T-antigen expression by polyoma mutants with modified RNA splicing

Polyoma virus mutants were constructed that could not express all the three T-antigens. The mutagenesis was directed to the two 5' splice sites utilized in the maturation of early RNA. The mutant bc1051 had a base change at the splice site of large T-antigen mRNA, and the mutants dl1061 and dl1062 had deletions at the corresponding splice point of small and middle T-antigen mRNA. The site was removed in mutant dl1061 and altered by fusion to upstream sequences in mutant dl1062. Analysis of viral RNA showed that dl1061 and dl1062 formed only large T-antigen mRNA, whereas bc1051 did not produce this RNA-species. However, the biological properties of dl1062 suggested that it also produced mRNA directing the synthesis of a small T-antigen-related polypeptide, at least in low amounts. Only mutant bc1051 could induce transformation of rat cells. In mouse 3T3 cells dl1062 multiplied to a limited extent, while bc1051 and dl1061 failed to produce virus. However, dl1061 DNA was synthesized at a low rate which could be increased to normal levels by co-transfection with mutant bc1051. This result suggests that polyoma small and middle T-antigen have a previously unrecognized function in the early phase of the infection process, or in viral DNA-synthesis.     

Activities of polyomavirus large-T-antigen proteins expressed by mutant genes

We constructed a set of polyomavirus mutants with alterations in the DNA sequences encoding large T-antigen. The mutant genomes were cloned and propagated as recombinants of plasmid pBR322, and the presence of the mutations was confirmed by nucleotide sequence analysis. To facilitate the analysis of defects in the function of large T-antigen, the dl1061 deletion was introduced into the mutant genomes. This deletion restricts the early gene expression to the synthesis of large T-antigen (Nilsson and Magnusson, EMBO J. 2:2095-2101, 1983). The mutant large T-antigens were identified after radioactive labeling. Their functional characterization was based on analysis of DNA binding, activity in the replication of viral DNA, and cellular localization. The native large T-antigen, which is 785 amino acid residues long, binds specifically to the regulatory region of polyomavirus DNA. This binding was significantly reduced by the deletion of amino acid residues 136 to 260. Nevertheless, this mutant large T-antigen was active in the initiation of viral DNA replication. Conversely, all of the mutants in this study that produced large T-antigens with alterations in the carboxy-terminal 146 amino acid residues had normal DNA-binding properties. However, these mutants were inactive in viral DNA synthesis and also inhibited the replication of wild-type DNA in cotransfected cells. The analysis of mutant dl2208 (Nilsson et al., J. Virol. 46:284-287, 1983) led to unexpected results. Its large T-antigen, missing amino acid residues 191 to 209, was overproduced. Although the protein had normal DNA-binding properties, it was not entering the cell nucleus normally. Furthermore, the dl2208 DNA replication was extremely low in the absence of small and middle T-antigens but was normal in the presence of these proteins.     

Immunization against the polyoma virus-induced tumor-specific transplantation antigen by early region mutants of the virus

To investigate the relation between the polyoma tumor-specific transplantation antigen and the virus-coded proteins, mice were immunized by inoculation of a variety of viable polyoma virus mutants and then challenged with polyoma virus-induced tumors. Two classes of early region mutants were used. One class produces a normal small T-antigen and truncated middle and large T-antigens. The second class (hr-t mutants) forms a normal large T-antigen together with N-terminal fragments of small and middle T-antigens. All mutants, transforming as well as nontransforming, induced protection against polyoma virus tumors. However, there were quantitive differences between the mutants. The finding that an hr-t mutant could induce tumor rejection suggests that full-length middle and small T-antigens are not necessary for the induction of this response. Since intact middle T-antigen is the only virus-coded protein known to associate with the plasma membrane, the possibility must be considered that the polyoma virus tumor-specific transplantation antigen consists of cellular components.     

Viable deletion mutant in the medium and large T-antigen-coding sequences of the polyoma virus genome.

A polyoma virus mutant that maps in the early region between the known hr-t and ts-a mutants has been isolated. Its 66-base-pair deletion results in structural changes in both medium and large T-antigens but causes no substantial alterations in viral replication or cell transformation.     

Recent progress in studies of polyomavirus tumour antigens

The polyomavirus tumour (T) antigens were originally identified by their reactivity with antisera from tumour-bearing animals. The primary structure of the three T-antigens has been established by combining the information from the nucleotide sequencing of DNA, RNA analysis, and peptide mapping. The functions of the T-antigens in productive infection and cellular transformation have largely been analysed by using virus mutants. The large T-antigen binds specifically to polyomavirus DNA. This binding is probably linked to the activity of the protein in the control of viral DNA and RNA synthesis. In addition, the large T-antigen has the ability to confer an unlimited growth potential to cells in culture. The middle T-antigen is a primary inducer of cellular transformation. The part of this protein that is located in the plasma membrane, is associated with a tyrosine kinase activity. The small T-antigen, finally, has not yet been studied extensively. However, small T-antigen has to be expressed to allow a complete productive infection cycle in mouse cells.     

Nucleotide sequence changes in polyoma virus A gene mutants.

The mutational alterations in polyoma virus mutants ts-a and ts-25E which cause their large T-antigens to be thermolabile have been identified. In ts-a, a G leads to A transition at nucleotide 2193 causes the replacement of Ala (GCT) by Thr (ACT). In ts-25E, a G leads to T transversion at nucleotide 2883 causes the replacement of Gly (GGC) by Cys (TGC). Revertants of both mutants have been isolated and shown to have the original nucleotides restored at these positions.     

Polyoma viral middle T-antigen is required for transformation.

To determine whether small or middle T-antigen (or both) of polyoma virus is required for transformation, we constructed mutants of recombinant plasmids which bear the viral oncogene and measured the capacity of these mutants to transform rat cells in culture. Insertion and deletion mutations in sequences encoding small and middle T-antigens (79.7, 81.3, and 82.9 map units) rendered the DNA incapable of causing transformation by the focus assay. Similar mutations in sequences that encoded middle but not small T-antigen (89.7, 92.1, and 96.5 map units) generally abolished the transforming activity of the DNA. However, two mutants (pPdl1-4 and PPd12-7) that carried deletions at 92.1 map units retained the capacity to transform cells; pPdl1-4 did so at frequencies equal to those of the parental plasmid, whereas pPdl2-7 transformed at 10% the frequency of its antecedent. From these studies we conclude that small T-antigen alone is insufficient to cause transformation and that middle T-antigen is required for transformation, either in combination with small T-antigen or by itself.     

Effect of deletions in adenovirus early region 1 genes upon replication of adeno-associated virus.

The growth of adeno-associated virus (AAV) is dependent upon helper functions provided by adenovirus. We investigated the role of adenovirus early gene region 1 in the AAV helper function by using six adenovirus type 5 (Ad5) host range mutants having deletions in early region 1. These mutants do not grow in human KB cells but are complemented by and grow in a line of adenovirus-transformed human embryonic kidney cells (293 cells); 293 cells contain and express the Ad5 early region 1 genes. Mutants having extensive deletions of adenovirus early region 1a (dl312) or regions 1a and 1b (dl313) helped AAV as efficiently as wild-type adenovirus in 293 cells, but neither mutant helped in KB cells. No AAV DNA, RNA, or protein synthesis was detected in KB cells in the presence of the mutant adenoviruses. Quantitative blotting experiments showed that at 20 h after infection with AAV and either dl312 or dl313 there was less than one AAV genome per cell. In KB cells infected with AAV alone, the unreplicated AAV genomes were detected readily. Apparently, infection with adenovirus mutant dl312 or dl313 results in degradation of most of the infecting AAV genomes. We suggest that at least an adenovirus region 1b product (and perhaps a region 1a product also) is required for AAV DNA replication. This putative region 1b function appears to protect AAV DNA from degradation by an adenovirus-induced DNase. We also tested additional Ad5 mutants (dl311, dl314, sub315, and sub316). All of these mutants were inefficient helpers, and they showed varying degrees of multiplicity leakiness. dl312 and dl313 complemented each other for the AAV helper function, and each was complemented by Ad5ts125 at the nonpermissive temperature. The defect in region 1 mutants for AAV helper function acts at a different stage of the AAV growth cycle than the defect in the region 2 mutant ts125.     

Efficient oligodeoxyribonucleotide-directed deletion mutagenesis using pEMBL vectors: removal of early region introns from polyoma virus mutants

We have used oligodeoxyribonucleotide-directed deletion mutagenesis to remove early region introns from polyoma virus mutants. To this end we compared single priming, double priming, and gapped duplex approaches using either priming at 37 degrees C or at the critical temperature. The gapped duplex approach, coupled with priming at the critical temperature, resulted in up to 70% yield of the desired product. In conjunction with the use of the pEMBL vector system this method was simplified to yield specific deletions from cloned large DNA fragments with high efficiency. The resulting mutant plasmids could be used directly for biological assays without retransformation or recloning. RNA and protein analyses showed that removal of the large T- or middle T-antigen introns from polyoma early region mutants dl23 and dl8 was specific and resulted in DNA competent for the synthesis of only one T antigen.     

Nucleotide sequence of the Hind-C fragment of simian virus 40 DNA. Comparison of the 5'-untranslated region of wild-type virus and of some deletion Mutants.

We report here the nucleotide sequence of the wild-type simian virus 40 (strain 776) restriction fragment Hind-C-P1 DNA and of the homologous region of various mutant DNAs which lack part of this fragment. During this work, we detected between EcoRII fragments N and G an additional, 17-base-pair EcoRII fragment, fragment P, which had previously been overlooked. Also, an additional dTpdG dinucleotide at residues L 339--340 was observed by sequence analysis of the DNA minus (E) strand; the presence of this dinucleotide was masked on sequencing patterns of the plus strand due to the persistence (during gel electrophoresis) of some secondary structures in the strand's 5'-terminal region. These nucleotide additions raise the total length of SV40 DNA to 5243 base pairs. The longest tandemly repeated segment in SV40 DNA now extends over 72 base pairs. SV40 deletion mutants dl 893 and dl 894 and SV40 strains Rh 911 and 1801 all lack an identical 72-base-pair-long DNA segment in the Hind-C region. This deletion corresponds precisely to one of the two aforementioned large tandemly repeated sequences. Mutant dl 895 lacks 66 base pairs, 63 of which are part of the former repetition. All these mutants, except dl 895, very probably were generated by an intramolecular, homologous recombination event. The 40-base-pair deletion in mutant dl 1811 includes the major capping site of SV40 late RNA. dl 1812 lacks only three base pairs, which are part of the overlapping HhaI and HpaII restriction sites at position 0.725--0.726.     

Mutant of simian virus 40 large T-antigen that is defective for viral DNA synthesis, but competent for transformation of cultured rat cells.

A mutant was isolated which demonstrates that the transforming activity of simian virus 40 large T-antigen is separable from its function in viral DNA replication. The mutant, SVR9D, is nonconditionally defective for viral DNA synthesis, but competent at wild-type level for morphological transformation of cultured rat cells. The lytic growth defect in SVR9D is complemented by the simian virus 40 A gene product present in the transformed CV1 cell line, COS1. The lesion in SVR9D DNA was mapped genetically by marker rescue of plaque formation and localized to a 214-base-pair segment of the viral genome bounded by nucleotide numbers 4100 and 4314. DNA sequence analysis showed the mutation to be an adenine-to-guanine transition at nucleotide number 4178. This change predicts a lysine-to-glutamic acid amino acid change at residue number 214 of the mutant large T-antigen polypeptide.     

The tumor antigens and the early functions of polyoma virus

Summary Polyoma virus (Py) tumor (T) antigens are the proteins specified by the early region of the viral genome. They are responsible for most biological effects caused by this oncogenic virus, i.e. induction of tumors, cell transformation and most of the virus-induced events observed in productive and transforming infection. By immunoprecipitation with antitumor serum followed by gel electrophoresis three major Py T-antigens have been characterized: large Tantigen (IT) with an apparent M_T of about 100 000, middle T-antigen (mT) of about 55 000 Mr and small T-antigen (sT) of about 23 000 Mr. In addition, there may exist one or more minor species by Py T-antigens. Analysis of the tryptic peptides showed that IT, mT and sT have a common N-terminal amino acid sequence, but differ from each other in the size and the sequence of the C-terminal part of the molecule as a consequence of different splicing of their mRNAs. With the nucleotide sequence of the Py genome being known, the coding regions for each of the Py T-antigens have been identified and consequently the amino acid sequence of IT, mT and sT was deduced. Cell fractionation experiments showed that the major part of 1T is located in the nucleus, mT was found in plasma membranes and sT is mainly present in the cytoplasm. Large T is a phosphoprotein and undergoes posttranslational modification. Two-dimensional gel electrophoresis of Py T-antigens revealed considerable charge heterogeneity particularly for mT and sT.All Py transformed cell lines analyzed contained mT and sT. Large T was not detected in virtually all Py transformed mouse cell lines and in about one third of Py transformed rat and hamster cell lines. Instead of 1T often new immunoreactive proteins were found which are probably truncated forms of 1T. These and other recent results suggest that IT is required neither for initiation nor for maintenance of cell transformation. For tumor induction in hamsters, similar conclusions were reached from analysis of Py T-antigens and viral DNA sequences in cell lines derived from tumors that had been induced either by virus or by viral DNA digested with various restriction enzymes. Experiments done with several deletion mutants indicated that mT is required for cell transformation by Py. In a protein kinase assay done in vitro with Py T-antigen immunoprecipitates, a kinase activity associated with Py mT was found which phosphorylates tyrosine residues mainly of mT and less frequently of 1T and of rat immunoglobulins. In all transformation defective mutants, kinase activity measured by this assay was absent or strongly reduced.In a concluding chapter I discuss the events occurring in wild-type virus and mutant infected cells trying to attribute specific functions to each of the three Py T-antigens. At least two functions are known for 1T, one is initiation of viral DNA replication, the other induces a mitotic response of the host cell, i.e. the events leading to and including host chromatin duplication. Middle T-antigen is certainly involved in cell transformation, possibly by its presence in the membrane. No function has been defined yet for sT. Since there are more virus-induced events observed in infected cells than Py T-antigens at least one of them must be a multifunctional protein.     

Simian virus 40 T-antigen: identification of tryptic peptides in the C-terminal region and definition of the reading frame

T-antigen (the simian virus 40 A cistron protein) was purified by immunoprecipitation and electrophoresis on polyacrylamide gels from monkey kidney CV-1 cells infected with simian virus S (SV-S), dl1263, or dl1265 and digested with trypsin. The tryptic peptides, labeled with [35S]methionine, [35S]cysteine, or [3H]proline, were fractionated either by chromatography on Chromobead-P resin or by two-dimensional electrophoresis and chromatography on cellulose thin layers. The T-antigen of SV-S was shown to give rise to a proline-rich (approximately 6 mol of proline) tryptic peptide which was absent in dl1265 T-antigen and hence, on the basis of DNA sequence data, must originate from the C-terminus of the SV-S protein. T-antigen from dl1265, but not SV-S, yielded a cysteine-rich terminal tryptic peptide. The presence of these cysteines caused the protein to be retarded during electrophoresis under the usual conditions in polyacrylamide gels. The T-antigen of dl1263 possessed the proline-rich tryptic peptide; the data are consistent with there being only one peptide altered by the deletion. Both deletion mutants produced a T-antigen that had a higher electrophoretic mobility than SV-S T-antigen but still a larger apparent molecular weight than was predicted by the DNA sequence. The major form of T-antigen found in several lines of 3T3 cells transformed by these mutants was indistinguishable from the T-antigen found in infected cells, and in addition seemed to associate normally with the host-coded 53,000-dalton protein. Except for a minor form of T-antigen with a slightly lower mobility in gels but the same C-terminus, no other polypeptides were detected among the extracted and immunoprecipitated proteins whose electrophoretic mobility was affected by either deletion.     

Recombinant retroviruses that transduce middle T antigen cDNAs derived from polyomavirus mutants: separation of focus formation and soft-agar growth in transformation assays and correlations with kinase activities in vitro.

To study correlations between cellular transformation and the biochemical properties of polyomavirus middle T antigen, middle T cDNAs have been derived from the polyomavirus mutants dl1015, dl23, and NG59b and have been introduced into rodent fibroblast cell lines by using a retrovirus vector. It was found that all three mutants are completely defective in inducing growth in soft agar but possess a range of activities in assays of focus formation on cell monolayers. Furthermore, when assays of middle T antigen-associated kinase activities were performed in vitro, a correlation between the level of associated phosphatidylinositol kinase activity and the ability of mutant middle T antigens to induce focus formation was observed. However, the association of this activity with middle T antigen does not appear to be sufficient to bring about full transformation, since the middle T antigen derived from dl1015 is completely defective for soft-agar growth but is associated with a level of phosphatidylinositol kinase activity which is comparable to that of the wild type. Therefore, some other unidentified middle T antigen function may also be required for full transformation.     

Polyoma virus T antigen. I. Synthesis of modified heat-labile T angiten in cells transformed with the ts-a mutant.

In mouse cells transformed with the ts-a mutant of polyoma virus (ts-a-3T3), only low amounts of the virus-specific T antigen were synthesized at high temperature (39 C). After a shift-down to the permissive temperature (31 C), these cells exhibited the same level of T-antigen production as wild-type polyoma transformants. The T antigen produced by ts-a-transformed cells was inactivated at 39 C in vitro at a faster rate than that produced by wild-type-transformed cells. These observations indicate that T antigen is, or includes, a virus-coded peptide.     

In Vitro Polyoma DNA Synthesis: Studies on an Early Temperature-Sensitive Mutant

The polyoma ts-a function was investigated by using an in vitro DNA-synthesizing system. A comparison of systems derived from ts25 (a ts-a group mutant)-and ts1260 (a late group mutant)-infected cells showed that the activation energies for DNA chain elongation and the mechanisms of discontinous growth were identical for both mutants.     

Mutant din-21, a variant of polyoma virus containing a mouse DNA sequence in the viral genome.

An unusual non-defective mutant of polyoma virus with an anomalously large genome, designated din-21, has been isolated. The viral chromosome lacks 49 base pairs of the putative control region between the origin of replication and the initiation codon for the early proteins, the T-antigens. In their stead , 95 base pairs, with limited homology to the deleted sequence and apparently of mouse origin, have been inserted. The primary sequence of the insert DNA has been determined and some of the biological properties of the mutant examined. It transforms rat-1 cells slightly better than wild-type virus and grows slightly less well in lytically infected mouse cells. It does not interfere with the growth of wild-type polyoma virus. The properties of this mutant suggest that it is a natural isolate of mouse cells. The mutant was presumably generated by reciprocal recombination between polyoma DNA and mouse host DNA. This could be associated with the integration of a viral DNA sequence into the host chromosome during the viral replicative cycle.     

The sequences between 0.59 and 0.54 map units on SV40 DNA code for the unique region of small t antigen.

To demonstrate directly that the carboxy terminal portion of simian virus 40 (SV40) small t is encoded by a sequence of nucleotides from the region between 0.59-0.54 map units on SV40 DNA, we characterized the putative shortened forms or fragments of small t produced by mutants of SV40 (dl 884, dl 885, dl 890) with deletions in this region of the genome. Attempts to isolate the putative fragments of small t from mutant-infected cells, or from cell-free systems primed with mRNA from mutant-infected cells, resulted in only low yields of the fragments. Experiments using purified SV40 mRNA in low background cell-free systems, in which large T and small t could be detected without immunoprecipitation, suggested that these low yields were accounted for by reduced amounts of mRNA coding for the shortened forms of small t present in the mutant-infected cells. Larger amounts of putative fragments of small t were produced by translation of deletion mutant cRNA (complementary RNA synthesized in vitro using purified deletion mutant DNA and E. coli RNA polymerase). Fingerprint analysis of the proteins produced showed that they contain most, if not all, of the methionine peptides common to small t and large T. Furthermore, the fragments of small t produced in response to dl 884 and dl 890 lack two methionine peptides that are present in small t but not in large T. These data provide direct evidence that the region between 0.59-0.54 map units on SV40 DNA codes for polypeptide sequences that are unique to small t, and establishes that the nucleotide sequences from the region between 0.59-0.54 map units are both a coding sequence (for small t) and an intervening sequence (for large T).     

Mapping of an adenovirus function involved in the inhibition of DNA degradation.

A function involved in the inhibition of DNA degradation has been assigned through complementation tests to a product of region E1b of the adenovirus genome (between 4.5 and 10.5 map units). DNA degradation induced by the adenovirus type 12 (Ad12) cyt mutant H12cyt70 and the Ad5 early deletion mutant dl313 (with the deletion between 3.5 and 10.7 map units) was inhibited by coinfection with Ad5 region E1a (between 0 and 4.5 map units) mutants dl312 and hr1 and region E1b mutant hr6. The defect of inhibition of DNA degradation in Ad5 dl313 was also complemented in 293 cells. This DNase-inhibitory function does not appear to involve polypeptide IX or the 58,000-dalton polypeptide. Wild-type Ad12 induced DNA degradation in hamster embryo cells, suggesting that the DNase-inhibitory function is not expressed in these nonpermissive cells. Additional evidence suggests the involvement of a second viral product which positively influences the DNase activity and which appears to be an early function.     

Mutations near the carboxyl terminus of the simian virus 40 large tumor antigen alter viral host range.

We report the characterization of three mutants of simian virus 40 with mutations that delete sequences near the 3' end of the gene encoding large tumor antigen (T antigen). Two of these mutants, dl1066 and dl1140, exhibit an altered viral host range. Wild-type simian virus 40 is capable of undergoing a complete productive infection on several types of established African green monkey kidney lines, including BSC40 and CV1P. dl1066 and dl1140 grow on BSC40 cells at 37 degrees C. However, both mutants fail to form plaques on BSC40 cells at 32 degrees C or on CV1P cells at any temperature. These mutants are capable of replicating viral DNA in the nonpermissive cell type, indicating a defect in an activity of T antigen not related to its replication function. Furthermore this defect can be complemented in trans by the wild type or by a variety of DNA replication-negative T antigen mutants, so long as they produce a normal carboxyl-terminal region of the molecule. Our data are consistent with the hypothesis that the C-terminal region of T antigen constitutes a functional domain. We propose that this domain encodes an activity that is required for simian virus 40 productive infection on the CV1P cell line, but not on BSC40.