Control of simian virus 40 gene expression at the levels of RNA synthesis and processing: thermally induced changes in the ratio of the simian virus 40 early mRNA's and proteins

Alternative splicing of SV40 early pre-mRNA in vitro

Simian virus 40 (SV40) early pre-mRNA is spliced using either of two alternative 5' splice sites and a common 3' splice site to produce two mRNAs that encode the T and t antigens. We have studied alternative splicing of SV40 early pre-mRNA in vitro using a HeLa cell nuclear extract. Synthetic SV40 early transcripts are processed to T and t antigen mRNAs in vitro. As in SV40-infected cells in vivo, cleavage at the T antigen 5' splice site is more efficient than cleavage at the t antigen 5' splice site in vitro, although both of these 5' splice sites are utilized relatively inefficiently in vitro. The ratio of cleavage at the T and t antigen 5' splice sites is not changed significantly by a number of alterations in the conditions under which the in vitro splicing reactions are carried out.

Fission yeast Schizosaccharomyces pombe correctly excises a mammalian RNA transcript intervening sequence

Study of heterologous gene expression in the budding yeast Saccharomyces cerevisiae has shown that this organism is incapable of correctly removing intervening sequences from transcripts of higher eukaryotic genes. This is probably due to the stringent requirement for the presence of a TACTAAC box close to the 3' end of the intervening sequence if splicing in S. cerevisiae is to occur. Comparison of the introns found in the fission yeast Schizosaccharomyces pombe has identified conserved sequences similar to those found in higher eukaryotes. Therefore, we have investigated whether Schiz. pombe is capable of accurately excising intervening sequences from the transcripts of higher eukarotic genes. We show here that both the 5' and 3' splice sites of the simian virus 40 (SV40) small-T antigen transcript are accurately utilized when cloned viral DNA is expressed in Schiz. pombe cells. These data suggest that Schiz. pombe may be a better model system than S. cerevisiae for the genetic study of RNA splicing and for expressing higher eukaryotic genes.

The expression of the SV40 early region in the transformed human cell line SV80

The one complete copy of the SV40 early region present in human cells of the transformed line SV80 carries a duplication of the 5' portion of the early region, including its transcriptional control region and splicing signals (W. Gish and M. Botchan, personal communication). Novel SV40-specific RNA species of sufficiently large size, 3.8 and 4.2 kb, to be expressed from the duplicated early transcriptional control region were detected in SV80 cytoplasmic and nuclear RNA preparations by blot hybridization. The results of transcription in a cell-free system of a plasmid, pSV80-04, representing this SV80 cell SV40 DNA integrate (W. Gish and M. Botchan, personal communication) and of nuclease protection experiments with end-labelled pSV80-04 DNA fragments support the conclusion that the duplicated early sequences are transcribed in SV80 cells. It has also been established that the duplicated early splicing signals are functional in SV80 cells. These results are discussed in relation to the large amounts of SV40 early mRNA and T-antigen synthesized in cells of the SV80 line.

Monoclonal antibody to simian virus 40 small t

A monoclonal antibody, PAb280, was produced that recognizes simian virus 40 (SV40) small t but does not react with SV40 large T. The specificity of the antibody was analyzed by immunoprecipitation of labeled cell extracts, Western blotting, and immunocytochemistry. Small t was found to accumulate late in the SV40 lytic cycle and was localized in both the cytoplasm and the nucleus of cells infected with wild-type SV40. Importantly, antibodies against determinants common to SV40 large T and small t did not appear to be able to recognize the cytoplasmic form of SV40 small t at the immunocytochemical level. The localization of small t within the nucleus appeared to be distinct from that of large T.

Splicing pathways of SV40 mRNAs in X. laevis oocytes differ in their requirements for snRNPs

To examine the role of small nuclear ribonucleoproteins (snRNPs) in mRNA splicing, we have injected SV40 DNA, in the presence or absence of anti-Sm or anti-(U1)RNP antibodies, into the nucleus of X. laevis oocytes, and analyzed the viral specific RNAs and proteins that were synthesized. In the absence of antibodies, the majority of the viral mRNAs were spliced, giving rise to transcripts and proteins analogous to those found in infected monkey cells. However, the relative efficiencies with which the various splice sites were utilized were different in the two cell types. When sera from systemic lupus erythematosus (SLE) patients containing anti-Sm or anti-(U1)RNP antibodies were coinjected with the viral DNA, splicing of L-strand-specific (late) mRNA was dramatically inhibited. Cleavage at both 5' and 3' splice sites was blocked, leading to an accumulation of unspliced primary transcripts. Neither the total amount of late RNA synthesized nor the formation of mature polyadenylated late mRNA 3' ends was affected. These results indicate that U1 snRNPs play a crucial role in mRNA splicing in vivo. Unexpectedly, the effects of the sera on E-strand-specific (early) viral mRNA splicing were different. All anti-Sm or -(U1)RNP sera tested had no detectable effect on the splicing of the mRNA coding for the small tumor antigen. A subset of these sera, however, inhibited large tumor antigen mRNA splicing. On the basis of these data it is suggested that different pre-mRNAs, or even different splice sites within the same pre-mRNA, have dissimilar interactions with snRNP particles in the splicing reaction.

Stabilization of the large T protein in temperature-independent (type A) FR 3T3 rat cells transformed with the simian virus 40 tsA30 mutant

The stabilities of in vivo [35S]methionine-labeled large T and small t proteins, synthesized in temperature-sensitive (type N) and temperature-insensitive (type A) FR 3T3 rat cells transformed by an early temperature-sensitive mutant of simian virus 40 (SV40), tsA30, were analyzed at the permissive and restrictive temperatures. The two polypeptides, detected in greatly reduced amounts in cells of the N type at the restrictive temperature, were also unstable at the permissive temperature. However, both were made in similar amounts and were apparently stable in cells of the A type, irrespective of the temperature. The structures of the viral RNAs present at the permissive temperature were analyzed for transformants representative of each type, and containing a single integration of viral DNA. The two cell lines synthesized transcripts identical to the large T and small t mRNAs identified in SV40-infected monkey cells. Similar amounts of viral RNA were found in A and N transformants in active growth at the permissive and restrictive temperatures, which argued against a control at a transcriptional level. Assay of a defined function of the protein, namely, the binding of nucleotide detected by affinity labeling with periodate-oxidized [alpha-32P]ATP, clearly showed that the large T proteins from both types of transformants exhibited, at least for that particular biochemical function, the same in vitro temperature sensitivity. In transformants of the A type only could a reduced binding activity be detected in extracts from cells grown at the restrictive temperature. Thus, the temperature-independent behavior of the A transformants may result from an in vivo partial stabilization of the newly synthesized large T protein, probably through interaction with a cellular component(s).

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.

Detection of simian virus 40 surface-associated large tumor antigen by enzyme-catalyzed radioiodination

To facilitate detection of SV40 surface-associated tumor antigen (T-ag), conditions were established to surface label T-ag on intact cells by lactoperoxidase-catalyzed radioiodination (125I/LPO). SDS-PAGE analysis of anti-T immunoprecipitates of SV40-transformed and -infected cells labelled with 125I/LPO revealed the presence of iodinated T-ag. Several types of control experiments were employed to guarantee the surface specificity of the 125I/LPO labelling technique. When SV40-transformed mouse cells were surface labelled with lactoperoxidase and glucose oxidase immobilized on insoluble beads, a preparation less readily internalized than soluble enzymes, T-ag was iodinated. Selective immunoprecipitation of surface antigens demonstrated that lactoperoxidase did not iodinate internally localized T-ag. A reconstruction experiment in which an extract of SV40-infected cells was added to uninfected cells prior to surface labelling suggested that T-ag released from lysed cells did not adhere significantly to monolayer surfaces and become iodinated. Finally, systematic omission of reactants from the iodination reaction revealed that exogenous addition of lactoperoxidase and H2O2 was necessary to generate an iodinated T-ag, indicating that endogenous host cell reactants do not contribute significantly to the iodination of T-ag. 125I-labelled T-ag was detectable on the surface of SV40 tsA-infected cells at the nonpermissive temperature 24 h post infection, indicating that the tsA lesion does not prevent the interaction of T-ag with the cell surface. When 125I/LPO-labelled transformed or infected cells were chased for 2.5 h after labelling, iodinated T-ag was no longer associated with the cell monolayer but was immunoprecipitable from culture supernatants. Cultures from which labelled T-ag had been shed could then be relabelled with 125I/LPO and surface-associated T-ag was again detectable. These data suggest that surface-associated T-ag is continuously shed from the cell surface and is rapidly replaced in the membrane by intracellular T-ag.

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.

New chimeric splice junction in adenovirus type 2-simian virus 40 hybrid viral mRNA

We have examined hybrid viral RNAs synthesized in both human and monkey cells infected by three nondefective adenovirus type 2 (Ad2)-simian virus 40 (SV40) hybrid viruses; Ad2+ND1, Ad2+ND2, and Ad2+ND4. Most of the hybrid viral RNA molecules appeared to be initiated within adenoviral sequences, but were polyadenylated on their 3' end at the early SV40 mRNA polyadenylation site. The Ad2+ND4 stock of virus was not homogeneous, but consisted of two principle populations of viral DNA. Both populations contained a segment of SV40 DNA extending from the SV40 map positions 0.63 to 0.11 in a left-to-right orientation at adenovirus map position 0.82. One population contained an intact SV40 segment, whereas the other (representing 80 to 85% of the population) has a 500-base pair deletion mapping from approximately 0.60 to 0.50 SV40 map units. This deletion encompassed the SV40 DNA segment which encodes the early SV40 splice sites. Cells infected by the mixed Ad2+ND4 population induced the synthesis of at least three major SV40 RNA species among the hybrid viral transcripts. The most abundant of these hybrid mRNA's appeared only late in the lytic cycle, after the onset of viral DNA replication. It contained an RNA splice junction which extended from a donor (5') nucleotide within the adenoviral RNA sequences to an acceptor (3') splice site within the early region of SV40 at 0.46 SV40 map units. This SV40 acceptor splice site was remarkable in that its use has not been detected in the spliced viral mRNA's of SV40-infected or -transformed cells.