Nucleotide sequence changes in polyoma virus A gene mutants

Site-specific in situ amplification of the integrated polyomavirus genome: a case for a context-specific over-replication model of gene amplification

The fate of the genome of the polyoma (Py) tumor virus following integration in the chromosomes of transformed rat FR3T3 cells was re-examined. The viral sequences were integrated at a single transformant-specific chromosomal site in each of 22 transformants tested. In situ amplification of the viral sequences was observed in 24 of 34 transformants analyzed. Large T antigen, the unique viral function involved in initiating DNA replication from the viral origin, was essential for the amplification process. There was an absolute requirement for a reiteration of viral sequences and the extent of the reiteration affected the degree of amplification. The reiteration may be important for homologous recombination-mediated resolution of in situ amplified sequences. Among 11 transformants harboring a 1 to 2 kb repeat, the degree of amplification was transformant-specific and varied over a wide range. At the high end of the spectrum, the genome copy number increased 1300-fold at steady state, while at the low end, amplification was below twofold. Some aspect of the host chromatin at the site integration that affected viral gene expression, also directly or indirectly modulated the amplification. Use of high-resolution electrophoresis for the analysis of the integrated amplified sequences revealed a recurring novel pattern, consisting of a ladder with numerous bands separated by a constant distance approximately the size of the Py genome. We suggest that this pattern was generated by conversion of the amplified viral genomes to head to tail linear arrays with cell to cell variations in the number of genome repeats at single, transformant-specific, chromosomal sites. In light of the known "out of schedule" firing of the Py origin, we propose an "onion skin" structure intermediate and present a homologous recombination model for the conversion from onion skins to linear arrays. The relevance of the in situ amplification of the Py genome to cellular gene amplification is discussed. Finally, these results clarify our understanding of the integration of the Py genome in rat cells. They suggest that, in most cases, the multiple bands previously described in Py-transformants are likely to reflect genome amplification rather than multiple independent integration events, as assumed in the past. This interpretation is congruent with the accepted view that the integration of the Py genome is a rare and rate-limiting event in transformation.

RNA footprint mapping of RNA polymerase II molecules stalled in the intergenic region of polyomavirus DNA

RNA polymerase II molecules that transcribe the late strand of the 5.3-kb circular polyomavirus genome stall just upstream of the DNA replication origin, in a region containing multiple binding sites for polyomavirus large T antigen. Stalling of RNA polymerases depends on the presence of functional large T antigen and on the integrity of large T antigen binding site A. To gain insight into the interaction between DNA-bound large T antigen and RNA polymerase II, we mapped the position of stalled RNA polymerases by analyzing nascent RNA chains associated with these polymerases. Elongation of RNA in vitro, followed by hybridization with a nested set of DNA fragments extending progressively farther into the stalling region, allowed localization of the 3' end of the nascent RNA to a position 5 to 10 nucleotides upstream of binding site A. Ribonuclease treatment of nascent RNAs on viral transcription complexes, followed by in vitro elongation and hybridization, allowed localization of the distal end of stalled RNA polymerases to a position 40 nucleotides upstream of binding site A. This RNA footprint shows that elongating RNA polymerases stall at a site very close to the position of DNA-bound large T antigen and that they protect approximately 30 nucleotides of nascent RNA against ribonuclease digestion.

Stalling by RNA polymerase II in the polyomavirus intergenic region is dependent on functional large T antigen

RNA polymerase II encounters an elongation block and stalls in vivo during transcription of the late strand of polyomavirus DNA. In this study, we performed transcriptional run-on assays and localized the stalling site to a 164-nucleotide region (nt 11-175) that contains specific binding sites for polyomavirus large T antigen. The effect of large T antigen on elongation by RNA polymerase II through this region was examined in cells infected with a mutant polyomavirus (AT3-ts25E) which encodes a thermolabile large T antigen. Removal of functional large T antigen by shifting to the nonpermissive temperature (39 degrees) eliminated stalling by RNA polymerase in this region, although RNA polymerases transcribing other regions of the viral genome were unaffected. RNA polymerase resumed stalling when functional large T antigen was again allowed to accumulate by shifting back to the permissive temperature (32 degrees). We conclude that stalling by RNA polymerase II in vivo is dependent on the presence of functional large T antigen.

Complete transformation of embryonal rat fibroblasts by polyomavirus occurs during passage in vitro

The tumorigenicity of secondary rat embryo fibroblasts transfected with a plasmid harboring a replication origin-defective polyomavirus was found to increase during in vitro propagation. Thus, polyomavirus-transfected cells were found to be more than 10,000-fold more tumorigenic when injected into syngenic rats at 3 months after transfection compared to those injected at an earlier time point. Furthermore, most clones of polyomavirus-transfected cells did not grow in semisolid medium at 52 days after transfection but did grow at 95 days. Addition of glucocorticoid hormones, but not of 25% fetal calf serum, to the growth medium of the early passage cells resulted in limited anchorage-independent growth. An altered level of expression of a number of proteins was found in cells analyzed at different times after transfection. Notably, the expression of a component of the actin filament system, tropomyosin 2, was shown to decrease during growth in vitro. The development of a more fully transformed phenotype at late passages correlated with loss of the requirement for large T-antigen for growth. Thus, cells transfected with a polyomavirus mutant encoding a thermolabile large T-antigen did not grow at the restrictive temperature at 6 weeks after transfection, but grew well at 5 months after transfection. We suggest that these phenomena may be explained by assuming that establishment of rodent fibroblasts, and thereby sensitivity to transformation by middle T-antigen, is not an immediate consequence of expression of large T-antigen but occurs after a period of growth in vitro.

A substitution in a nonconserved region of polyomavirus large T antigen which causes a thermosensitive mutation

The temperature-sensitive defect of the tsP155 mutant of polyomavirus (Py) maps in the large T antigen (LT) coding sequence of a viral DNA diverging markedly from that of extensively characterized wild-types (WTs) such as A2 and CSP. We have sequenced about 600 base pairs (bp) "early" DNA encompassing the mutated site in tsP155, as well as the corresponding DNA segment from a revertant virus (RtsP155). As expected, tsP155 was found to be more closely related to CSP than to A2. Out of 3 single bp differences between tsP155 and CSP, 2 were common to tsP155 and RtsP155. The only substitution exclusive to tsP155 was a G----C transversion at bp 2658 which canceled the HaeIII site at bp 2657. Heteroduplexes inclusive of tsP155 DNA and of a 312-bp-long fragment of RtsP155 DNA yielded recombinant viruses growing under restrictive conditions whose DNAs had all regained the HaeIII site at bp 2657. These findings clearly identify the ts mutation with the tranversion at bp 2658, which is expected to change Ala 701 for a Pro in LT. We discuss this substitution in relation to the phenotype of tsP155.

Transformation by polyoma ts-a mutants. I. Characterization of the transformed phenotype

Seven clonal lines of Fischer rat cells transformed with ts-a mutants of polyoma virus were studied. Four clones are characterized by a temperature-sensitive (ts) and three clones by a temperature-insensitive-transformed phenotype. Six clones have retained a functional though temperature-sensitive large T antigen, as judged by a 10- to 20-fold amplification of viral sequences in clones grown at low temperature compared to those grown at high temperature. No amplification is observed in one non-ts clone. As analyzed by Southern blotting, no obvious difference appears in the integration pattern of viral sequences in ts and non-ts clones concerning the number of sites of genome integration, the presence or absence of tandem repeats of the viral genome, or the absence of specific viral sequences. In autoradiograms of gel-electrophoresed immunoprecipitates, no correlation can be drawn between the amounts of either large T antigen or middle T antigen and the type of transformed state of the clones under the conditions tested. In assays of the middle T-antigen-associated kinase, no reproducible difference can be observed between the non-ts and ts clones. Finally, no correlation was observed between a temperature-insensitive phenotype and the production of an N-terminal fragment of large T antigen. Thus the molecular basis for the difference between ts-a transformants with ts or non-ts phenotypes remains elusive.

Simian virus 40 and polyomavirus large tumor antigens have different requirements for high-affinity sequence-specific DNA binding

By using a DNA fragment immunoassay, the binding of simian virus 40 (SV40) and polyomavirus (Py) large tumor (T) antigens to regulatory regions at both viral origins of replication was examined. Although both Py T antigen and SV40 T antigen bind to multiple discrete regions on their proper origins and the reciprocal origin, several striking differences were observed. Py T antigen bound efficiently to three regions on Py DNA centered around an MboII site at nucleotide 45 (region A), a BglI site at nucleotide 92 (region B), and another MboII site at nucleotide 132 (region C). Region A is adjacent to the viral replication origin, and region C coincides with the major early mRNA cap site. Weak binding by Py T antigen to the origin palindrome centered at nucleotide 3 also was observed. SV40 T antigen binds strongly to Py regions A and B but only weakly to region C. This weak binding on region C was surprising because this region contains four tandem repeats of GPuGGC, the canonical pentanucleotide sequence thought to be involved in specific binding by T antigens. On SV40 DNA, SV40 T antigen displayed its characteristic hierarchy of affinities, binding most efficiently to site 1 and less efficiently to site 2. Binding to site 3 was undetectable under these conditions. In contrast, Py T antigen, despite an overall relative reduction of affinity for SV40 DNA, binds equally to fragments containing each of the three SV40 binding sites. Py T antigen, but not SV40 T antigen, also bound specifically to a region of human Alu DNA which bears a remarkable homology to SV40 site 1. However, both tumor antigens fail to precipitate DNA from the same region which has two direct repeats of GAGGC. These results indicate that despite similarities in protein structure and DNA sequence, requirements of the two T antigens for pentanucleotide configuration and neighboring sequence environment are different.

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.

Small and middle T antigens contribute to lytic and abortive polyomavirus infection

Using three different polyomavirus hr-t mutants and two polyomavirus mlT mutants, we studied induction of S-phase by mutants and wild-type virus in quiescent mouse kidney cells, mouse 3T6 cells, and FR 3T3 cells. At different times after infection, we measured the proportion of T-antigen-positive cells, the incorporation of [3H]thymidine, the proportion of DNA-synthesizing cells, and the increase in total DNA, RNA, and protein content of the cultures. In permissive mouse cells, we also determined the amount of viral DNA and the proportion of viral capsid-producing cells. In polyomavirus hr-t mutant-infected cultures, onset of host DNA replication was delayed by several hours, and a smaller proportion of T-antigen-positive cells entered S-phase than in wild-type-infected cultures. Of the two polyomavirus mlT mutants studied, dl-23 behaved similarly to wild-type virus in many, but not all, parameters tested. The poorly replicating but well-transforming mutant dl-8 was able to induce S-phase, and (in permissive cells) progeny virus production, in only about one-third of the T-antigen-positive cells. From our experiments, we conclude that mutations affecting small and middle T-antigen cause a reduction in the proportion of cells responding to virus infection and a prolongation of the early phase, i.e., the period before cells enter S-phase. In hr-t mutant-infected mouse 3T6 cells, production of viral DNA was less than 10% of that in wild-type-infected cultures; low hr-t progeny production in 3T6 cells was therefore largely due to poor viral DNA replication.

Regulation of polyomavirus transcription by large tumor antigen

We have analyzed the regulation of viral transcription by the large tumor antigen in cells infected by several viable deletion and insertion mutants of polyomavirus. We find that deletion of the early promoter "TATA box" and associated large tumor antigen binding site has only a small effect on the balance of early and late mRNAs. Furthermore, transcription of a polyomavirus containing a heterologous adenovirus promoter in place of the normal TATA box and cap sites is regulated by the large tumor antigen. We conclude that repression of polyomavirus early transcription cannot occur simply by binding of the large tumor antigen to DNA sequences at the site of transcription initiation and must involve the interaction of the large tumor antigen binding at other sites.

Sequence homology between the large tumor antigen of polyoma viruses and the putative E1 protein of papilloma viruses

The large tumor antigen of simian virus 40, mouse polyoma virus, and human BK virus is compared to the E1 protein of human and bovine papilloma viruses. It is suggested that large T and E1 are evolutionarily related and that the region of major homology forms part of an alpha/beta domain involved in nucleotide binding.

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.

Simian virus 40 large T-antigen point mutants that are defective in viral DNA replication but competent in oncogenic transformation

The large T antigen of simian virus 40 (SV40) is a multifunctional protein that is essential in both the virus lytic cycle and the oncogenic transformation of cells by SV40. To investigate the role of the numerous biochemical and physiological activities of T antigen in the lytic and transformation processes, we have studied DNA replication-deficient, transformation-competent large T-antigen mutants. Here we describe the genetic and biochemical analyses of two such mutants, C2/SV40 and C11/SV40. The mutants were isolated by rescuing the integrated SV40 DNA from C2 and C11 cells (CV-1 cell lines transformed with UV-irradiated SV40). The mutant viral early regions were cloned into the plasmid vector pK1 to generate pC2 and pC11. The mutations that are responsible for the deficiency in viral DNA replication were localized by marker rescue. Subsequent DNA sequencing revealed point mutations that predict amino acid substitutions in the carboxyl third of the protein in both mutants. The pC2 mutation predicts the change of Lys----Arg at amino acid 516. pC11 has two mutations, one predicting a change of Pro----Ser at residue 522, and another predicting a Pro----Arg change at amino acid 549. The two C11 mutations were separated from each other to form two distinct viral genomes in pC11A and pC11B. pC2, pC11, pC11A, and pC11B are able to transform both primary and established rodent cell cultures. The C11 and C11A T antigens are defective in ATPase activity, suggesting that wild-type levels of ATPase activity are not necessary for the oncogenic transformation of cells by T antigen.

Simian virus 40 large T-antigen point mutants that are defective in viral DNA replication but competent in oncogenic transformation

The large T antigen of simian virus 40 (SV40) is a multifunctional protein that is essential in both the virus lytic cycle and the oncogenic transformation of cells by SV40. To investigate the role of the numerous biochemical and physiological activities of T antigen in the lytic and transformation processes, we have studied DNA replication-deficient, transformation-competent large T-antigen mutants. Here we describe the genetic and biochemical analyses of two such mutants, C2/SV40 and C11/SV40. The mutants were isolated by rescuing the integrated SV40 DNA from C2 and C11 cells (CV-1 cell lines transformed with UV-irradiated SV40). The mutant viral early regions were cloned into the plasmid vector pK1 to generate pC2 and pC11. The mutations that are responsible for the deficiency in viral DNA replication were localized by marker rescue. Subsequent DNA sequencing revealed point mutations that predict amino acid substitutions in the carboxyl third of the protein in both mutants. The pC2 mutation predicts the change of Lys----Arg at amino acid 516. pC11 has two mutations, one predicting a change of Pro----Ser at residue 522, and another predicting a Pro----Arg change at amino acid 549. The two C11 mutations were separated from each other to form two distinct viral genomes in pC11A and pC11B. pC2, pC11, pC11A, and pC11B are able to transform both primary and established rodent cell cultures. The C11 and C11A T antigens are defective in ATPase activity, suggesting that wild-type levels of ATPase activity are not necessary for the oncogenic transformation of cells by T antigen.

T-antigen-independent replication of polyomavirus DNA in murine embryonal carcinoma cells

Expression of wild-type polyomavirus (Py) is restricted in murine embryonal carcinoma (EC) cells. The block appears to be located at the level of early transcription. Since no T antigen is produced, we investigated the fate of viral DNA upon infection of these cells; we showed that wild-type Py DNA replicates efficiently in all EC cells, probably via a T-antigen-independent mechanism. Furthermore, we studied, at permissive and restrictive temperatures, the replication of tsa (thermosensitive for T antigen) viral DNA of an in vitro-constructed deletion mutant lacking part of the early region coding sequences and of a double mutant carrying both the tsa mutation and the PyEC F9 mutation (allowing expression of early and late viral functions in EC cells). Our results imply that replication of wild-type A2 strain Py DNA can occur in EC cells in the absence of a functional T antigen. However, this protein clearly enhances viral DNA replication and is absolutely required in differentiated cells.

T-antigen-independent replication of polyomavirus DNA in murine embryonal carcinoma cells

Expression of wild-type polyomavirus (Py) is restricted in murine embryonal carcinoma (EC) cells. The block appears to be located at the level of early transcription. Since no T antigen is produced, we investigated the fate of viral DNA upon infection of these cells; we showed that wild-type Py DNA replicates efficiently in all EC cells, probably via a T-antigen-independent mechanism. Furthermore, we studied, at permissive and restrictive temperatures, the replication of tsa (thermosensitive for T antigen) viral DNA of an in vitro-constructed deletion mutant lacking part of the early region coding sequences and of a double mutant carrying both the tsa mutation and the PyEC F9 mutation (allowing expression of early and late viral functions in EC cells). Our results imply that replication of wild-type A2 strain Py DNA can occur in EC cells in the absence of a functional T antigen. However, this protein clearly enhances viral DNA replication and is absolutely required in differentiated cells.

Comparison of the DNA sequence of the Crawford small-plaque variant of polyomavirus with those of polyomaviruses A2 and strain 3

The DNA sequence of two wild-type strains of polyomavirus (A2 and strain 3) are known. We have determined the majority of the DNA sequence of a third strain, the Crawford small-plaque virus. This virus has been noted for its capacity to induce readily detected tumor-specific transplantation antigen in hamster cells, a property that is most likely attributable to an altered middle T-antigen. A comparison of its DNA sequence with those of the A2 and strain 3 viruses reveals numerous nucleotide substitutions, insertions, and deletions throughout the genome. Most sequence changes in coding regions are silent mutations; however, variability in proteins can be predicted from these sequence data at 5 locations in middle T-antigen, 10 in large T-antigen, and 10 in VP1. The Crawford small-plaque virus noncoding regulatory region contains, in addition to nucleotide substitutions, a 44-base-pair tandem repeat of sequences on the late side of the origin of DNA replication.

Expression of the large T protein of polyoma virus promotes the establishment in culture of "normal" rodent fibroblast cell lines

Transfer into mouse and rat embryo fibroblasts in primary culture of cloned polyoma virus genes encoding only the large T protein led to the establishment of flat colonies in sparse subcultures at a frequency equal to that of transformation by wild-type virus. Cell lines could be derived from such colonies and maintained in culture for large numbers of generations without entering crisis. They exhibited a normal phenotype, by the criteria of growth on plastic to a low saturation density and of anchorage dependency. However, they required a lower serum concentration for growth than spontaneously established 3T3 cells. Similar results were obtained after transfer of recombinant DNA molecules encoding only the amino-terminal 40% of the large T protein, suggesting that this "immortalization" function corresponds to the activity of an amino-terminal domain of the protein. Immunoprecipitation analysis of T antigens in cell lines established after transfer of the full-size and of the truncated large T genes demonstrated the expression of the full-size large T protein and of a Mr 40,000 antigen expressed from the amino-terminal part of the gene, respectively. After transfer of a "large T only" plasmid that carries a tsa mutation, cell lines were established at 33 degrees C with the same efficiency as with the wild-type large T gene, but their growth was arrested after a shift to 40 degrees C, with a progressive loss in cell viability. This result indicates a continuous requirement for a large T function in the maintenance of "immortality."

Loss of polyoma virus infectivity as a result of a single amino acid change in a region of polyoma virus large T-antigen which has extensive amino acid homology with simian virus 40 large T-antigen

The polyoma virus (Py) transformed cell line 7axB, selected by in vivo passage of an in vitro transformed cell, contains an integrated tandem array of 2.4 genomes and produces the large, middle, and small Py T-antigen species, with molecular weights of 100,000, 55,000, and 22,000, respectively (Hayday et al., J. Virol. 44:67-77, 1982; Lania et al., Cold Spring Harbor Symp. Quant. Biol. 44:597-603, 1980). The integrated viral and adjacent host DNA sequences have been molecularly cloned as three EcoRI fragments (Hayday et al.). One of these fragments (7B-M), derived from within the tandem viral sequences, is equivalent to an EcoRI viral linear molecule. Fragment 7B-M has been found to be transformation competent but incapable of producing infectious virus after DNA transfection (Hayday et al.). By constructing chimerae between 7B-M and Py DNA and by direct DNA sequencing, the mutation responsible for the loss of infectivity has been located to a single base change (adenine to guanine) at nucleotide 2503. This results in a conversion of an aspartic acid to a glycine in the C-terminal region of the Py large T-antigen but does not appear to affect the binding of the Py large T-antigen to Py DNA at the putative DNA replication and autoregulation binding sites. The mutation is located within a 21-amino acid homology region shared by the simian virus 40 large T-antigen (Friedmann et al., Cell 17:715-724, 1979). These results suggest that the mutation in the 7axB large T-antigen may be involved in the active site of the protein for DNA replication.

Polyoma mutants that productively infect F9 embryonal carcinoma cells do not rescue wild-type polyoma in F9 cells

Mouse embryonal carcinoma cells are refractory to infection by wild-type polyoma virus, the infection process apparently being blocked at a stage after adsorption and penetration but before early protein synthesis. Polyoma virus mutants capable of productive infection of mouse embryonal carcinoma cells have been isolated and these mutants all have DNA sequence alterations in a noncoding region near the origin of replication of the viral genome. PyF101 and PyF441 are two mutants selected for their ability to infect the embryonal carcinoma cell line F9. Here we show that these PyF mutants do not rescue replication of wild-type polyoma during a mixed infection of F9 cells. The mutant and wild-type DNAs were distinguished on the basis of restriction fragments obtained by digestion with Msp I or BstNI, and no wild-type DNA was detected in F9 cells coinfected with wild-type polyoma and with either PyF101 or PyF441. The mutant viruses do not appear to inhibit wild-type replication during a mixed infection because both mutant and wild-type DNAs can replicate efficiently in coinfected 3T6 cells which are permissive for both mutant and wild-type viruses. A double mutant having the PyF101 mutation and the ts-25E temperature-sensitive mutation in polyoma large tumor antigen was constructed and found to be temperature-sensitive for replication in F9 cells. This double mutant, designated PyFts-1, can be rescued in F9 cells at the restrictive temperature by coinfection with PyF441. These results suggest that the PyF mutations affect two processes in F9 cells, one involving expression of polyoma early genes and a second involving viral DNA replication.

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.

Nucleotide sequence changes in polyoma ts-a mutants: correlation with protein structure

The mutations in three polyoma ts-a mutants have been determined. Two mutants, ts-25 and ts-52, have different single-base changes at the same position (2883) in the early region corresponding to a conserved glycine residue very near the C-terminus of the polyoma large T antigen. Mutant ts-48 has a single-base change at position 2341, as well as a second change at position 1228, in the region of large T antigen shared with medium T antigen.