Properties of simian virus 40 small t antigen overproduced in bacteria

Determination of the optimal aligned spacing between the Shine-Dalgarno sequence and the translation initiation codon of Escherichia coli mRNAs

The prokaryotic mRNA ribosome binding site (RBS) usually contains part or all of a polypurine domain UAAGGAGGU known as the Shine-Dalgarno (SD) sequence found just 5' to the translation initiation codon. It is now clear that the SD sequence is important for identification of the translation initiation site on the mRNA by the ribosome, and that as a result, the spacing between the SD and the initiation codon strongly affects translational efficiency (1). It is not as clear, however, whether there is a unique optimal spacing. Complications involving the definition of the spacing as well as secondary structures have obscured matters. We thus undertook a systematic study by inserting two series of synthetic RBSs of varying spacing and SD sequence into a plasmid vector containing the chloramphenicol acetyltransferase gene. Care was taken not to introduce any secondary structure. Measurements of protein expression demonstrated an optimal aligned spacing of 5 nt for both series. Since aligned spacing corresponds naturally to the spacing between the 3'-end of the 16S rRNA and the P-site, we conclude that there is a unique optimal aligned SD-AUG spacing in the absence of other complicating issues.

Simian virus 40 small-t antigen binds two zinc ions

Six cysteine residues of the simian virus 40 small-t antigen (small-t) are important for stability of the protein. Stability has been shown to be related to the ability of small-t to bind zinc ions in vitro. Purified small-t expressed either in bacteria or from baculovirus vectors binds two molecules of zinc per molecule of protein. Thus, small-t may resemble GAL4, which contains a Zn(II)2Cys6 binuclear cluster.

Effect of zinc ions on the biochemical behavior of simian virus 40 small-t antigen expressed in bacteria

The simian virus 40 small-t antigen contains 10 cysteine residues, 6 of which are organized in two CysXCysXXCys clusters. Mutation of individual Cys residues in the two clusters or mutation of specific residues found between these clusters causes pronounced instability of the protein in animal cells. Protein instability correlates with failure of the bacterially expressed mutant proteins to bind zinc ions, an interaction which allows purification of large amounts of small-t antigen in monomeric form.

Identification of a binding site in the hepatitis B virus RNA pregenome for the viral Pol gene product

The hepatitis B virus, although containing a DNA genome, replicates by reverse transcription of an RNA pregenome. The viral Pol gene encodes the reverse transcriptase which catalyzes viral DNA synthesis. To study the interaction of this protein with HBV RNA, the entire Pol gene product was expressed except its eight amino-terminal codons in Escherichia coli as fusion protein with beta-galactosidase. In the absence of competing nucleic acids full-length expression products were able to nonspecifically bind in vitro synthesized HBV RNAs of different polarity and length. However, if competed with an excess of unspecific RNA, only those HBV RNAs were bound which contained besides the direct repeats 1 and 2 nucleotide sequences downstream of direct repeat 1. The corresponding binding site was found to be located within the adjacent 134 nucleotides downstream of DR1. We conclude from our data that this region which is in part homologous to the U5 region of retroviral genomes may be important for the binding of the HBV Pol gene product to the viral pregenome.

Control of protein phosphatase 2A by simian virus 40 small-t antigen

Soluble, monomeric simian virus 40 (SV40) small-t antigen (small-t) was purified from bacteria and assayed for its ability to form complexes with protein phosphatase 2A (PP2A) and to modify its catalytic activity. Different forms of purified PP2A, composed of combinations of regulatory subunits (A and B) with a common catalytic subunit (C), were used. The forms used included free A and C subunits and AC and ABC complexes. Small-t associated with both the free A subunit and the AC form of PP2A, resulting in a shift in mobility during nondenaturing polyacrylamide gel electrophoresis. Small-t did not interact with the free C subunit or the ABC form. These data demonstrate that the primary interaction is between small-t and the A subunit and that the B subunit of PP2A blocks interaction of small-t with the AC form. The effect of small-t on phosphatase activity was determined by using several exogenous substrates, including myosin light chains phosphorylated by myosin light-chain kinase, myelin basic protein phosphorylated by microtubule-associated protein 2 kinase/ERK1, and histone H1 phosphorylated by protein kinase C. With the exception of histone H1, small-t inhibited the dephosphorylation of these substrates by the AC complex. With histone H1, a small stimulation of dephosphorylation by AC was observed. Small-t had no effect on the activities of free C or the ABC complex. A maximum of 50 to 75% inhibition was obtained, with half-maximal inhibition occurring at 10 to 20 nM small-t. The specific activity of the small-t/AC complex was similar to that of the ABC form of PP2A with myosin light chains or histone H1 as the substrate. These results suggested that small-t and the B subunit have similar qualitative and quantitative effects on PP2A enzyme activity. These data show that SV40 small-antigen binds to purified PP2A in vitro, through interaction with the A subunit, and that this interaction inhibits enzyme activity.

Control of protein phosphatase 2A by simian virus 40 small-t antigen

Soluble, monomeric simian virus 40 (SV40) small-t antigen (small-t) was purified from bacteria and assayed for its ability to form complexes with protein phosphatase 2A (PP2A) and to modify its catalytic activity. Different forms of purified PP2A, composed of combinations of regulatory subunits (A and B) with a common catalytic subunit (C), were used. The forms used included free A and C subunits and AC and ABC complexes. Small-t associated with both the free A subunit and the AC form of PP2A, resulting in a shift in mobility during nondenaturing polyacrylamide gel electrophoresis. Small-t did not interact with the free C subunit or the ABC form. These data demonstrate that the primary interaction is between small-t and the A subunit and that the B subunit of PP2A blocks interaction of small-t with the AC form. The effect of small-t on phosphatase activity was determined by using several exogenous substrates, including myosin light chains phosphorylated by myosin light-chain kinase, myelin basic protein phosphorylated by microtubule-associated protein 2 kinase/ERK1, and histone H1 phosphorylated by protein kinase C. With the exception of histone H1, small-t inhibited the dephosphorylation of these substrates by the AC complex. With histone H1, a small stimulation of dephosphorylation by AC was observed. Small-t had no effect on the activities of free C or the ABC complex. A maximum of 50 to 75% inhibition was obtained, with half-maximal inhibition occurring at 10 to 20 nM small-t. The specific activity of the small-t/AC complex was similar to that of the ABC form of PP2A with myosin light chains or histone H1 as the substrate. These results suggested that small-t and the B subunit have similar qualitative and quantitative effects on PP2A enzyme activity. These data show that SV40 small-antigen binds to purified PP2A in vitro, through interaction with the A subunit, and that this interaction inhibits enzyme activity.

A baculovirus vector can express intron-containing genes

A simjan virus 40 genomic fragment containing the genes coding for the large T and small t antigens was inserted into the genome of the baculovirus Autographa californica nuclear polyhedrosis virus downstream of the strong polyhedrin promoter. Infection of eucaryotic Spodoptera frugiperda (SF9) cells with this recombinant virus produced significant amounts of small t antigen and little or no large T protein. Analysis by Northern blotting and S1 nuclease digestion revealed correct and preferential utilization of the small t splicing signals.

The t-unique coding domain is important to the transformation maintenance function of the simian virus 40 small t antigen

The small t antigen (t) of simian virus 40, a 174-amino-acid-containing protein, when present together with the other early viral protein, large T antigen (T), plays an important role in the maintenance of simian virus 40-induced neoplastic phenotype in certain cells. Indeed, each protein functions in a complementary manner in this process. The t coding unit is composed of two segments, a 5' region of 246 nucleotides which is identical to that of the corresponding 5' region of the T coding unit and a 3' segment of 276 nucleotides which is unique. Two mutant, t-encoding genomes, one bearing a missense and the other a nonsense mutation at the same point in the t-unique coding region were constructed in vitro and found to be defective in their ability to dissolve the actin cytoskeleton of rat fibroblasts and to complement T in the growth of mouse fibroblasts in soft agar. Therefore, the unique segment of the t gene encodes a portion of the t molecule which is essential to its transformation maintenance function.

Precore sequence of hepatitis B virus inducing e antigen and membrane association of the viral core protein

Hepatitis B virus (HBV) DNA contains a precore (pre-c) sequence of 29 codons with unknown function upstream of its gene for the major core protein. Its significance was studied by expression of core proteins with and without pre-c in Escherichia coli. Core protein without pre-c, P22c, assembled spontaneously to core particles and formed core antigen. It had the same size and antigenicity as core particles from infected liver. Core protein with pre-c, P25e, instead formed membrane-associated e antigen (HBeAg). The data suggest that pre-c functions as a signal peptide for the attachment of core protein P25e to cellular membranes. This hypothesis can explain the not yet understood relation between viremia and HbeAg and the protective role of anti-HBe antibody.

Cellular proteins which can specifically associate with simian virus 40 small t antigen

When crude, radiolabeled extracts of various cells were applied to homogeneous simian virus 40 small t antigen-Sepharose adsorbents, three cell proteins (57, 32, and 20 kilodaltons [kDa]) bound specifically. Each also bound to an insoluble, truncated t derivative composed of the COOH-terminal 123 residues of the protein. The binding of these proteins was greatly inhibited after reduction and alkylation of the t ligand. Therefore, some element of native conformation, but not all of the primary structure of t, is necessary for this binding property, which may constitute a discrete, in vitro biochemical function of this protein. Results of cell fractionation experiments suggested that the 57- and 32-kDa proteins are nonnuclear cell constituents, whereas the 20-kDa protein was closely associated with a detergent-washed nuclear fraction. Specific immunoblotting and comparative partial proteolytic digestion analyses indicated that the 57-kDa protein is tubulin, a major component of the cytoskeleton. In this regard, t and tubulin were observed to coimmunoprecipitate from crude cell extracts after incubation with monospecific anti-t antibody. Therefore, it is possible that t and tubulin interact in vivo.

The t-unique coding domain is important to the transformation maintenance function of the simian virus 40 small t antigen

The small t antigen (t) of simian virus 40, a 174-amino-acid-containing protein, when present together with the other early viral protein, large T antigen (T), plays an important role in the maintenance of simian virus 40-induced neoplastic phenotype in certain cells. Indeed, each protein functions in a complementary manner in this process. The t coding unit is composed of two segments, a 5' region of 246 nucleotides which is identical to that of the corresponding 5' region of the T coding unit and a 3' segment of 276 nucleotides which is unique. Two mutant, t-encoding genomes, one bearing a missense and the other a nonsense mutation at the same point in the t-unique coding region were constructed in vitro and found to be defective in their ability to dissolve the actin cytoskeleton of rat fibroblasts and to complement T in the growth of mouse fibroblasts in soft agar. Therefore, the unique segment of the t gene encodes a portion of the t molecule which is essential to its transformation maintenance function.

Interaction of simian virus 40 small-T antigen produced in bacteria with 56K and 32K proteins of animal cells

Small-t antigen produced in bacteria interacted with two animal cell proteins with molecular weights of 56,000 and 32,000, as did the viral antigen from infected cells. Demonstration of this specific interaction required the enrichment of native, monomeric small-t antigen from extracts in which much of the small-t antigen was highly aggregated.

An adenovirus vector system used to express polyoma virus tumor antigens

We have used a generalized adenovirus vector system to express the three polyoma tumor (T) antigen proteins under the control of the adenovirus major late promoter. One hybrid virus, Ad-PySVR498, expresses high levels of polyoma middle and small T antigens. A second hybrid virus, Ad-LTSVR545, which contains a cDNA copy of the polyoma A gene, overproduces large T antigen. The T antigens produced are indistinguishable from their authentic polyoma counterparts as determined by immunoprecipitation and partial cleavage by V8 protease. Analysis of polyoma mRNAs encoded by the recombinant viruses showed that they initiate from the adenovirus major late promoter and contain the tripartite leader at their 5' ends. Large T antigen isolated from Ad-LTSVR545-infected cells by immunoaffinity was shown to bind selectively to polyoma DNA sequences that contain the origin of viral DNA replication as well as the sites for transcription initiation.

A novel strategy for constructing clustered point mutations

We have modified the synthetic linker mutagenesis procedure (1,2) in order to facilitate both the construction and the analysis of deletions, insertions and clustered point mutations generated in DNA in vitro. The protocol as originally described by McKnight and Kingsbury (1) involved attaching a synthetic linker sequence to each 5' or 3' deletion endpoint in DNA. We have designed specific plasmid vectors that can be used to generate nested sets of deletion mutations in the DNA being analyzed. The utility of these vectors is that a linker sequence of choice can be inserted at the endpoint of a deletion in a single intramolecular ligation without the use of synthetic linker DNA. In a second modification of the original procedure, we have adopted a rapid method for sequencing supercoiled plasmid DNAs from 10 ml cultures by primer extension. The site-directed mutagenesis strategy outlined here is suited for studying regulatory regions of DNA, such as origins of DNA replication, transcriptional promoters, enhancer elements, and activator binding sites. We have used this rapid and efficient strategy to generate deletions, insertions, and clustered point mutations in the transcriptional control region of a gene encoding the major human ribosomal RNAs.

Bacterial gene expression and biotechnology

This brief review article has been intended to give a few up-to-date examples of the dramatic impact that our knowledge of gene expression (especially bacterial gene expression) has had in the area of biotechnology. This area is in a state of such rapid growth that it has only been possible to present a limited overview of the subject matter. We have tried to illustrate our points with examples from work in which we have had some direct involvement. It should be apparent that continued increase in our understanding of gene expression should provide additional opportunities for expanded application of the new methodology.

Plasmid-directed synthesis of genuine adenovirus 2 early-region 1A and 1B proteins in Escherichia coli

The transforming region of human adenovirus 2 is located in the left 11.2% of the viral genome and is comprised of two distinct genetic units termed E1A and E1B. cDNAs containing the entire nucleotide sequence of the mature E1A 13S and E1B 22S mRNAs that are complementary to these genetic units have been introduced into bacterial plasmids a short distance downstream from the Escherichia coli lac promoter. Upon transformation into appropriate E. coli hosts, one of these plasmids, pKHAO, directed the synthesis of a 45-kilodalton (kd) protein, and the other, pKHBO, synthesized a protein of 54.9 kd. Both of these plasmid-encoded proteins constituted 0.1 to 0.3% of the total cellular protein and were virtually identical to the authentic adenovirus 2 E1A 42- to 50-kd and E1B 53- to 58-kd tumor antigens (T antigen) as determined by gel electrophoresis, immunoprecipitation, and tryptic fingerprint analysis. With the use of our pKHBO expression plasmid we were also able to demonstrate that the second AUG sequence appearing in the E1B 22S mRNA corresponded to the start of the gene encoding the large adenovirus 2 T antigen. This confirms theoretical deductions based on DNA sequencing analysis that translation of the large T antigen initiates translation at an internal ATG rather than at the 5'-proximal AUG.

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.

Plasmid-directed synthesis of genuine adenovirus 2 early-region 1A and 1B proteins in Escherichia coli

The transforming region of human adenovirus 2 is located in the left 11.2% of the viral genome and is comprised of two distinct genetic units termed E1A and E1B. cDNAs containing the entire nucleotide sequence of the mature E1A 13S and E1B 22S mRNAs that are complementary to these genetic units have been introduced into bacterial plasmids a short distance downstream from the Escherichia coli lac promoter. Upon transformation into appropriate E. coli hosts, one of these plasmids, pKHAO, directed the synthesis of a 45-kilodalton (kd) protein, and the other, pKHBO, synthesized a protein of 54.9 kd. Both of these plasmid-encoded proteins constituted 0.1 to 0.3% of the total cellular protein and were virtually identical to the authentic adenovirus 2 E1A 42- to 50-kd and E1B 53- to 58-kd tumor antigens (T antigen) as determined by gel electrophoresis, immunoprecipitation, and tryptic fingerprint analysis. With the use of our pKHBO expression plasmid we were also able to demonstrate that the second AUG sequence appearing in the E1B 22S mRNA corresponded to the start of the gene encoding the large adenovirus 2 T antigen. This confirms theoretical deductions based on DNA sequencing analysis that translation of the large T antigen initiates translation at an internal ATG rather than at the 5'-proximal AUG.

Effects of alterations in the translation control region on bacterial gene expression: use of cat gene constructs transcribed from the lac promoter as a model system

The region controlling translation of the cat gene, which codes for chloramphenicol acetyltransferase, has been varied structurally in a series of plasmids that place the gene under control of the lac promoter. These plasmid constructs have enabled study of the structural features that affect the efficiency of mRNA translation. Altering the potential for secondary structure formation within the translation control region caused a tenfold variation in the synthesis of CAT enzyme, whereas varying the distance between the Shine-Dalgarno sequence (SD) and the translation start codon from 7 to 13 bases did not significantly affect the yield of CAT. If the SD was situated in a region of mRNA that is capable of base pairing, the efficiency of translation was decreased; however, the translation start codon, AUG, can initiate translation efficiently even when located in a segment capable of duplex formation. Overlapping of the cat translation control region by translation initiated upstream markedly affected initiation of translation within the cat gene: out-to-frame overlapping translation reduced CAT production by 90%; in-frame overlapping translation prevented detectable initiation of protein synthesis at the cat gene translation start codon, and yielded only fusion proteins. The enzymatic activity of such proteins was influenced by the length of the adventitious peptide segment added to the amino-terminus of the CAT polypeptide.

Genetic and biochemical characterization of the Escherichia coli K-12 fhuB mutation

The fhuB region of Escherichia coli K-12 was subcloned from pLC4-44 into pP lac to obtain pCPN1. Deletions of this recombinant plasmid were made, and a 1.4-kilobase PstI fragment was further subcloned into the vector plasmid pKK177-2 to obtain pCPN12. The response of tonA and tonB strains and fhuB strains containing the plasmids to 15 hydroxamate siderophores were assayed. Results showed that tonA strains were deficient only in the utilization of ferrichrome-type siderophores, whereas fhuB strains were deficient in the utilization of all hydroxamate-type siderophores. The response of the plasmid-containing fhuB strains to the siderophores showed that the fhuB gene resides on a 1.4-kilobase PstI fragment of DNA. The proteins synthesized by these plasmids were examined in maxicells of strain CSR603. Plasmid pCPN1 expressed five proteins of molecular weights 78,000, 40,000, 30,000, 24,000, and 13,700. By the use of deletions of pCPN1, the approximate order of the genes for these proteins was determined. Plasmid pCPN12 expressed no proteins other than the beta-lactamase proteins in maxicell strain CSR603. However, in maxicell strain BN660, a lon mutant, it expressed a 20,000-molecular-weight protein. Inner membrane vesicles made from tonB and fhuB strains were able to transport [55Fe]ferrichrome and [55Fe]rhodotorulate at rates similar to those obtained in vesicles from tonB+ and fhuB+ strains.

Recognition of messenger RNA during translational initiation in Escherichia coli

The structural aspects of recognition by E. coli ribosomes of translational initiation regions on homologous messenger RNAs have been reviewed. Also discussed is the location of initiation region on mRNA, its confines, typical nucleotide sequences responsible for initiation signal, and the influence of RNA macrostructure on protein synthesis initiation. Most of the published DNA nucleotide sequences surrounding the start of various E. coli genes and those of its phages have been collected.

Portable Shine-Dalgarno regions: a system for a systematic study of defined alterations of nucleotide sequences within E. coli ribosome binding sites

We have developed a gene expression system in Escherichia coli that contains a portable Shine-Dalgarno region. Transcription of this system is under the direction of a hybrid promoter (tacII) derived from trp and lac-UV5 promoter sequences which is followed by a region that encodes a portable Shine-Dalgarno region (PSDR). Using a series of synthetic PSDRs, we varied the four bases that follow the Shine-Dalgarno (SD) region. We found that the presence of four A residues or four T residues in this position gives the highest translational efficiency. The presence of four C residues reduces the translation efficiency by 50% as compared with PSDRs with A or T residues. The presence of four G residues following the SD region lowers the translational efficiency by at least 75% with respect to PSDRs with A or T residues.

Translational control of SV40 T antigen expressed from the adenovirus late promoter

We have constructed four novel adenovirus-SV40 hybrid viruses that contain the SV40 A gene at different positions downstream from the adenoviral major late promoter, within the region that encodes the second and third segments of the late tripartite RNA leader. The SV40 insert was precisely positioned in preselected regions of the adenoviral genome by using a combination of in vitro and in vivo recombination. As expected, all four recombinants produce equally high levels of SV40-encoded RNA that initiates at the adenovirus late promoter and contains two or three leader segments at the 5' end. Yet, in spite of this efficient transcription, only one virus, Ad-SVR284, directs the synthesis of high levels of SV40 large T antigen in infected cells; the other recombinants all produce approximately 20-fold less T antigen. This differential expression is, however, not seen in vitro, where equal amounts of hybrid T mRNA direct the synthesis of equal amounts of SV40 T antigen. Thus, some form of translational regulation is present in adenovirus-infected cells that is missing from the in vitro translation reaction.

Biochemical activities of T-antigen proteins encoded by simian virus 40 A gene deletion mutants

We have analyzed T antigens produced by a set of simian virus 40 (SV40) A gene deletion mutants for ATPase activity and for binding to the SV40 origin of DNA replication. Virus stocks of nonviable SV40 A gene deletion mutants were established in SV40-transformed monkey COS cells. Mutant T antigens were produced in mutant virus-infected CV1 cells. The structures of the mutant T antigens were characterized by immunoprecipitation with monoclonal antibodies directed against distinct regions of the T-antigen molecule. T antigens in crude extracts prepared from cells infected with 10 different mutants were immobilized on polyacrylamide beads with monoclonal antibodies, quantified by Coomassie blue staining, and then assayed directly for T antigen-specific ATPase activity and for binding to the SV40 origin of DNA replication. Our results indicate that the T antigen coding sequences required for origin binding map between 0.54 and 0.35 map units on the SV40 genome. In contrast, sequences closer to the C terminus of T antigen (between 0.24 and 0.20 map units) are required for ATPase activity. The presence of the ATPase activity correlated closely with the ability of the mutant viruses to replicate and to transform nonpermissive cells. The origin binding activity was retained, however, by three mutants that lacked these two functions, indicating that this activity is not sufficient to support either cellular transformation or viral replication. Neither the ATPase activity nor the origin binding activity correlated with the ability of the mutant DNA to activate silent rRNA genes or host cell DNA synthesis.

Construction of a synthetic messenger RNA encoding a membrane protein

We have synthesized microgram quantities of a functional eucaryotic mRNA by in vitro transcription. For this purpose, we constructed a plasmid in which the Escherichia coli lactose promoter was 5' to the vesicular stomatitis virus (VSV) G protein gene (Rose, J. K., and C. J. Gallione, 1981, J. Virol., 39:519-528). This DNA served as the template in an in vitro transcription reaction utilizing E. coli RNA polymerase. The RNA product was capped using the vaccinia guanylyltransferase. A typical preparation of the synthetic G mRNA was equivalent to the amount of G mRNA that can be isolated from approximately 10(8) VSV-infected cells. This synthetic mRNA was translated by a wheat germ extract in the presence of microsomes, producing a polypeptide that was indistinguishable from G protein in its size, antigenicity, degree of glycosylation, and its membrane insertion. This technique should aid in identifying features needed by proteins for insertion into membranes.

Cloning of the aerobactin-mediated iron assimilation system of plasmid ColV

The high-affinity iron assimilation system of plasmid ColV-K30 was cloned on the vector plasmid pPlac. Plasmid pABN1 was isolated by means of sensitivity to cloacin, a bacteriocin using the same outer membrane receptor as ferric aerobactin. Restriction maps were determined for this plasmid and for a subclone, pABN5. Plasmid pABN1 codes for the complete gene complex, whereas plasmid pABN5 encodes only the biosynthetic genes for aerobactin. Regulation of the uptake system by iron is retained in cloned sequences of pABN1.

Biochemical activities of T-antigen proteins encoded by simian virus 40 A gene deletion mutants

We have analyzed T antigens produced by a set of simian virus 40 (SV40) A gene deletion mutants for ATPase activity and for binding to the SV40 origin of DNA replication. Virus stocks of nonviable SV40 A gene deletion mutants were established in SV40-transformed monkey COS cells. Mutant T antigens were produced in mutant virus-infected CV1 cells. The structures of the mutant T antigens were characterized by immunoprecipitation with monoclonal antibodies directed against distinct regions of the T-antigen molecule. T antigens in crude extracts prepared from cells infected with 10 different mutants were immobilized on polyacrylamide beads with monoclonal antibodies, quantified by Coomassie blue staining, and then assayed directly for T antigen-specific ATPase activity and for binding to the SV40 origin of DNA replication. Our results indicate that the T antigen coding sequences required for origin binding map between 0.54 and 0.35 map units on the SV40 genome. In contrast, sequences closer to the C terminus of T antigen (between 0.24 and 0.20 map units) are required for ATPase activity. The presence of the ATPase activity correlated closely with the ability of the mutant viruses to replicate and to transform nonpermissive cells. The origin binding activity was retained, however, by three mutants that lacked these two functions, indicating that this activity is not sufficient to support either cellular transformation or viral replication. Neither the ATPase activity nor the origin binding activity correlated with the ability of the mutant DNA to activate silent rRNA genes or host cell DNA synthesis.

Purification of biologically active simian virus 40 small tumor antigen

The simian virus 40 small tumor antigen (t antigen) gene has been cloned downstream from a hybrid Escherichia coli trp-lac promoter and a suitable ribosome binding site. A bacterial clone (865i) transformed by such a plasmid (pTR865) expresses this gene and, under optimal conditions, can produce greater than or equal to 5% of its total protein as t antigen. Soluble extracts of such a clone were relatively depleted in t antigen, which was found in the initial pellet fraction. The protein was recovered from this fraction in a significantly purified form by extraction with urea-containing buffer. After gel filtration of such t antigen-enriched solutions, highly purified protein was obtained. When either this fraction (freed of urea) or NaDodSO4 gel-purified 865i t antigen (rendered free of detergent) was injected into untransformed rat cells, dissolution of intracellular actin cable networks was observed.

Self-splicing RNA: autoexcision and autocyclization of the ribosomal RNA intervening sequence of Tetrahymena

In the macronuclear rRNA genes of Tetrahymena thermophila, a 413 bp intervening sequence (IVS) interrupts the 26S rRNA-coding region. A restriction fragment of the rDNA containing the IVS and portions of the adjacent rRNA sequences (exons) was inserted downstream from the lac UV5 promoter in a recombinant plasmid. Transcription of this template by purified Escherichia coli RNA polymerase in vitro produced a shortened version of the pre-rRNA, which was then deproteinized. When incubated with monovalent and divalent cations and a guanosine factor, this RNA underwent splicing. The reactions that were characterized included the precise excision of the IVS, attachment of guanosine to the 5' end of the IVS, covalent cyclization of the IVS and ligation of the exons. We conclude that splicing activity is intrinsic to the structure of the RNA, and that enzymes, small nuclear RNAs and folding of the pre-rRNA into an RNP are unnecessary for these reactions. We propose that the IVS portion of the RNA has several enzyme-like properties that enable it to break and reform phosphodiester bonds. The finding of autocatalytic rearrangements of RNA molecules has implications for the mechanism and the evolution of other reactions that involve RNA.

Syntheses and stabilities of proteins related to the polyoma small T antigen in Escherichia coli

We compared the syntheses and turnovers of two proteins related to the polyoma small T antigen synthesized in Escherichia coli from plasmids containing polyoma genomic segments joined to lac control elements. A protein with an authentic polyoma N terminus was more unstable than a protein with N-terminal amino acids derived from beta-galactosidase. Both were more unstable than most bacterial proteins.

Cellular proteins reactive with monoclonal antibodies directed against simian virus 40 T-antigen

Several recently isolated monoclonal antibodies which reacted with simian virus 40 T antigens also reacted with proteins found in uninfected and untransformed cells. The proteins were different from each other, PAb419 reacting with a 35,000-molecular-weight protein, PAb427 reacting with a 75,000-molecular-weight phosphoprotein, PAb405 reacting with a 150,000-molecular-weight phosphoprotein, and PAb204 reacting with a 68,000-molecular-weight protein. It is suggested that although some of these cross-reactions may be fortuitous, they may, as an alternative, reflect similarities of shape and perhaps function between domains of the viral T antigen and the relevant host proteins.

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.

Syntheses and stabilities of proteins related to the polyoma small T antigen in Escherichia coli

We compared the syntheses and turnovers of two proteins related to the polyoma small T antigen synthesized in Escherichia coli from plasmids containing polyoma genomic segments joined to lac control elements. A protein with an authentic polyoma N terminus was more unstable than a protein with N-terminal amino acids derived from beta-galactosidase. Both were more unstable than most bacterial proteins.

Systematic alteration of the nucleotide sequence preceding the translation initiation codon and the effects on bacterial expression of the cloned SV40 small-t antigen gene

In the preceding paper (Derom et al., 1981) we described the cloning in bacterial plasmids of the simian virus 40 (SV40) small-t antigen gene under transcriptional control of the bacteriophage lambda pL promoter. Systematic variation of the distance and/or nucleotide sequence between the Shine-Dalgarno ribosome interaction sequence and the small-t translation initiation codon leads to considerable differences in production of small-t by the different plasmids. Secondary structure models derived for the different mRNAs confirm our previous conclusions about the requirement first for an accessible start codon and second for an accessible ribosome interaction site for efficient translation initiation. Secondary structure models for mRNAs from plasmids containing the small-t gene under control of the lac promoter are in agreement with these conclusions.

High-level synthesis in Escherichia coli of the SV40 small-t antigen under control of the bacteriophage lambda pL promoter

Several plasmids were constructed in which the SV40 small-t antigen gene was inserted in close proximity downstream from the thermoinducible leftward promoter (pL) of bacteriophage lambda. Upon temperature induction the best of our constructions expressed a small-t-related 19 000-dalton polypeptide in an amount corresponding to approx. 2.5% of total de novo protein synthesis. This 19 000-dalton protein was identified as small-t by specific immunoprecipitation with anti-T serum and by two-dimensional fingerprint analysis. In addition to the 19 000-dalton product, representative plasmids expressed fairly large amounts (up to 7% of total de novo protein synthesis) of a protein with an apparent Mr of 14 500. This 14 500-dalton polypeptide was shown to be related to authentic small-t. Presumably the secondary structure of the mRNA starting at pL is such that translation initiation at an internal AUG codon of the small-t gene is favored over initiation at the true initiating codon.

Construction of a general vector for efficient expression of mammalian proteins in bacteria: use of a synthetic ribosome binding site

With the premise that mRNAs transcribed in Escherichia coli from cloned eukaryotic DNA inserts do not possess the necessary regulatory signals for recognition by prokaryotic ribosomes, we have constructed a general plasmid vector carrying a chemically synthesized prokaryotic ribosome binding site that will ensure the efficient expression of eukaryotic proteins in E. coli. In addition to the regulatory signals necessary for ribosome recognition, the synthetic segment contains, at one end, a Pst I cleavage site which will direct its insertion to pBR322 DNA and, at the other end, a HindIII site to facilitate attachment of the passenger eukaryotic gene. Using simian virus 40 (SV40) tumor (t) antigen as a model system, we have ligated the SV40 DNA fragment containing the entire t antigen gene in tandem with the synthetic ribosome binding site to pBR322 DNA at the Pst I site, which lies within the coding sequence of the beta-lactamase gene. Initiation of transcription at the beta-lactamase promoter would produce a chimeric mRNA with the synthetic ribosome binding signals and the SV40 sequence flanked by beta-lactamase coding sequences. Utilization of the synthetic regulatory signals for initiation of translation is demonstrated by the efficient synthesis, in bacterial transformants, of authentic SV40 t antigen. Excision of the entire SV40 insert by HindIII from those clones that have retained intact HindIII sites at the junction between the ribosome binding site and the SV40 sequence would allow insertion of other heterologous DNAs by using HindIII linkers. The efficient expression of any DNA insert would require that the entire coding sequence be contiguous and that its termini be randomized by treatment with exonuclease III and nuclease S1 to vary the distance between the translational initiation codon and the synthetic ribosome binding site.

Initiation factor-independent translation of mRNAs from Gram-positive bacteria

Initiation factor-independent translation of mRNA derived from bacillus phage phi29 DNA occurs with translation systems derived from Bacillus subtilis or Escherichia coli. This is in sharp contrast to the strict dependence on ribosome salt wash fraction of E. coli ribosomes for the translation of T7 and other mRNAs derived from Gram-negative organisms.