Strong and regulated expression of Escherichia coli beta-galactosidase in insect cells with a baculovirus vector

Expression of cauliflower mosaic virus gene I using a baculovirus vector based upon the p10 gene and a novel selection method

A new baculovirus expression vector based upon the p10 gene of Autographa californica nuclear polyhedrosis virus (AcNPV) and a novel system for the screening of p10 recombinants have been developed. The insertion of a cassette containing the lacZ gene under the control of a heat-shock promoter of Drosophila melanogaster downstream from the cloning site in p10 transfer vectors allows the convenient identification of putative recombinants by virtue of their expression of beta-galactosidase. Using this p10 transfer vector an AcNPV recombinant was engineered with a cDNA copy of gene I of cauliflower mosaic virus (CaMV) in place of the p10 coding sequence. This p10 recombinant expressed CaMV gene I at levels equivalent to those of p10 and polyhedrin, and was shown to be as effective in producing this protein as recombinants exploiting the polyhedrin promoter. CaMV gene I protein formed large numbers of hollow fiber-like structures in the cytoplasm of infected cells. Because the polyhedrin gene remains intact, these p10 expression vectors may be exploited for the expression of heterologous proteins in insects infected per os and for the enhancement of baculovirus pathogenicity for insect control.

Use of Early Baculovirus Promoters for Continuous Expression and Efficient Processing of Foreign Gene Products in Stably Transformed Lepidopteran Cells

Baculoviruses are currently used as vectors for the transient high-level expression of foreign gene products in insect cells. In this study, we demonstrate that baculoviruses can also be made to continuously express a foreign gene product by using the promoter from IE1, an immediate early viral gene, to produce stably-transformed insect cells. This approach gave levels of foreign gene expression lower than those usually obtained with the lytic baculovirus expression vector system. Expression, however, was continuous and stable, and a complex human glycoprotein (tissue plasminogen activator) was processed more efficiently. We conclude that stable transformation is a feasible approach for baculovirus-mediated foreign gene expression in lepidopteran cells, particularly for products that are relatively poorly-expressed and/or processed in lytically infected cells.

Bioreactor development for production of viral pesticides or heterologous proteins in insect cell cultures

The insect cell-baculovirus expression system has significant potential for producing proteins requiring some degree of posttranslational modification. T. ni cells appear to be as good a host as S. frugiperda cells for heterologous protein production as demonstrated by production of beta-galactosidase. Attachment-dependent cells of T. ni can be effectively cultured in a packed-bed reactor using glass beads. When cell in such a reactor were infected, they produced 35% of the total protein as beta-galactosidase. No cell detachment was observed even 70 h postinfection. A model of viral entry has been proposed and tested.

Infected cell specific protein and viral DNA synthesis in productive and abortive infections of Spodoptera frugiperda nuclear polyhedrosis virus

This study examines viral protein and DNA synthesis in Spodoptera frugiperda multicapsid nuclear polyhedrosis virus (SfMNPV) infections of S. frugiperda and Trichoplusia ni cells. A total of 28 infected cell specific polypeptides (ICSPs) were detected in the productive S. frugiperda cells. Of these, 14 were identified as structural polypeptides. Based on the change in their rate of synthesis during the replication cycle, these ICSPs were grouped into four classes. Only a 97k and a 29k ICSP were detected in SfMNPV infections of T. ni cells. Inhibition of host protein synthesis occurred in productive infections only, beginning at 10 h postinfection (p.i.) and reaching maximal levels by 20 h p.i. The rate of viral DNA synthesis in the productive cells was maximal between 8 to 16 h postinfection, and only low levels of viral DNA were synthesized in T.ni cells. The data suggest that the productive SfMNPV/S.frugiperda cell infection has a gene expression program similar but not identical to that of Autographa californica MNPV infections. The SfMNPV/T.ni cell infection is nonpermissive and is restricted at the earliest phase of the viral gene expression program.

Regulation of expression of a baculovirus ecdysteroid UDPglucosyltransferase gene

The Autographa californica nuclear polyhedrosis virus egt gene encodes an ecdysteroid UDPglucosyltransferase which catalyzes the transfer of glucose from UDPglucose to ecdysteroid insect molting hormones. Expression of this gene allows the virus to block molting and pupation of infected insect larvae. In this study, we present the nucleotide sequence of the A. californica nuclear polyhedrosis virus egt gene and characterize egt gene expression at the transcriptional and translational levels. egt was transcribed as two 5'-coterminal mRNAs early in infection. Transferase activity was detected in infected cells and in the extracellular fluid by 3 h after infection. The majority of the activity accumulated in the extracellular fluid. We show that the egt gene product is a 60-kilodalton protein which is secreted from the infected cell. The egt gene is located in a region of the A. californica nuclear polyhedrosis virus genome which exhibited hypervariability in serially passaged virus stocks. Nucleotide sequence analysis revealed that the most common deletion occurring in these serially passaged virus isolates is located in the egt gene.

Synthesis of the membrane fusion and hemagglutinin proteins of measles virus, using a novel baculovirus vector containing the beta-galactosidase gene

An improved baculovirus expression vector was developed to expedite screening and facilitate oligonucleotide-directed mutagenesis. This vector contained twin promoters derived from the P10 and polyhedrin genes of Autographica californica nuclear polyhedrosis virus. The P10 promoter directed the synthesis of beta-galactosidase, whereas the polyhedrin promoter controlled the synthesis of foreign gene products. These two genes recombined with wild-type virus genome to yield recombinants which were polyhedrin negative, produced the foreign gene product, and formed blue plaques when beta-galactosidase indicator was present in the agarose overlay. An origin of replication derived from M13 or f1 bacteriophage was also included in the plasmid to permit the synthesis of single-stranded DNA. This template DNA was used to introduce or delete sequences through the process of site-specific mutagenesis. The measles virus virion possesses a membrane envelope which contains two glycoproteins: the hemagglutinin (H) and membrane fusion (F) proteins. The H polypeptide has receptor-binding and hemagglutinating activity, whereas the F protein mediates virus penetration of the host cell, formation of syncytia, and hemolysis of erythrocytes. Genes for these two glycoproteins were inserted into the NheI cloning site of the modified expression vector described above. The vector and purified wild-type viral DNA were introduced into Sf9 insect cells by calcium phosphate precipitation. A mixture of wild-type and recombinant virus was generated and used to infect Sf9 cells, which were subsequently overlaid with agarose. After 3 days, 0.1 to 1% of the plaques became blue in the presence of beta-galactosidase indicator. At least 70% of these blue viral colonies contained the foreign gene of interest as determined by dot blot analysis. Recombinant virus was separated from contaminating wild-type virus through several rounds of plaque purification. Insect cells were then infected with the purified recombinants, and synthesis of H and F proteins were verified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by immunoblot detection and Coomassie blue staining. Glycosylation of the proteins appeared to be impaired somewhat, and the precursor to the F protein was not completely cleaved by the proteases present in insect host cells. On the other hand, both proteins appeared to be active in hemagglutination, hemolysis, and cell fusion assays. Levels of synthesis were in the order of 50 to 150 mg of protein per 10(8) cells.

Insect baculoviruses: powerful gene expression vectors

Baculovirus vectors have proven useful in producing high levels of biologically active eukaryotic proteins and providing cellular fractions which are enriched in the protein of interest. Expression occurs in infected insect cells which also provide a suitable environment for post-translational modification and folding of the protein product. Stable baculovirus vectors can be constructed rapidly with a minimum of viral manipulation.

Transfection of cloned Heliothis zea cell lines with the DNA genome of the Heliothis zea nuclear polyhedrosis virus

Transfection conditions were optimized for the cloned UND-K derivative of the IPLB-HZ 1075 cell line using the calcium-phosphate co-precipitation technique and the DNA genome of the Heliothis zea S-type nuclear polyhedrosis virus. Optimal efficiencies were obtained using supercoiled viral DNA, and by extending the adsorption period for the diluted precipitate to 12 h. Transfection efficiencies ranging from 0.5 to 1.3 x 10(3) plaque forming units per microgram of supercoiled viral DNA were routinely obtained for UND-K cells and HzS-15 viral DNA. Transfection efficiencies were compared for 10 other cloned Heliothis cell strains and the uncloned parental IPLB-HZ 1075 cell line. The cloned cell strains UND-F, L, and U were incapable of transfection, while UND-I and G were 3 and 131 fold (respectively) less efficient than UND-K. The UND-K cells and the calcium phosphate transfection procedure permit relatively efficient in vitro manipulation of the Heliothis zea NPV virus genome.

Transposon mutagenesis of baculoviruses: analysis of Trichoplusia ni transposon IFP2 insertions within the FP-locus of nuclear polyhedrosis viruses

The transposable IFP2 element of Trichoplusia ni was originally isolated as a host DNA insertion in spontaneous FP mutants of Galleria mellonella or Autographa californica nuclear polyhedrosis viruses (NPVs). The termini of IFP2 insertions from five independently isolated FP mutants were sequenced. In all cases IFP2 is flanked by 13-bp terminal inverted repeats and has additional inverted repeats of 19 bp in length located asymmetrically with respect to the ends of the element. Insertion of IFP2 into the viral genome always generated a duplication of the tetranucleotide target site, TTAA. There was an apparent preference for insertion within a 12-bp A + T-rich imperfect palindromic sequence surrounding the target site. Sequence analysis of three independent IFP2 elements revealed an internal domain of 2.475 kb containing an RNA polymerase II promoter region and two large open reading frames. Primer extension analysis of IFP2-specific mRNA positioned the 5' terminus of the transcript. The element is present in DNA isolated from T. ni cell lines TN-368 and TN-5B1, but is not apparent in DNAs isolated from the TN-R2 cell line or our laboratory colony of T. ni larvae, suggesting IFP2 was recently introduced into the T. ni genome.

Conservation of polyhedrin gene promoter function between Autographa californica and Mamestra brassicae nuclear polyhedrosis viruses

The DNA sequence of the polyhedrin gene of the Mamestra brassicae multiple nucleocapsid nuclear polyhedrosis virus (MbMNPV) was determined and compared with the polyhedrin genes of Autographa californica (Ac) and Panolis flammea (Pf) MNPVs. Using this information, a transfer vector was constructed based on the EcoRI I fragment of AcMNPV in which the polyhedrin promoter was replaced by the homologous region extending 481 nucleotides upstream from the MbMNPV polyhedrin coding sequence. The Escherichia coli lacZ gene was also included downstream from the putative MbMNPV promoter. Cotransfection of this transfer vector with wild-type AcMNPV DNA produced stable recombinant viruses expressing the lacZ gene under the control of the MbMNPV polyhedrin promoter. The levels of beta-galactosidase produced by these recombinants in infected cells were 30% lower than the expression level obtained from viruses with the authentic AcMNPV promoter in front of the lacZ gene. The MbMNPV promoter has thus been shown to function efficiently in the genetic environment of AcMNPV. The implications of this finding for the release of genetically manipulated baculovirus insecticides and for the construction of baculovirus multiple expression vectors are discussed.

Bombyx mori nuclear polyhedrosis virus-based vectors for expressing passenger genes in silkmoth cells under viral or cellular promoter control

The polyhedrin gene of the nuclear polyhedrosis virus of the silkmoth Bombyx mori (BmNPV) has been subjected to deletion mutagenesis. A number of clones containing partially deleted polyhedrin genes were characterized and four clones containing limited deletions of the 5'-untranslated or 5'-flanking sequences of the gene were further analyzed with respect to polyhedrin promoter activity. The functional characterization of the deletion mutants was achieved through the insertion of a chloramphenicol acetyl transferase (CAT) gene (cat) into each deletion junction. The resultant cat constructs were introduced into the genome of BmNPV through homologous recombination and the effect of each deletion on the activity of the polyhedrin promoter was evaluated by measurements of CAT enzymatic activity in extracts of tissue culture cells infected with the corresponding recombinant BmNPVs as well as by primer extension assays. Removal of the entire leader region and eleven adjacent residues of the 5'-flanking sequences of the polyhedrin gene results in a dramatic decrease in promoter activity, which, however, remains detectable through CAT activity measurements. Elimination of an additional 30 nucleotides (nt) of the upstream sequences results in the complete inactivation of the polyhedrin promoter. The functional characterization of a deletion mutant lacking 41 nt of the 5'-flanking sequences has demonstrated that no functions necessary for viral infectivity, replication or assembly are disrupted by this deletion, since the corresponding recombinant viruses propagate in the cells with the same kinetics and to the same extent as wild-type BmNPV. As a result of the deletion mutagenesis, two classes of transfer vectors have become available. The first class can be used for introducing into the viral genome foreign nucleotide sequences under polyhedrin promoter control, while the second one can be used for obtaining recombinant viruses harboring foreign genetic material in an environment which is devoid of polyhedrin promoter activity.

Protein purification

This monograph summarizes recent developments in the purification and analysis of natural and recombinant proteins. The basic strategies employed in protein purification are reviewed with regards to the characteristics of the protein of interest that may aid its isolation, choice of the starting material, and use of denaturants. Preparation of cell-free extracts followed by bulk precipitation and/or phase partition constitute the initial steps of many purification schemes. Chromatographic methods (size exclusion, ion exchange, hydroxylapatite, reversed phase, hydrophobic interaction and affinity based) utilizing either traditional, low pressure or high-performance liquid chromatography instrumentation are discussed. Electrophoretic techniques used to analyze the homogeneity of the protein product include SDS-PAGE, isotachophoresis, IEF and two dimensional gel electrophoresis.

Synthesis of a gene coding for an insect-specific scorpion neurotoxin and attempts to express it using baculovirus vectors

We have explored the possibility of improving baculovirus pesticides by incorporating an insect-specific neurotoxin gene into a baculovirus genome. A 112-bp gene (BeIt) encoding insectotoxin-1 of the scorpion Buthus eupeus was synthesized and cloned in Escherichia coli. For expression, BeIt was transferred to the DNA genome of Autographa californica nuclear polyhedrosis virus (AcMNPV). Three different recombinant AcMNPVs, carrying BeIt under the control of the strong AcMNPV polyhedrin promoter, were constructed and expression of BeIt was monitored upon infection of Spodoptera frugiperda (Sf) cells. Toxin expression was low using a recombinant virus in which BeIt was inserted 6 nucleotides (nt) downstream from the intact polyhedrin mRNA leader. More expression was observed when a signal-peptide was attached in-frame to the N terminus of BeIt. The highest level of expression was observed with a fusion gene comprised of the 58 N-terminal codons of polyhedrin fused to BeIt; however, the level of expression was ten- to twenty-fold below that for polyhedrin. Polyhedrin promoter-directed transcripts of all three recombinants accumulated to levels similar to those of wild-type polyhedrin transcripts, indicating that the limitation to expression of unfused BeIt was not at the level of transcription but rather at the posttranscriptional level including translation or protein stability. Paralytic activity of the toxin products was not detected.

Signals important for high-level expression of foreign genes in Autographa californica nuclear polyhedrosis virus expression vectors

The transcriptional and translational signals required for efficient expression of the chloramphenicol acetyltransferase, beta-galactosidase, and tissue plasminogen activator genes, under the control of the polyhedrin promoter in Spodoptera frugiperda cells infected with Autographa californica nuclear polyhedrosis virus, were investigated by SDS-PAGE and RNA dot blot analysis. The recombinant baculoviruses all contained alterations in the leader sequence or 5' proximal coding region of the polyhedrin gene. Highest levels of foreign proteins and polyhedrin-linked mRNAs were observed when portions of the coding sequence of the polyhedrin gene were fused in phase with the foreign gene. Recombinant viruses in which the foreign gene was inserted upstream from the polyhedrin ATG start codon expressed nonfused products but at lower levels than contructs which produced fusion proteins. A corresponding decrease in the levels of mRNAs produced by such constructs was also observed. Some constructs in which the foreign gene was inserted out of phase downstream from the polyhedrin start codon expressed nonfused protein products at low levels but produced polyhedrin-linked mRNA at levels comparable to vectors which produced protein fusions. These data suggest that reinitiation of translation can take place at AUG start codons a short distance downstream from the primary polyhedrin start codon. These results indicate that sequences immediately upstream from the polyhedrin start codon are important for regulation of transcription and that additional sequences near the AUG start codon can have a dramatic influence on the levels of translation observed.

Eight base pairs encompassing the transcriptional start point are the major determinant for baculovirus polyhedrin gene expression

The effects of mutations within the 92-bp region immediately upstream from the translational initiation ATG of the polyhedrin gene of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) were determined by plasmid transient expression assays in the presence of wild-type (wt) AcMNPV DNA. Clustered point mutations were generated by substitution of 10-bp stretches of the polyhedrin promoter/leader region with a 10-bp HindIII linker. Three of these linker scan (LS) mutations in the region from nucleotides (nt) -62 to -84 (relative to the original polyhedrin ATG at +1, +2, +3) had no effect or a mild stimulatory effect on reporter gene expression. One mutation immediately upstream (nt -52 to -60) from the transcription start point (at nt -50) reduced expression four-fold. Three overlapping mutations affecting 8 bp from nt -44 to -51 (encompassing the transcriptional start point) reduced expression by 1000-fold. A 1000-fold reduction was also observed for a total deletion of nt -1 and -92. Five LS mutations between nt -1 and -43 each reduced expression two- to five-fold, whereas combining an LS mutation and a 9-bp deletion mutation in the leader reduced expression approx. nine-fold. Reversing the orientation of the reporter gene along with the wt 92-bp upstream polyhedrin promoter/leader sequences resulted in slightly higher expression levels than those observed for the normal orientation indicating that all the essential cis-acting promoter elements, with the possible exception of long-range enhancer sequences, are located downstream from nt -92. Sequences of the AcMNPVhr5 enhancer (homologous region No. 5 of AcMNPV) had only a minor effect on expression from the polyhedrin promoter in transient assays. The results show that 8 bp encompassing the transcriptional start point, a sequence which is conserved in all late AcMNPV promoters, is essential for polyhedrin gene expression. Additional nucleotides within the leader region are necessary for optimal expression.

Regulation of host RNA levels during baculovirus infection

During infection of the permissive host insect cell line Spodoptera frugiperda IPLB-SF-21 by the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV), the levels of host actin, histone, and heat shock 70 (hsp70) RNAs are reduced substantially. Reduction of the host RNA levels occurs primarily during a narrow window of the replication process, from approximately 12 to 18 hr postinfection (p.i.), corresponding to the phase in which the extracellular form of the virus buds into the media. A late viral protein appears to be required for this reduction since cycloheximide, an inhibitor of cytosolic protein synthesis, and aphidicolin, an inhibitor of host and viral DNA polymerases, inhibit the reduction of actin and histone RNA levels. A cDNA corresponding to the carboxyl half of the S. frugiperda mitochondrial cytochrome c oxidase subunit III (COIII) gene was isolated, sequenced, and characterized. Two differentially regulated mitochondrial transcripts of this gene are observed. The level of the larger of these transcripts, which is dependent on active cytosolic protein synthesis, is reduced during virus infection in a fashion similar to that of the nuclear host genes. The smaller COIII transcript is stable until at least 24 hr p.i. but the level of this RNA eventually declines by 48 hr p.i.

Synthesis of functional bovine opsin in insect cells under control of the baculovirus polyhedrin promoter

In vitro expression of cDNA encoding bovine opsin is accomplished using the baculovirus expression vector system. Full-length opsin was synthesized which was recognized by poly- and monoclonal antisera raised against bovine rhodopsin. Upon infection with a recombinant virus, 1 x 10(6) insect cells produced up to 3 micrograms opsin. Incubation of the in vitro synthesized opsin with 11-cis retinal produced a hydroxylamine-stable, photosensitive pigment.

Expression of the E2 open reading frame of papillomaviruses BPV1 and HPV6b in silkworm by a baculovirus vector

To produce and characterize the nature of the E2 protein of papillomaviruses, we have developed a system to express the DNA sequence containing an open reading frame (ORF) as a fusion protein in cultured insect cells and silkworms. The DNA fragments of the E2 ORF predicted from the DNA sequence of bovine papillomavirus type 1 and human papillomavirus type 6b were linked to the N-terminal part of polyhedrin gene of the Baculovirus Bombyx mori nuclear polyhedrosis virus (BmNPV) vector. Hybrid proteins composed of polyhedrin protein (52 amino acids) and the E2 proteins of BPV1 (410 amino acids) or HPV6b (346 amino acids) were efficiently produced in B. mori cells and silkworm larvae infected with recombinant viruses. The amount of E2 fusion proteins produced by recombinant viruses was comparable to that of polyhedrin produced by wild type BmNPV. The hybrid proteins were immunologically reactive to antiserum against polyhedrin.

Detection and purification of a recombinant human B lymphotropic virus (HHV-6) in the baculovirus expression system by limiting dilution and DNA dot-blot hybridization

A recombinant virus Ac373-HB was detected and purified from the transfection mixture of wild type and recombinant virus in the baculovirus expression system using a combination of limiting dilution and DNA dot-blot hybridization. This method allows for a quick and convenient way of detection and purification of recombinant virus without the need to use a plaque purification step. It is generally applicable to other expression systems besides the baculovirus system described here.

Baculoviruses for foreign gene expression in insect cells

Insect baculovirus vector systems are rapid and convenient means of obtaining substantial quantities of many different foreign gene products. The vector systems are proving useful in academic research and in industry. Numerous examples of the successful high-level expression of biologically active vertebrate proteins have been reported. Cleavage of mammalian signal sequences occurred correctly in the cases studied. Vertebrate and invertebrate core glycosylation appears to be similar, but terminal glycosylation differs substantially. Nevertheless, many biologically active glycosylated vertebrate proteins can be expressed in biologically active form using this system. The ease and rapidity of the vector systems are two of their most valuable features. The vectors will be particularly convenient for the rapid analysis of the products of genes subjected to site-specific modifications.

Active human erythropoietin expressed in insect cells using a baculovirus vector: a role for N-linked oligosaccharide

Biologically active recombinant human erythropoietin has been expressed at high levels in an insect cell background. Expression involved the preparation of a human erythropoietin cDNA, the transfer of this cDNA to the Autographa californica nuclear polyhedrosis virus (AcNPV) genome under the polyhedrin gene promoter, and the subsequent infection of Spodoptera frugiperda cells with recombinant AcNPV. Erythropoietin cDNA was prepared through the expression of the human erythropoietin gene in COS cells using pSV2 and the construction of a COS cell cDNA library in bacteriophage Lambda GT10. Prior to transfer to the AcNPV genome, erythropoietin cDNA isolated from this library was modified at the 3'-terminus in order to replace genomic erythropoietin for SV40 cDNA derived from pSV2. Transfer of this cDNA to AcNPV and the infection of S. frugiperda cells with cloned recombinant virus led to the secretion of erythropoietin: based on bioassay, rates of hormone secretion (over 40 U/ml per h) were 50-fold greater than observed for COS cells. The purified recombinant product possessed full biological activity (at least 200,000 U/mg), but was of lower Mr (23,000) than human erythropoietin produced in COS cells (30,000) or purified from urine (30,000 to 38,000). This difference was attributed to the glycosylation of erythropoietin in S. frugiperda cells with oligosaccharides of only limited size. Further removal of N-linked oligosaccharides from this Mr 23,000 hormone using N-Glycanase yielded an apo-erythropoietin (Mr 18,000) which possessed substantially reduced biological activity. These results indicate that glycosylation, but not the normal processing of oligosaccharides to complex types, is required for the full hormonal activity of human erythropoietin during red cell development.

Expression of envelope glycoproteins of human immunodeficiency virus by an insect virus vector

The envelope gene of human immunodeficiency virus was inserted into the genome of an insect virus vector (Autographa californica nuclear polyhedrosis virus). Upon infection of tissue culture cells, this recombinant virus produced immunoreactive polypeptides related to the envelope glycoproteins of human immunodeficiency virus. Serological survey indicates such polypeptides would be of value as antigens in diagnostics for acquired immunodeficiency syndrome.

Expression of biologically active and antigenically authentic parainfluenza type 3 virus hemagglutinin-neuraminidase glycoprotein by a recombinant baculovirus

The hemagglutinin-neuraminidase (HN) gene of human type 3 parainfluenza virus has been inserted into a baculovirus expression vector under the control of the polyhedrin promoter. HN protein produced in insect cells by the recombinant baculovirus appeared to be glycosylated, was transported to the cell surface, and was biologically active. All of the HN epitopes previously mapped functionally to a region(s) involved in neuraminidase and/or hemagglutination activities were conformationally unaltered on the recombinant protein. The HN produced in this system also induced a protective immune response in immunized cotton rats. From these studies we conclude that the HN expressed in insect cells represents a source of authentic HN glycoprotein suitable for structural analysis and immunization.

Effects of serial passage of Autographa californica nuclear polyhedrosis virus in cell culture

A study of the major genomic alterations occurring during serial passage of Autographa californica nuclear polyhedrosis virus (AcNPV) in a Trichoplusia ni cell line was conducted. Progeny viruses from 24 independent passages were randomly selected and analyzed with restriction endonucleases. Specific deletion mutations were generated repeatedly in the PstI-G (7.6 to 13.1%) and the PstI-I (14.4-17.9%) regions; these mutations became predominant in the serially passaged stocks in which they arose. The deletions in the PstI-G region and two different insertions in this region were mapped to a 1 Kb PvuII-Bg/II fragment (9.85-10.70%) reflecting a high degree of sequence specificity in the initiation or selection of genomic alterations in this region. Insertional mutations were observed frequently and repeatedly within the PstI-E/HindIII-I region (33.6-37.2%) of the AcNPV genome. Individual examples of insertional mutations were observed in several other regions of the genome.

Expression of the human immunodeficiency virus gag gene in insect cells

Regions of the gag-pol gene of human immunodeficiency virus (HIV), the causative agent of AIDS, have been cloned into the polyhedrin gene of the baculovirus Autographa californica nuclear polyhedrosis virus. When these recombinant viruses were used to infect insect cells, the cells produced gag-related proteins which could be immunoprecipitated with serum from AIDS patients. The major proteins produced by Acgag1, which contained the entire gag gene and a small portion of the pol gene, had molecular weights of 55,000 and 40,000 Da. Acgag2, which contained a larger portion of the pol gene in addition to the gag coding sequences, produced a major protein of 24,000 Da and only minor amounts of the 55,000- and 40,000-Da proteins. The implications of these results with respect to proteolytic processing of HIV gag proteins as well as the potential diagnostic use of this system are discussed.

Histochemical staining of clonal mammalian cell lines expressing E. coli beta galactosidase indicates heterogeneous expression of the bacterial gene

An evaluation has been made of the E. coli beta-galactosidase (beta-gal) gene for use as a reporter gene in mammalian cells in culture. We have adopted a histochemical procedure which enables identification of those cells within a population that express the introduced bacterial gene. Data is presented concerning the sensitivity of the histochemical method relative to an immunological method of detection. It has been found that several clonal cell lines generated after transfection of human 293 cells with a Rous sarcoma virus (RSV) long terminal repeat (LTR) promoter-beta-gal construction are mosaic for expression of the introduced mini-gene. Furthermore, after treatment of these clonal cell lines with the nucleoside analog 5-aza-cytidine (5-aza-C), an increase in production of beta-gal under control of this promoter element was observed.

Cell-surface expression of influenza virus haemagglutinin in insect cells using a baculovirus vector

A baculovirus transfer vector was constructed using a cloned copy of the Autographa californica nuclear polyhedrosis virus (AcNPV) polyhedrin gene. The ATG translation initiation codon and 32 bases of the polyhedrin mRNA 5' leader sequences were deleted and replaced with a cDNA clone of the A/PR/8/34 influenza virus haemagglutinin (HA) gene. The hybrid HA/polyhedrin gene was transferred to AcNPV and polyhedrin-negative recombinants identified which expressed HA activity. The insect cell-derived HA bound a monoclonal antibody specific for the HA gene in immunofluorescence tests and was shown to be plasma membrane-bound by haemadsorption assays. A 65 kDa protein was identified in infected cells by immunoprecipitation. The mRNA from recombinant-infected cells was analysed and revealed a major transcript of 2.95 kilobases consistent with transcription initiation and termination at the authentic polyhedrin sites. Other transcripts of different length were also detected so it was not possible to conclude which was responsible for producing the protein. The results demonstrated that AcNPV can be used to express plasma membrane-bound virus glycoproteins.

Construction of an infectious pseudorabies virus recombinant expressing a glycoprotein gIII-beta-galactosidase fusion protein

An infectious herpesvirus mutant has been constructed in which a major structural envelope glycoprotein gene was replaced by a hybrid gene encoding a novel fusion protein consisting of the N-terminus of the viral glycoprotein joined to Escherichia coli beta-galactosidase (beta Gal). Specifically, we fused DNA encoding the first 157 amino acids of the structural glycoprotein gIII from pseudorabies virus strain Becker to the E. coli lacZ gene in a bacterial expression vector. The resulting hybrid gene was then used to replace the wild-type gIII gene in the virus by cotransfection of plasmid and viral DNA. The desired viral recombinants were identified by their inability to react with specific monoclonal antibodies that recognized only wild-type gIII protein. One such mutant virus, PRV-Z1, was chosen for further analysis. PRV-Z1 expressed a glycosylated gIII-beta Gal fusion protein after infection of PK15 cells. The fusion protein has no demonstrable beta Gal activity and, although glycosylated, remains sensitive to the enzyme endo-beta-N-acetylglucosaminidase H, unlike the mature gIII gene product, indicating that the fusion protein was incompletely processed.

lacZ gene fusions and insertion mutagenesis in the TL-region of Agrobacterium rhizogenes Ri plasmid

Agrobacterium rhizogenes induces root formation and inserts a fragment of its plasmid into the genome of infected plants. A part of the transferred region (TL-region) of the Ri plasmid of A. rhizogenes strain A4 was cloned in pBR322. Insertions of the Escherichia coli lacZ coding region into the hybrid plasmids were made in vivo using mini-Mu-duction. Two mini-Mus were used, one with the Mu A and B transposase genes (MudII1681) and the other without (MudII1734). Two inserts which result in E. coli lacZ expression where shown to be located in the T-DNA region. This indicates that portions of the T-DNA are capable of expression in bacteria. When these two hybrid plasmids were transformed into Agrobacterium only the one harboring MudII1734 insert gave transformants which correspond to homologous recombination. These results indicate that gene fusion and insertion directed mutagenesis can be simultaneously obtained with this mini-Mu and could be used to study Agrobacterium gene expression.

Production of human alpha-interferon in silkworm using a baculovirus vector

Microorganisms are generally used for mass production of foreign gene products, but multicellular organisms such as plants have been proposed as an economical alternative. The silkworm may be useful in this context as it can be cultured easily and at low cost. We have therefore developed a virus vector to introduce foreign genes, for example, the gene for human alpha-interferon (IFN-alpha), into silkworms. We used the baculovirus Bombyx mori nuclear polyhedrosis virus (BmNPV) which has a large (greater than 100 kilobases, kb) double-stranded circular DNA genome within its rod-shaped capsid. Baculoviruses have been used previously as vectors for expression of beta-interferon and beta-galactosidase in established cell lines. Although BmNPV has not been used previously as an expression vector, it has an advantage over the baculovirus Autographa californica NPV in that it has a narrower host range and will not grow in wild insect pests in the field. In the present study, the polyhedrin gene encoding the major inclusion body protein of BmNPV was identified by hybridization with complementary DNA and cloned in a plasmid. For insertion of foreign genes, we constructed a recombinant plasmid carrying a polylinker linked to the promoter of the polyhedrin gene, and inserted the IFN-alpha gene into this plasmid. The resulting plasmid and the BmNPV genomic DNA were co-transfected into BM-N cells, and stable recombinant viruses isolated by plaque assay on BM-N cells. The recombinant virus replicated in silkworm larvae, which synthesized as much as 5 X 10(7) units (approximately 50 micrograms) of interferon in their haemolymph.