shown to contain both 21-S ribosomal RNA genes. 14 S ribosomal RNA hybridizes to a region in the central part of the DNA, about 8000 nucleotides or 20% of the total DNA length apart from the nearest 21 S ribosomal RNA gene. 3. We have confirmed this position of the three ribosomal RNA genes by electron microscopical analysis of DNA . RNA hybrid molecules and R-loop molecules. 4. Hybridization of 21 S ribosomal RNA with duplex mtDNA digested either with phage lambda-induced exonuclease or exonuclease III of Escherichia coli, shows that the 21-S ribosomal RNA genes are located on the 5'-ends of each DNA strand. Electron microscopy of denaturated mtDNA hybridized with a mixture of 14-S and 21-S ribosomal RNAs show that the 14 S ribosomal RNA gene has the same polarity as the nearest 21 S ribosomal RNA gene. 5. Tetrahymena mtDNA is (after Saccharomyces mtDNA) the second mtDNA in which the two ribosomal RNA cistrons are far apart and the first mtDNA in which one of the ribosomal RNA cistrons is duplicated.

Chemical and physical properties of mammalian mitochondrial aminoacyl-transfer RNAs. I. Molecular weights of mitochondrial leucyl- and methionyl-transfer RNAs

The sedimentation and electrophoretic properties of Syrian hamster cytosolix and mitochondrial methionyl- and leucyl- +RNAs have been compared under denaturing conditions. Mitochondrial leucyl-tRNA could be separated into three species by chromatography on RPC-5. Their apparent molecule weights as determined by polyacrylamide slab gel elecltrophoresis were 23 000 for one species and 24 000 for the other two compared to the five cytosolic leucyl-tRNA species whose apparent molecular weights ranged from 26 000 to 28 000. Mitochondrial leucyl-tRNAs sedimented more slowly than their cytosolic counterparts, again indicating a lower molecular weight. The apparent molecular weights of the mitochondrial methionyl-tRNAs were identical or only slightly lower than their cytosolic counterparts as determined by polyacrylamide slab gel electrophoresis but both mitochondrial methionyl-tRNA and formylmethionyl-tRNA sedimented slightly more slowly than cytolsolic methionyl-tRNA. It is suggested that mitochondrial tRNAs fall into the size range of other t RNAs and might be uniform in size.

A restriction endonuclease cleavage map of mitochondrial DNA from transformed hamster cells

Mitochondrial DNA from cultured C13/B4 hamster cells was cleaved by the restriction endonucleases Hpa II, Hind III, Eco RI and Bam HI into 7, 5, 3 and 2 unique fragments, respectively. The summed molecular weights of fragments obtained from electrophoretic mobilities in agarose-ethidium bromide gels (with Hpa I-cleaved T7 DNA as standard) and electron microscopic analysis of fragment classes isolated from gels (with SV40 DNA as standard) were in good agreement with the size of 10.37 +/- 0.22 x 10(6) daltons (15,700 +/- 330 nucleotide pairs) determined for the intact circular mitochondrial genome. Cyclization of all Hind III, Eco RI and Bam HI fragments was observed. A cleavage map containing the 17 restriction sites (+/- 1% s.d.) was constructed by electrophoretic analysis of 32P-labeled single- and double-enzyme digestion products and reciprocal redigestion of isolated fragments. The 7 Hpa II sites were located in one half of the genome. The total distribution of the 17 cleavages around the genome was relatively uniform. The position of the D-loop was determined from its location and expansion on 3 overlapping restriction fragments.

Isolation and partial characterization of poly (A)-containing 7.5S messenger RNA from rat liver mitochondria

Poly(A)-containing low molecular weight (7.5S) messenger RNA was isolated in a highly purified form from both polyribosomes and post-polysomal supernatant of rat liver mitochondria. Both mRNA's contain rather short poly(A) tracts (40-70 mononucleotides) according to a profile of their elution from poly(U)-Sepharose column with a gradient of formamide concentration. Both mRNA's when added to a preincubated mitochondrial lysate programmed the synthesis of a hydrophobic polypeptide of a molecular weight about 9000 daltons which was soluble in the neutral chloroform-methanol mixture.

The ribosomal RNA genes on Neurospora crassa mitochondrial DNA are adjacent

Hybridization of separated 24 S and 17 S ribosomal RNA from Neurospora crassa mitochondrial ribosomes to restriction fragments of mitochondrial DNA leads to the conclusion that the large and small ribosomal RNA are adjacent on the restriction endonuclease cleavage map of the DNA. The distance between the two genes is estimated at 900 basepairs. This result is consistent with the existence of a ribosomal precursor RNA in N. crassa mitochondria and is in contrast to the situation in yeast, where the ribosomal genes are far apart on the mitochondrial DNA. The position of the ribosomal RNA genes on the cleavage map of N. crassa mtDNA provides a start for ordering the Hind III restriction fragments.

Translation in Escherichia coli mini-cells containing hamster mitochondrial DNA-Co1E1 - Ampr recombinant plasmids

Hamster mitochondrial DNA is cleaved into two fragments (4.2 and 11.4 kilobase pairs of DNA (kb)) by the restriction enzyme, Eco RI. Recombinant DNA molecules formed in vitro between an Escherichia coli plasmid, Co1E1 - Ampr, and Eco RI-digested hamster mitochondrial DNA were transformed into E. coli K12. The translation products of the parent plasmid, Co1E1 - Ampr, and recombinant plasmid DNAs containing (i) the 4.2 kb mitochondrial DNA fragment and (ii) the 11.4 kb fragment were characterized on sodium dodecyl sulfate-polyacrylamide gels using bacterial mini-cell lysates. The Co1E1 - Ampr plasmid specifies at least six polypeptides whose structural genes comprise 56% of the plasmid DNA. Insertion of hamster mitochondrial DNA at the Eco RI site of the plasmid alters the relative rate of synthesis of these six polypeptides and induces the occurrence of a new band on sodium dodecyl sulfate-polyacrylamide gels which is probably not specified by the inserted mitochondrial DNA sequences.

Mapping of mitochondrial DNA of individual sheep and goats: rapid evolution in the D loop region

Mitochondrial DNA (mtDNA) from sheep and goat was compared by restriction endonuclease analysis and heteroduplex mapping in the electron microscope. The fragment patterns produced by endonuclease Hae III from three individual sheep and two goat mtDNAs all differed from each other. The three sheep mtDNAs had identical Eco RI and Hind III fragments, but the two goat mtDNA patterns differed from each other as well as from sheep mtDNA. We estimate that each sheep mtDNA differs from each other by 0.5-1% of its nucleotide sequences, the two goat mtDNAs by 1-2%, and there is a 6-11% sequence difference between sheep and goat mtDNAs. We have mapped the Eco RI and Hind III sites of goat and sheep mtDNA and determined the positions of the D loop, which marks the replication origin, relative to the restriction map. The D loops are at homologous positions on the mtDNAs from both species, but the goat D loop is only 75% as long as the sheep D loop. Regions with a high degree of sequence divergence occur at both ends of the D loop. We suggest that a duplication of about 150 base pairs has occurred in the region where the sheep and goat D loops differ in length. We discuss mtDNA evolution in terms of divergence of isolated "mitochondrial DNA clones."

Analysis of Euglena gracilis chloroplast deoxyribonucleic acid with a restriction endonuclease, EcoRI

Cleavage of chloroplast deoxyribonucleic acid (DNA) of Euglena gracilis Z with restriction endonuclease RI from Escherichia coli (EcoRI) yielded 23 bands upon electrophoresis in gels of agarose. Four of the bands contained twice the stoichiometric amount of DNA. One of these bands contained two similarly sized fragments. The sum of the molecular weight of the 24 different fragments equaled the molecular weight of the circular molecule. The restriction fragments had different buoyant densities, with four having distinctly heavy densities in CsCl. Restriction fragments with a high buoyant density were preferentially lost when broken chloroplast DNA was purified by equilibrium density gradient centrifugation. Hybridization of chloroplast ribosomal ribonucleic acid to intact chloroplast DNA determined that there are two cistrons for 16S and 23S ribosomal ribonucleic acid. These two cistrons are located on six restriction fragments, all of which have buoyant densities greater than the intact molecule of chloroplast DNA.

Molecular and genetic approaches to the analysis of the informational content of the mitochondrial genome in mammalian cells

Our laboratory has been involved in the last few years in investigations aiming at analysing by molecular approaches the informational content of the mitochondrial genome in mammalian cells and the mechanisms and control of its expression, H eLa cells and other mammalian cell lines have been utilized for these studies. These investigations, as well as work carried out in other laboratories, have yielded a considerable amount of information concerning the mechanism, products and regulation of transcription of mitochondrial DNA (mit-DNA), the apparatus and products of mitochondria-specific protein synthesis in animal cells, and the number and topology of the sites on mit-DNA which code for the primary gene products identified so far. It is the purpose of the present report to summarize the latest observations in this area, as well as some recent results on the isolation and characterization of chloramphenicol-resistant variants of a human cell line. Reference is made to previous review articles 1,2,3 for the earlier work.

The structural organization of ribosomal DNA in Drosophila melanogaster

The genes coding for the large rRNAs (rDNA) were purified from Drosophila melanogaster embryos by two different methods. Purified rDNA was analyzed by gel electrophoresis after digestion with restriction endonucleases and by electron microscopy. Each repeating unit of rDNA consists of a gene region that codes for a transcript which includes the 18S and 28S rRNA sequences plus a nontrascribed spacer. Analysis of rRNA/rDNA hybrids in the electron microscope reveals that two types of repeating units exist in the rDNA from this fly. One of these is characterized by insertions of various lengths within the 28S gene. Insertions range in size from about 0.5 kilo base pairs (kb) to 6.0 kb and occur in distinct size classes which are multiples of 0.5 kb. Figure 10 summarizes the structure of the rDNA repeating unit. Repeats with insertions constitute about two thirds, and repeats without insertions about one third of all repeating units in the sample studied where 86% of the rDNA is derived from the X chromosome nucleolus organizer. Statistical analysis of pairs of of nearest-neighbor repeats demonstrates that repeats with and without insertions are interspersed in the X chromosome nucleolus organizer, and that scrambling is close to that expected for random arrangement. Insertions are the major source of length heterogeneity in this rDNA. A minor length heterogeneity also exists in thie nontranscribed spacer of Drosophila rDNA as demonstrated by heteroduplex mapping of restriction fragments in the electron microscope.

Characterization and mapping of mitochondrial ribosomal RNA and mitochondrial DNA in Drosophila melanogaster

We report the isolation and some properties of the mitochondrial ribosomal RNA (mt-rRNA) of Drosophila melanogaster, and a restriction map of the mtDNA of this organism which shows the position of the rRNA genes and of the A + T-rich region in the DNA. The mt-rRNAs are about 860 and 1500 nucleotides long and have the unusual composition of about 20% G+C. Some 25% of the mtDNA is very rich in A+T and is visualized as a contiguous early melting region in denaturation mapping. We mapped the three cleavage sites of the restriction endonuclease Hae III and the four sites of Hind III on mtDNA relative to each other and relative to the early melting region. The rRNA genes have been positioned on this map. The two rRNA genes are next to each other, separated by a gap of about 160 bases. We determined the polarity of the rRNA molecules: transcription proceeds in the direction small-to-large rRNA. Despite great divergence in nucleotide composition and sequence, the properties of mt-rRNA and the arrangement of mt-rRNA genes are very similar in Drosophila and vertebrate animals.

An electron microscope study of the relative positions of the 4S and ribosomal RNA genes in HeLa cells mitochondrial DNA

The 4S RNA genes in HeLa mitochondrial DNA (mtDNA) have been mapped by electron microscopy using the electron-opaque label ferritin. This method is based on the high affinity interaction between the protein, avidin,and biotin. 4S RNA, covalently coupled to biotin, was hybridized to single-stranded mtDNA. The hybrids were then labeled with ferritin-avidin conjugates. The positions of ferritin-labeled 4S RNA genes were determined relative to the rRNA genes on both heavy (H) and light (L) strands of mtDNA. This region was recognized as a duplex segment after hybridization either with rRNA in the case of H strands or with DNA complementary to rRNA in the case of L strands. Our studies suggest that at least nineteen 4S RNA genes are present in the HeLa mitochondrial genome. On the H strand, we have confirmed the nine map positions found in a previous electron microscope mapping study (Wu et al., 1972) and obtained evidence for three additional 4S RNA genes. On the L strand, seven 4S RNA genes have been mapped. The nineteen genes are distributed more or less uniformly around the genome. There is a pair of closely spaced genes, approximately 150 nucleotides apart, on the H strand, and another closely spaced pair on the L strand.

Transfer RNA genes between 16S and 23S rRNA genes in rRNA transcription units of E. coli

We have identified genes for tRNAGLU/2 on the transducing phages o80d3ilvsu7+ (see Ohtsubo et al., 1974) and lambdarifd18 (Kirschbaum and Konrad, 1973), and a gene for tRNAlle/1 on the transducing phage o80rifr (Konrad, Kirschbaum, and Austin, 1973). All these phages have previously been shown to carry genes for rRNA (Ohtsubo et al., 1974; Lindahl et al., 1975; Jaskunas et al., 1975a). We have analyzed the position of these tRNA genes by hybridizing purified RNAs to restriction fragments of the phage DNA. The tRNA genes are located inside the rRNA transcription unit in the spacer region between the 16S and 23S rRNA genes.