Does simian virus 40 T antigen unwind DNA?

Transgenic mouse models and peptide producing endocrine tumours: morpho-functional aspects

Three transgenic mouse models which proved to develop endocrine tumours are reviewed and discussed. The neoplasms were induced through the production of the transforming oncoprotein simian virus 40 (SV40) large T-antigen. The SV40/metallothionein-growth hormone (MGH), the insulin/SV40 (INS/SV40) and the vasopressin/SV40 (AVP/SV40) transgenic mice models all developed endocrine tumours of pancreas mainly composed of insulin-producing B cells, with a minor PP cell component. In the pancreata of INS/SV40 and AVP/SV40 transgenic mice, non-tumour lesions (hyperplasia and dysplasia) were also described. AVP/SV40 transgenic mice presented tumour genesis in anterior pituitary too. The usefulness of transgenic mouse models in reproducing human pathology is outlined with special reference to genetically dependent tumours.

Simian virus 40 (SV40) T antigen binds specifically to double-stranded DNA but not to single-stranded DNA or DNA/RNA hybrids containing the SV40 regulatory sequences

Simian virus 40 T antigen has been shown previously to bind specifically with high affinity to sites within the regulatory region of double-stranded simian virus 40 DNA. Using competition filter binding and the DNA-binding immunoassay, we show that T antigen did not bind specifically to either early or late single-stranded DNA containing these binding sites. Moreover, T antigen did not bind these sequences present in single-stranded RNA, RNA/RNA duplexes, or RNA/DNA hybrids. T antigen did, however, bind as efficiently to single-stranded DNA-cellulose as to double-stranded DNA-cellulose. This binding was nonspecific because it was independent of the presence of T-antigen-binding sites. The implications of these observations are discussed.

Bovine seminalplasmin [corrected] is a DNA unwinding protein

The duplex DNA unwinding ability of seminalplasmin [corrected] from bovine semen was examined by treatment of plasmid-protein complexes with calf thymus topoisomerase I and resolution of the topoisomer distributions by agarose gel electrophoresis. Binding of seminalplasmin [corrected] results in a moderate degree of unwinding of supercoiled plasmid. The elongation of the RNA chain by E. coli RNA polymerase over promoter containing template is not inhibited by seminalplasmin [corrected]. However, the reinitiation of transcription is blocked in such cases indicating that seminalplasmin [corrected] inhibits transcription by binding to the initiation site of RNA polymerase.

The AT-rich sequence of the SV40 control region influences the binding of SV40 T antigen to binding sites II and III

During lytic infection SV40 T antigen binds specifically to three different regions of the SV40 DNA to initiate DNA replication and to regulate early and late transcription. We constructed plasmids containing either 23-bp synthetic oligonucleotides representing site I or II or SV40 DNA fragments with combinations of binding sites II and III with or without SV40 specific flanking regions. These plasmids were used to determine which sequences are sufficient for specific binding to isolated regions II and III. Under identical conditions T antigen bound in a sensitive in vitro binding assay efficiently to site I but not to the corresponding oligonucleotide of site II. Binding to site II could only be observed in the presence of the adjacent 17-bp AT-rich region of the SV40 DNA. On account of the markedly low affinity for binding site II, T antigen concentrations were required which exceeded those necessary to achieve saturation of binding to site I. The very low affinity for isolated site III could be slightly raised by the same AT-rich region. An increased binding to site II at 37 degrees compared to 0 degree in the presence of this region points to an indirect influence on the DNA structure of the binding site.

Identification and biochemical analysis of DNA replication-defective large T antigens from SV40-transformed cells

Nine commonly studied Simian virus 40 (SV40)-transformed rodent cell lines were screened for tumor (T) antigens defective in SV40 DNA replication using a simple polyethylene glycol-mediated cell fusion assay. Each line contained a functional origin of SV40 DNA replication, as shown by fusion with Cos 1 cells. Fusion with uninfected monkey cells revealed that T antigens from two lines lacked detectable replicative activity, while T antigens from five other lines exhibited only very weak replicative activity. One line, and a tumor cell line derived from it, expressed T antigen with wild-type replication activity. Biochemical analysis of these proteins revealed defects in DNA binding activity and ATPase activity. One line expressed large T antigen defective in both activities. All of the lines contained complexes of T antigen with the cellular protein p53 and all of the T antigens exhibited nucleotide-binding activity. The results indicate that some of these lines may constitute a useful source of new replication-defective T antigens.

Genetic and biochemical analysis of transformation-competent, replication-defective simian virus 40 large T antigen mutants

To study the role of the biochemical and physiological activities of simian virus 40 (SV40) large T antigen in the lytic and transformation processes, we have analyzed DNA replication-defective, transformation-competent T-antigen mutants. Here we describe two such mutants, C8/SV40 and T22/SV40, and also summarize the properties of all of the mutants in this collection. C8/SV40 and T22/SV40 were isolated from C8 and T22 cells (simian cell lines transformed with UV-irradiated SV40). Early regions encoding the defective T antigens were cloned into a plasmid vector to generate pC8 and pT22. The mutations responsible for the defects in viral DNA replication were localized by marker rescue, and subsequent DNA sequencing revealed missense and one nonsense mutation. The T22 mutation predicts a change of histidine to glutamine at residue 203. C8 has two mutations, one predicts lysine224 to glutamamic acid and the other changes the codon for glutamic acid660 to a stop codon; therefore, C8 T antigen lacks the 49 carboxy-terminal amino acids. pC8A and pC8B were constructed to contain the C8 mutations separately. Plasmids pT22, pC8, pC8A, and pC8B were able to transform primary rodent cell cultures. T22 T antigen is defective in binding to the SV40 origin. C8B (49-amino-acid truncation) is a host-range mutant defective in a late function in CV-1 but not BSC cells. Analysis of T antigens in mutant SV40-transformed mouse cells suggests that the replicative function of T antigen is important in generating SV40 DNA rearrangements that allow the expression of "100K" variant T antigens in the transformants.

A DNA topoisomerase activity copurifies with the DNA polymerase induced by herpes simplex virus

A DNA-relaxing enzyme was found to copurify along with herpes simplex virus type I (HSV-1)-induced DNA polymerase throughout a multistep purification scheme. Both the enzymes had similar sedimentation velocity, required high ionic strength for optimal enzymatic activities and showed time dependence of reaction. The DNA-relaxing enzyme however, differed from the HSV-1 DNA polymerase in its requirement for higher Mg2+ concentration, rATP and much broader pH dependence. Furthermore, phosphonoacetic acid, a potent inhibitor of HSV-1 DNA polymerase did not influence the DNA-relaxing activity even at a much higher concentration. On the other hand, the DNA-relaxing enzyme associated with the DNA polymerase may be specified by HSV-1 since IgG fraction of rabbit antisera against the virus-infected cells but not against the mock-infected cells strongly inhibited both the enzymatic activities. Thus, HSV-1-induced DNA polymerase which is known to be associated with a 3' to 5' exonuclease may also be associated with yet another enzymatic activity involved in DNA metabolism.

Specific DNA binding activity of T antigen subclasses varies among different SV40-transformed cell lines

Large tumor antigen (T antigen) occurs in at least three different oligomeric subclasses in cells infected or transformed by simian virus 40 (SV40): 5-7 S, 14-16 S, and 23-25 S. The 23-25 S form is complexed with a host phosphoprotein (p53). The DNA binding properties of these three subclasses of T antigen from nine different cell lines and free p53 protein were compared using an immunoprecipitation assay. All three subclasses of T antigen bound specifically to SV40 DNA sequences near the origin of replication. However, the DNA binding activity varied between different cell lines over a 40- to 50-fold range. The 23-25 S and 14-16 S forms from most of the cell lines tested bound much less SV40 origin DNA than 5-7 S T antigen. The free p53 phosphoprotein did not bind specifically to any SV40 DNA sequences.

SV40 gene expression is modulated by the cooperative binding of T antigen to DNA

We analyzed the DNA binding properties of wild-type simian virus 40 large T antigen and found that the protein binds cooperatively to three tandem sites at a regulatory region of SV40 DNA. One consequence of this T antigen:DNA interaction is the specific repression of SV40 early RNA synthesis in vitro. We mapped a region of 85 base pairs that is necessary and sufficient to initiate early SV40 transcription in vitro. This promoter region lies directly adjacent to the third T antigen binding site but does not include that "TATA" sequence. To determine how T antigen interacts with its binding sites to repress RNA synthesis, we analyzed transcription directed by a variety of wild-type, mutant and hybrid template DNAs. Our findings suggest that the cooperative binding of T antigen to its sites is directly responsible for inhibiting the initiation, rather than blocking elongation, of early RNA synthesis. A model is presented to explain the role of T antigen binding in the regulation of viral transcription and DNA replication during SV40 lytic infection.