Transcription of simian virus 40. VI. SV 40 DNA-RNA polymerase complex isolated from productively infected cells transcribed in vitro

T-antigen is not bound to the replication origin of the simian virus 40 late transcription complex

Simian virus 40 tumor antigen (T-antigen) plays a central role in determining which gene is transcribed from viral DNA late in infection. Results from several studies have led to a model in which the binding of T-antigen to the viral origin of replication results in repression of transcription from the stronger early gene promoter and stimulation of transcription from the late gene promoter. We have tested this model by determining directly the occupancy of the T-antigen binding site in the origin of replication of the late transcription complex. Thus, viral transcription complexes were digested with BglI, a restriction enzyme that cuts in the viral replication origin. The enzyme cleaved 78(+/- 12)% of the late transcription complexes. Control experiments demonstrated that cleavage is blocked when T-antigen is bound to the origin site, that exogenously added T-antigen can bind to the site in the transcription complex, and that T-antigen is not released during isolation of the complex. These results indicate that most of the late transcription complexes do not have T-antigen bound to the origin site, and are therefore inconsistent with models that require this site to be occupied by T-antigen to maintain proper regulation of gene transcription late in infection.

The block to transcription elongation at the SV40 attenuation site is decreased in vitro by oligomers complementary to segments of the attenuator RNA

We have previously reported that a mechanism resembling attenuation in prokaryotes regulates simian virus 40 (SV40) late gene expression. We have suggested that modulation of the attenuator RNA secondary structure is an integral element regulating the elongation block at the attenuation site [Hay et al., Cell 29 (1982) 183-193]. In the present study, oligodeoxyribonucleotides (oligos), 13-19 nucleotides long, were used to probe the involvement of the attenuator RNA secondary structure in the control of elongation block at the SV40 attenuation site. These oligos are complementary to segments of the attenuator RNA suggested to play a role in the regulation of attenuation. The oligos were added to an in vitro transcription reaction containing SV40 transcription complexes, and their effect on transcription through the attenuation site was measured. As predicted, the three oligos caused specific decreases in the elongation block at the SV40 attenuation site. These results provide direct evidence for the involvement of RNA secondary structure in the attenuation mechanism in SV40.

Transcriptionally active SV40 minichromosomes are restriction enzyme sensitive and contain a nucleosome-free origin region

A nucleosome-free region or gap containing the origin of replication and the transcriptional promoter elements is observed on 20 to 25% of the SV40 minichromosomes isolated at physiological ionic strength late in infection. We used the preferential sensitivity of the gapped minichromosomes to restriction enzymes to obtain sucrose gradient fractions containing 50 to 80% of gapped molecules. The same fractions are also enriched in RNA polymerase B (II) molecules engaged in transcription. Using electron microscopy, we demonstrate here that the transcriptional complexes are preferentially sensitive to restriction enzyme digestion, which indicate that they represent a subpopulation of the gapped minichromosomes.

Efficient and accurate in vitro processing of simian virus 40-associated small RNA

Nuclei were isolated from simian virus 40 (SV40)-infected cells with a hypotonic, detergent-free buffer and incubated in vitro in a high-ionic-strength buffer containing [alpha-32P]UTP. The labeled viral RNAs produced were analyzed by gel electrophoresis together with 3-h-labeled viral RNAs extracted from SV40-infected cells. The in vitro-synthesized RNA contained a major RNA species of 62 to 64 nucleotides that appeared on the gel at the same position as in vivo-synthesized SV40-associated small RNA (SAS-RNA). Analyses of the in vitro-synthesized 62- to 64-nucleotide RNA by hybridization to restriction fragments and by the use of an SAS-RNA deletion mutant clearly identified it as SAS-RNA. The intensity of the band of the in vitro-synthesized SAS-RNA increased with an increase in the labeling time or when a short pulse was followed by a chase. Moreover, the SAS-RNA band disappeared when ITP replaced GTP in the transcription reaction mixture. These results indicate that SAS-RNA is processed from a precursor molecule and that an RNA secondary structure could be an element recognized by the processing enzyme.

Attenuation of late simian virus 40 mRNA synthesis is enhanced by the agnoprotein and is temporally regulated in isolated nuclear systems

Studies were performed to verify the physiological significance of attenuation in the life cycle of simian virus 40 and the role of agnoprotein in this process. For these purposes, nuclei were isolated at various times after infection and incubated in vitro in the presence of [alpha-32P]UTP under the standard conditions which lead to attenuation. Attenuation was evident by the production of a 94-nucleotide attenuator RNA, revealed by gel electrophoresis. In parallel, the synthesis of agnoprotein was studied at various times after infection by labeling the cells for 3 h with [14C]arginine, lysing them, and analyzing the labeled proteins by gel electrophoresis. Both attenuation and the synthesis of agnoprotein were predominant towards the end of the infectious cycle. At earlier times, there was almost no attenuation and no synthesis of agnoprotein. Moreover, there was almost no attenuation even at the latest times after infection in nuclei isolated from cells infected with simian virus 40 deletion mutants that do not synthesize agnoprotein. Finally, analysis by dot blot hybridization showed higher amounts of cytoplasmic viral RNA in cells infected with an agnoprotein gene insertion mutant, delta 79, that does not produce agnoprotein, compared with cells infected with wild-type virus. The present studies indicate that attenuation is temporally regulated and suggest that agnoprotein enhances attenuation in isolated nuclei and that may also enhance it in vivo.

Attenuation of late simian virus 40 mRNA synthesis is enhanced by the agnoprotein and is temporally regulated in isolated nuclear systems

Studies were performed to verify the physiological significance of attenuation in the life cycle of simian virus 40 and the role of agnoprotein in this process. For these purposes, nuclei were isolated at various times after infection and incubated in vitro in the presence of [alpha-32P]UTP under the standard conditions which lead to attenuation. Attenuation was evident by the production of a 94-nucleotide attenuator RNA, revealed by gel electrophoresis. In parallel, the synthesis of agnoprotein was studied at various times after infection by labeling the cells for 3 h with [14C]arginine, lysing them, and analyzing the labeled proteins by gel electrophoresis. Both attenuation and the synthesis of agnoprotein were predominant towards the end of the infectious cycle. At earlier times, there was almost no attenuation and no synthesis of agnoprotein. Moreover, there was almost no attenuation even at the latest times after infection in nuclei isolated from cells infected with simian virus 40 deletion mutants that do not synthesize agnoprotein. Finally, analysis by dot blot hybridization showed higher amounts of cytoplasmic viral RNA in cells infected with an agnoprotein gene insertion mutant, delta 79, that does not produce agnoprotein, compared with cells infected with wild-type virus. The present studies indicate that attenuation is temporally regulated and suggest that agnoprotein enhances attenuation in isolated nuclei and that may also enhance it in vivo.

Attenuation may regulate gene expression in animal viruses and cells

In eukaryotes, an abundant population of promoter-proximal RNA chains have been observed and studied, mainly in whole nuclear RNA, in denovirus type 2, and in SV40. On the basis of these results it has been suggested that a premature termination process resembling attenuation in prokaryotes occurs in eukaryotes. Moreover, these studies have shown that the adenosine analog 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) enhances premature termination, but its mode of action is not understood. The determination of the nucleotide sequences of SV40 and other viruses and cellular genes provide means for elucidating the nucleotide sequences involved in the attenuation mechanism. A model has recently been described in which attenuation and mRNA modulation in a feedback control system quantitatively regulate SV40 gene expression. The suggested mechanism described in this model opens up approaches to the investigation of attenuation and mRNA modulation as a possible mechanism whereby eukaryotes may regulate transcription in a variety of different circumstances.

Transcription of minute virus of mice, an autonomous parvovirus, may be regulated by attenuation

To characterize the transcriptional organization and regulation of minute virus of mice, an autonomous parvovirus, viral transcriptional complexes were isolated and cleaved with restriction enzymes. The in vivo preinitiated nascent RNA was elongated in vitro in the presence of [alpha-32P]UTP to generate runoff transcripts. The lengths of the runoff transcripts were analyzed by gel electrophoresis under denaturing conditions. On the basis of the map locations of the restriction sites and the lengths of the runoff transcripts, the in vivo initiation sites were determined. Two major initiation sites having similar activities were thus identified at residues 201 +/- 5 and 2005 +/- 5; both of them were preceded by a TATAA sequence. When uncleaved viral transcriptional complexes or isolated nuclei were incubated in vitro in the presence of [alpha-32P]UTP or [alpha-32P]CTP, they synthesized labeled RNA that, as determined by polyacrylamide gel electrophoresis, contained a major band of 142 nucleotides. The RNA of the major band was mapped between the initiation site at residue 201 +/- 5 and residue 342. We noticed the potential of forming two mutually exclusive stem-and-loop structures in the 142-nucleotide RNA; one of them is followed by a string of uridylic acid residues typical of a procaryotic transcription termination signal. We propose that, as in the transcription of simian virus 40, RNA transcription in minute virus of mice may be regulated by attenuation and may involve eucaryotic polymerase B, which can respond to a transcription termination signal similar to that of the procaryotic polymerase.

Mapping in vivo initiation sites of RNA transcription and determining their relative use

Runoff transcripts were generated on viral transcriptional complexes cleaved with restriction enzymes and incubated in vitro with [alpha-32P]UTP under pulse-chase conditions. As viral transcriptional complexes in vitro elongated the nascent RNA preinitiated in vivo, size analysis by gel electrophoresis of the runoff transcripts allowed identification of the in vivo initiation sites. Moreover, scanning the intensities of the runoff bands as they appeared in the autoradiogram of the gel allowed determination of the relative use of these sites. A model system in which the initiation sites of simian virus 40 late RNA were identified and their relative use determined is presented.

SV40 deletion mutant (d1861) with agnoprotein shortened by four amino acids

d1861 is an SV40 deletion mutant which was thought to lack the agnoprotein coding region and was used to verify the role of agnoprotein in the life cycle of SV40. In the present study the region flanking the deletion was sequenced and, in contrast to the available information, it was found that d1861 lacks 12 nt in phase, downstream from the AUG start codon of agnoprotein (residues 347-358). Using the runoff protocol with viral transcriptional complexes (VTC), that in vitro elongate the in vivo preinitiated nascent RNA, it was found that in vivo the major initiation site for late transcription is at residue 325, the same as in wild type (WT). In comparison with WT, d1861 encodes information for agnoprotein shortened by four amino acids and it has been identified in d1861 infected cells. However, pulse-chase experiments indicated that the rate of synthesis of d1861 agnoprotein is slower than that of WT agnoprotein and that it has a turnover rate of 1 hr as compared to 3 hr of WT agnoprotein. The reduced rate of synthesis of d1861 agnoprotein can be explained by nuclease S1 analyses in which the major leader of d1861 16 S RNA, that encodes the agnoprotein, appeared in significantly lower amounts as compared to the major leader of WT 16 S RNA. Furthermore, analysis of the potential secondary structures at the 5' end of the leader of d1861 16 S RNA has revealed stable structures in which the start codon of agnoprotein is sequestered in a stem. The involvement of RNA secondary structures in regulating the synthesis of agnoprotein is discussed.

S1 mapping of purified nascent transcripts of simian virus 40

We purified nascent simian virus 40 late transcripts by incubating viral transcriptional complexes, isolated from infected BSC-1 cells, in a reaction mixture that contained mercurated CTP; RNA molecules that had incorporated mercurated residues in vitro were isolated by sulfhydrylcellulose affinity chromatography. The nascent RNA was hybridized to an end-labeled HindIII C probe fragment (0.646 to 0.86 map unit), and the hybrids were analyzed by S1 mapping. Most of the products of digestion corresponded to unspliced transcripts with 5' ends mapping at nucleotides 325, 260, and 195, which are positions of the 5' ends of mature, cytoplasmic late mRNA species. In addition, two minor products diagnostic of splicing at the acceptor junctions mapping at nucleotides 556 and 443 were detected. Because the abundance of these products was not diminished by repurifying the nascent RNA through a second round of sulfhydrylcellulose chromatography, these products did not originate from contaminating non-nascent RNA. Moreover, the generation of these products was not affected when a higher salt concentration and lower temperature were used for S1 digestion, conditions that should decrease artifactual cleavage by S1 in A + U-rich regions of colinear hybrids. Therefore, it is likely that some simian virus 40 RNA chains are spliced before release from the template.

Attenuation in SV40 as a mechanism of transcription-termination by RNA polymerase B

Nuclei which were isolated from SV40 infected cells with a hypotonic detergent-free buffer were used to establish in vitro conditions which lead to transcription-termination at the attenuation site of SV40. This system allowed us to identify regulatory elements involved in transcription-termination by RNA polymerase B transcribing SV40. Transcription-termination at the attenuation site was found to be ionic strength dependent. Efficient termination occurred at low (100 mM NaCl) but not at high (100 mM (NH4)2 SO4 or 300 mM NaCl) ionic strength. When nuclei were prewashed with 300 mM NaCl, the efficiency of transcription-termination was low even when transcription was carried out at low ionic strength (100 mM NaCl). Efficient transcription-termination in the high salt prewashed nuclei was reconstituted by complementation with a high salt (300 mM NaCl) soluble factor extracted from nuclei of uninfected cells. In addition, the efficiency of transcription-termination was significantly reduced when ITP replaced GTP in the transcription reaction mixture. Our data indicate that a nuclear factor and RNA secondary structure are essential regulatory elements involved in transcription-termination by RNA polymerase B.

Control of late simian virus 40 transcription by the attenuation mechanism and transcriptionally active ternary complexes are associated with the nuclear matrix

Isolated nuclei derived from simian virus 40 (SV40)-infected cells and incubated with [alpha-32P]UTP can elongate the in vivo preinitiated SV40 late RNA, synthesizing a viral RNA species 94 nucleotides long (attenuator RNA) as well as longer RNA molecules. In contrast to newly synthesized SV40 RNA, the attenuator RNA is not associated with the nuclear matrix. Pretreating the cells with 5,6-dichloro-1-beta-ribofuranosylbenzimidazole before the incubation of isolated nuclei in vitro, enhances the accumulation of the attenuator RNA, but again it is removed from nuclei by DNase and high salt. In contrast, pretreating the cells with proflavine, an intercalating drug that interferes with RNA secondary structure, prevents the accumulation of the attenuator RNA and increases the amount of the long RNA molecules. These RNA molecules become associated with the nuclear matrix. Isolated nuclear matrices from SV40-infected cells are highly enriched in transcriptionally active ternary complexes. Thus, isolated nuclear matrices that contain from 2 to 6% of SV40 DNA are capable of synthesizing at least 35% of the viral RNA synthesized in isolated nuclei after 2 to 15 minutes incubation with [alpha-32P]UTP. The RNA synthesized in vitro on purified nuclear matrices and isolated nuclei is derived from the same regions of the viral genome, suggesting that there is an association between transcribed DNA sequences and the nuclear matrix. The results suggest a major role for the nuclear matrix in controlling SV40 gene expression.

Separation and properties of two kinds of simian virus 40 late transcription complexes

Simian virus 40 (SV40) transcription complexes were labeled in cells with 3-min pulses of [(3)H]uridine 48 h after infection and were extracted from nuclei in isotonic buffer or in a buffer containing Sarkosyl. In sucrose gradients, the labeled complexes sedimented faster than both free RNA and most SV40 nucleoproteins. Most of the pulse-labeled nascent RNA hybridized to the entire late region of SV40, remained bound to viral DNA in Cs(2)SO(4) gradients, and ranged in size from a few nucleotides to about 5,000 nucleotides, with a peak at about 700. In contrast, the SV40-associated RNA polymerase activity in the same preparations sedimented near the major peak of SV40 nucleoproteins and was clearly separated from the transcription complexes bearing pulse-labeled nascent RNA. The two kinds of transcription complexes were released from isolated nuclei at different rates. Complexes bearing pulse-labeled RNA were released immediately when the nuclei were agitated in a Dounce homogenizer in isotonic buffer, whereas most of the complexes bearing RNA polymerase active in vitro were released more slowly, during subsequent incubation of the nuclei at 0 degrees C. Since the complexes bearing pulse-labeled nascent RNA were virtually inactive in vitro, the blocked complexes described by Laub et al. (Proc. Natl. Acad. Sci. U.S.A. 77:3297-3301, 1980) probably account for almost all the SV40-associated RNA polymerase activity studied previously by many investigators. New procedures must be developed to preserve the activity of the pulse-labeled complexes if the many advantages of the SV40 system for studying transcription by nucleoprotein complexes in vitro are to be realized fully.

Spliced and unspliced virus specific RNA sequences are associated with purified simian virus 40 chromatin

We have analyzed SV40 specific RNA sequences which are associated with purified viral chromatin. Three discrete size classes of large unprocessed transcripts were identified: 4.8, 3.6 and 3.0 kb. Furthermore, two polyadenylated species of 2.6 and 1.6 kb were found, the latter of which representing mature 16 S mRNA. S1 analysis revealed both the presence of spliced and unspliced RNA sequences in transcriptionally active SV40 chromatin.

Site of premature termination of late transcription of simian virus 40 DNA: enhancement by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole

Sedimentation analysis of pulse-labeled RNA synthesized in nuclei isolated from simian virus 40-infected cells revealed an abundance of short cellular and viral RNAs. The relative amount of the short chains is increased in nuclei isolated from cells treated with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). The short viral RNAs were purified by hybridization to and elution from simian virus 40 DNA on filters, and their sizes were determined by gel electrophoresis. A major band of 93- to 95-nucleotide-long RNA was observed along with additional minor bands. Identical bands were revealed when the viral RNA was purified from nuclei of cells pretreated with DRB. The major band was identified as an aborted transcript of a RNA that initiated at the major initiation site (nucleotide 243). We have found that the DNA region where the RNA stops is A+T rich and is immediately preceded by a G+C-rich region that exhibits dyad symmetry, resembling the termination signal in prokaryotes. These observations show that RNA polymerase II responds to the same termination signal as the prokaryotic enzyme and suggest that a mechanism of attenuation regulates simian virus 40 late transcription.

Attenuation in the control of SV40 gene expression

Nuclei and viral transcriptional complexes were prepared from cells infected with simian virus 40 and incubated in vitro in the presence of alpha- 32P-UTP. The in vitro elongated viral RNA appeared with a peak of 5S in sucrose gradients and hybridized preferentially to a promoter-proximal region of SV40 DNA. Treatment of infected cells with proflavine led to transcription of elongated RNA, while treatment of cells with 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole, a drug known to enhance premature termination, augmented accumulation of the promoter-proximal RNA. The in vitro elongated RNA produced a major band of 93-95 nucleotides in length in acrylamide gel. This RNA was found to map between the major initiation site at nucleotide 243 and nucleotides 335-337. The significance of these observations with respect to the transcription termination signal and the control of SV40 gene expression is discussed.

Transforming genes and gene products of polyoma and SV40

The small DNA-containing viruses, SV40 and polyoma, transform cells in vitro and induce tumors in vivo. For both viruses two genes required for transformation have been found. The genes required for transformation are also involved in productive infection. Although the two viruses are similar in their effects on cells, the organization of the transforming genes and gene products is different. The purpose of this review is to compare what is known about the biology and the biochemistry of the early regions of the two viruses. The genetic and biochemical studies defining the sequences important for transformation will be reviewed. Then, the products of the transforming genes, called T antigens, will be discussed in detail. There is a substantial body of descriptive information on those products, and studies on the function of the T antigens have also begun.

Simian virus 40 transcriptional complexes incorporate mercurated nucleotides into RNA in vitro

Simian virus 40 Sarkosyl transcriptional complexes incorporated mercurated nucleotide precursors into preinitiated RNA chains. Synthesis with mercurated precursors was three- to fivefold slower than with nonmercurated nucleotides (12 to 20 pmol per 10(6) cells per h at 25 degrees C), and 50 to 70% of the RNA product bound specifically to sulfhydryl-agarose. The amount of mercurated CMP incorporated into RNA was measured by specific binding of [35S]cysteine to the mercury residues. More than 90% of the mercurated RNA hybridized to simian virus 40 DNA.

Escherichia coli RNA polymerase in vitro mimics simian virus 40 in vivo transcription when the template is viral nucleoprotein

We have used a low-salt detergent-free extraction procedure on cells infected with simian virus 40 to obtain viral nucleoprotein late after infection. Addition of EScherichia coli RNA polymerase and ribonucleotide triphosphates to the viral minichromosomes permitted transcription of RNA from viral templates. This synthesis was initiated predominantly within a fragment of DNA spanning 0.67 to 0.76 map unit on the genome. The synthesis from this region proceeded primarily along the "late" strand in a clockwise direction. These results were in contrast to the synthesis obtained with naked viral DNA in which initiation occurred on other regions of the genome and from which transcription proceeded counterclockwise along the early strand. These findings indicate that the nucleoprotein template or factors tightly associated with it may be responsible for site(s) and strand selection in transcription of simian virus 40.

5,6-dichloro-1-beta-ribofuranosylbenzimidazole enhances premature termination of late transcription of simian virus 40 DNA

Short RNA chains initiating at the major promoter sites for simian virus 40 (SV40) late transcription are elongated to approximately 450 nucleotides in a molar ammount greater than that from any other region of the viral DNA. This conclusion is based on the following observations: (i) Transcriptional complexes isolated by Sarkosyl and by hypotonic leaching (minichromosomes) from nuclei of cells infected with SV40 as well as intact nuclei were pulse labeled in vitro with [alpha-32P]TUP and were observed to synthesize short RNA transcripts that hybridized predominantly to a SV40 DNA fragment spanning between 0.67 and 0.76 map units. (ii) In the presence of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), a drug known to accentuate premature transcriptional termination, accumulation of these short SV40 RNA chains was enhanced. When SV40-infected cells were pretreated with DRB and then labeled in vivo or in vitro, they synthesized short labeled viral RNAs that hydridized almost exclusively with the DNA fragment spanning between 0.67 and 0.76 map units. These observations suggest a mechanism in the regulation of SV40 late transcription.

Extraction of transcribing SV40 chromosomes in very low salt

SV40 chromosomes capable of continued RNA synthesis in vitro have been extracted from infected cells in solutions of very low ionic strength (5 mM HEPES, 0.25 mM MgCl2). The RNA made is of high molecular weight, and synthesis is sensitive to alpha-amanitin. More RNA is made than from previously described (high salt-extracted) transcription complexes. Transcribing SV40 chromosomes sediment at a rate intermediate between replicating and mature chromosomes, and have a higher ratio of protein to nucleic acid than either. They retain proteins that are lost during exposure to high salt, and might prove valuable in identifying proteins involved in SV40 transcription.

Characterization of simian virus 40 transcriptional intermediates in infected CV1 cell nuclei

Simian Virus (SV40) transcriptional intermediates (T.I.) were isolated from infected cell nuclei incubated in vitro in the presence of the four ribonucleoside triphosphates. The nascent mRNA strands in the viral DNA-RNA hybrid molecules were hydrogen bonded to their template by 200-250 nucleotides on the average, as judged from the extent of their RNase resistance and the aspect of T.I. under electron microscope after treatment with 50 per cent formamide. The RNA polymerase involved (RNA polymerase II) synthesized up to full length transcripts at a rate of approximately 150 nucleotides/min. at 25 degrees C. Each SV40 infected cell was found to contain about 200 active T.I. molecules at the peak of late transcription. The DNA in the T.I. molecules was exclusively form I DNA only in cell infected with the tsA30 mutant of SV40 that had been transferred to non-permissive temperature in order to arrest DNA replication, but both form I DNA and molecules behaving as replicative intermediates (R.I.) in wild type infected cells.

Biochemical and electron microscopic characterization of DNA-RNA complexes from HeLa cell mitochondria

The previous electron microscopic investigations on the occurrence in HeLa cell mitochondria of transcription complexes of mitochondrial DNA [Aloni, Y., and Attardi, G. (1972a), J. Mol. Biol. 70, 363-373] have been extended with the aim of obtaining these complexes in a reasonably pure form for biochemical analysis. By using conditions designed to minimize losses of such structures and any possible contamination by nuclear DNA, it has been shown that a substantial fraction (40 to 50%) of mitochondrial DNA can be isolated from exponentially growing HeLa cells in the form of fastsedimenting complexes with RNA. These complexes have been characterized with respect to density and sedimentation properties, content in newly synthesized RNA, stability of the association of RNA with DNA, presence of different forms of mitochondrial DNA, and electron microscopic appearance. The properties of these complexes, as well as the results of reconstruction experiments, strongly suggest that the majority of such structures represent true transcriptional intermediates. The occurrence in this fraction of replicating or newly replicated mitochondrial DNA molecules has been observed. Although the presence of single-stranded DNA segments makes the replicative intermediates particularly susceptible to aggregation with free RNA, electron microscopic observations point to the possibility that these intermediates may be recruited for transcription.