RNA is synthesized at the nuclear cage

Synthesis and processing of simian virus 40-specific RNA in adenovirus-infected, simian virus 40-transformed human cells

Human simian virus 80 (SV80) cells transformed by simian virus 40 (SV40) synthesize substantial quantities of the SV40 large T-antigen (Henderson & Livingston, 1974; Tjian, 1978) and cytoplasmic, poly(A)-containing RNA species that exhibit spliced structures characteristic of the SV40, early messenger RNA species that encode both large and small T-antigens (Flint & Beltz, 1979). When SV80 cells were infected with type C adenovirus, both the synthesis of SV40 large T-antigen and the appearance in the cytoplasm of newly synthesized, SV40-specific RNA sequences were inhibited during the late phase of infection. The results of hybridization to SV40 DNA of SV80 nuclear RNA, prepared from mock- or adenovirus-infected cells after labeling for short periods in vivo or in vitro, indicated that transcription of integrated SV40 was, by contrast, not disrupted during the late phase of adenovirus infection. Poly(A)-containing, nuclear RNA species that hybridized to SV40 DNA sequences and exhibited the sizes of spliced, large and small T-antigen mRNA species were also synthesized in infected cells at a time when the corresponding mRNA sequences did not leave the nucleus. These results suggest that the failure of non-adenoviral mRNA sequences to enter the cytoplasm of adenovirus-infected cells does not reflect inhibition of either their transcription or the normal enzymatic processing reactions to which pre-mRNA species are subject. Several lines of evidence do, however, establish that nuclear, SV40-specific RNA sequences are less stable in adenovirus-infected compared to mock-infected SV80 cells.

Generation of an internal matrix in mature avian erythrocyte nuclei during reactivation in cytoplasts

When fused with mouse L-cell cytoplasts, chick erythrocyte nuclei enlarge, take up proteins from the host cytoplasm, and recommence RNA synthesis. We found that during this transition the erythrocyte nuclei gain an internal nuclear matrix, thus providing a novel approach to questions concerning the nature of the salt-resistant intranuclear skeleton. A new method for preparation and examination of the nuclear matrix in situ is also described.

94,000- and 100,000-molecular-weight simian virus 40 T-antigens are associated with the nuclear matrix in transformed and revertant mouse cells

A small fraction of the 94,000-molecular-weight multifunctional large T-antigen of simian virus 40 was associated with the nuclear protein matrix derived from simian virus 40-transformed mouse cells. The interaction between this fraction of T-antigen and the matrix was largely or entirely independent of nuclear DNA. Similar amounts of T-antigen were retained by the nuclei of transformed and revertant cell lines. A 100,000-molecular-weight variant of T-antigen, which has been found to correlate specifically with anchorage-independent growth, was present in the nuclear protein matrix of a transformed cell line. A T-antigen-containing revertant selected for the reacquisition of a high serum requirement and an anchorage requirement for growth retained T-antigen in association with its matrix.

Distribution of interchromatin granules in nuclear matrices obtained from nuclei exhibiting different degree of chromatin condensation

Female Wistar rats were injected intraperitoneally with thioacetamide or Ledakrin (1-nitro-9/3'-dimethylpropyloamine/acridine . 2 HCl) to provoke chromatin decondensation or condensation, respectively. Liver nuclei of each experimental group were used to isolate nuclear matrices. The results provide evidence that interchromatin granules are components of the nuclear matrix obtained by the procedure used in these experiments. They also reveal that the general ultrastructural pattern of the nuclear matrix is related to the degree of chromatin condensation in the nuclei used for its isolation. The distribution of interchromatin granules characteristic for the source nuclei is maintained in the nuclear matrices.

Processing of SV40 RNA is associated with the nuclear matrix and is not followed by the accumulation of low-molecular-weight RNA products

Nuclear matrices from SV40-infected cells were prepared by treating purified nuclei with DNase and salt to extract DNA and histones. After a 10-min pulse with [5,6(-3)H]uridine over 85% of the viral RNA was found in association with the nuclear matrix. Following a 3-hr chase with glucosamine and unlabeled uridine, 2-4 S nuclear viral components accumulate, but they are not associated with the nuclear matrix. The 2-4 S components had been characterized previously as viral RNA processing products (N. H. Chiu, M. F. Radonovich, M. M. Thoren, and N. P. Salzman, J. Virol. 28, 590-599, 1978). The results of the present study identify the 2-4 S components as DNA rather than RNA, thus indicating that the synthesis and the majority of the processing products of the viral RNA are associated with the nuclear matrix.

Mitotic architecture of the cell: the filament networks of the nucleus and cytoplasm

The skeletal framework of cells at the various stages of mitosis are prepared by extraction with nonionic detergent and examined by stereoscopic whole mount electron microscopy. The insoluble filament network remaining after the detergent-extraction and the depolymerization of microtubules is shown. The nonchromatin filament network of the nucleus, or nuclear matrix, becomes visible as the chromatin condenses at prophase. Filaments are associated with the chromosomes throughout mitosis. Parts of the chromosomes are associated with or are near the nuclear lamina at early stages. The nuclear lamina disappears at metaphase while chromosomes remain associated with filaments now continuous with the cytoplasmic network. Microtubules appear to be unnecessary for maintaining the chromosome position in these preparations since comparison of cells with and without microtubules shows no gross change in chromosome arrangement. The cellular filament network at metaphase and anaphase appears continuous from the plasma lamina to the chromosomes. The filament networks visualized here may be responsible for the prometaphase chromosome movement and participate in the formation of the midbody.

Nonrandom distribution of repeated DNA sequences with respect to supercoiled loops and the nuclear matrix

The DNA in a eukaryotic nucleus is arranged into a series of supercoiled loops that are anchored at their bases to the nuclear matrix. We have analyzed the DNA sequences that are closest to the matrix attachment points for their relative content of specific repeated sequences. Sequences were enriched (mouse satellite, human Alu family) or depleted (mouse EcoRI repeat, monkey alpha component), depending on the specific sequence and species examined. These results can be understood in terms of a nonrandom arrangement of DNA sequences with respect to nuclear DNA loops.

The changes in proviral chromatin that accompany morphological variation in avian sarcoma virus-infected rat cells

The clone All of avian sarcoma virus B77-infected Rat-1 cells comprises both morphologically normal and morphologically transformed derivatives. Transformed subclones, in which virus-specific RNA is readily detectable, contain a provirus that is very sensitive to DNase 1 digestion of chromatin, and show DNase 1 hypersensitive sites at the 5' end of the provirus and in 5' flanking cell DNA. Normal subclones with no detectable virus-specific RNA, whether infected cells that have never been transformed or revertants derived from transformed cells, contain a provirus that is far more resistant to DNase 1 digestion. Moreover this provirus lacks hypersensitive sites at its 5' end, although DNase 1 hypersensitive sites were detected at the 3' end of the provirus in either normal or transformed clones. The pattern of cytosine methylation in the proviral restriction sites of the isoschizomers Msp I and Hpa II differed between transformed and revertant clones; the revertants show additional methylation at some CpG doublets.

Distribution of tightly bound proteins in the chicken ovalbumin gene region

A method for investigating the distribution of the DNA-tightly bound proteins (TBP) in the chicken ovalbumin gene regions is described. The TBP are operationally defined as the proteins that remain bound to the DNA in the presence of 2 M NaCl. Nuclei from chicken erythrocytes, livers, and oviducts were lysed in 2 M NaCl and sheared, the lysed chromatin was chromatographed through a Sepharose 4B column to separate protein and DNA which contains the TBP. The DNA fraction, after digestion with restriction endonuclease EcoRI, was passed through a GF/C glass fiber filter. The filter retains the DNA-TBP but not the protein-free DNA. In all cases, only about 0.5% of the total genomic DNA was retained on the filter. The ovalbumin gene sequences in the DNA-TBP were analyzed by the Southern blot. It was found that the copy number of the ovalbumin gene in the DNA-TBP isolated from erythrocyte or liver nuclei is not significantly different from that in the total unfractionated nuclear DNA. Liquid hybridization of nick-translation-labeled DNA-TBP with a large excess of total chicken nuclear DNA also demonstrated that the DNA is not a specific subset of the genome. In the nuclei from laying hen oviducts in which the ovalbumin gene but not the globin is actively expressed, approximately 3-fold enrichment of the ovalbumin gene was found in the DNA-TBP. The enrichment could have been due to a contamination of transcriptional complexes during the purification of the DNA-TBP, since no depletion in the globin gene sequences was found in the same sample. These results suggest a random distribution of the TBP in that genome, due possibly to a transient interaction between DNA and TBP. Precautions for evaluation of the results dealing with the TBP published in the literature are also discussed.

The ovalbumin gene is associated with the nuclear matrix of chicken oviduct cells

The DNA in a eucaryotic nucleus is arranged into a series of supercoiled loops that are anchored at their bases to the nuclear matrix. Using nuclease digestion, one can progressively cleave DNA from the loops, thereby isolating residual DNA that is progressively closer to the nuclear matrix anchorage sites. We have determined that the ovalbumin gene is preferentially associated with the nuclear matrix of chicken oviduct cells, but is not preferentially associated with the nuclear matrix of chicken liver cells. As a control, the beta-globin gene, which is not transcribed in oviduct cells, was found not to be preferentially associated with the oviduct nuclear matrix. The observation that the transcriptionally active ovalbumin gene is preferentially associated with the nuclear matrix may have significant implications for gene expression and the organization of nuclear DNA into supercoiled-loop domains.

Expression of a beta-globin gene is enhanced by remote SV40 DNA sequences

We have studied the transient expression of a cloned rabbit hemoglobin beta 1 gene after its introduction into HeLa cells. Two and one-half days after transfection using the calcium phosphate technique we extracted RNA from the entire cell population and analyzed it by the S1 nuclease hybridization assay. Transcripts were barely detectable when beta-globin gene-plasmid recombinants were used. However, 200 times more beta-globin gene transcripts were found when the beta-globin gene recombinants also contained SV40 DNA, and 90% of these transcripts (about 1000 per cell) had the same 5' end as authentic rabbit globin mRNA. In the latter case, abundant production of beta-globin protein was readily detected in a fraction of transfected cells by immunofluorescent staining. Enhancement of globin gene expression was dependent on SV40 sequences acting in cis, but independent of the viral origin of DNA replication. The enhancing activity was associated with the 72 bp repeated sequence element located at the beginning of the viral late gene region. Viral DNA fragments containing the transcriptional enhancer element could act in either orientation at many positions, including 1400 bp upstream or 3300 bp downstream from the transcription initiation site of the rabbit beta-globin gene. These studies define a class of DNA elements with a mode of action that has not been heretofore described. The activation of genes by specific enhancer elements seems to be a widespread mechanism that may be used for the regulation of gene expression.