A new method for the isolation of recombinant baculovirus

A Baculovirus Expression Vector Derived Entirely from Non-Templated, Chemically Synthesized DNA Parts

Baculovirus expression system1s are a widely used tool in recombinant protein and biologics production. To enable the possibility of genome modifications unconstrained through low-throughput and bespoke classical genome manipulation techniques, we set out to construct a baculovirus vector (>130 kb dsDNA) built from modular, chemically synthesized DNA parts. We constructed a synthetic version of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) through two steps of hierarchical Golden Gate assembly. Over 140 restriction endonuclease sites were removed to enable the discrimination of the synthetic genome from native baculovirus genomes. A head-to-head comparison of our modular, synthetic AcMNPV genome with native baculovirus vectors showed no significant difference in baculovirus growth kinetics or recombinant adeno-associated virus production-suggesting that neither baculovirus replication nor very-late gene expression were compromised by our design or assembly method. With unprecedented control over the AcMNPV genome at the single-nucleotide level, we hope to ambitiously explore novel AcMNPV vectors streamlined for biologics production and development.

A Highly Efficient and Simple Construction Strategy for Producing Recombinant Baculovirus Bombyx mori Nucleopolyhedrovirus

The silkworm baculovirus expression system is widely used to produce recombinant proteins. Several strategies for constructing recombinant viruses that contain foreign genes have been reported. Here, we developed a novel defective-rescue BmNPV Bacmid (reBmBac) expression system. A CopyControl origin of replication was introduced into the viral genome to facilitate its genetic manipulation in Escherichia coli and to ensure the preparation of large amounts of high quality reBmBac DNA as well as high quality recombinant baculoviruses. The ORF1629, cathepsin and chitinase genes were partially deleted or rendered defective to improve the efficiency of recombinant baculovirus generation and the expression of foreign genes. The system was validated by the successful expression of luciferase reporter gene and porcine interferon γ. This system can be used to produce batches of recombinant baculoviruses and target proteins rapidly and efficiently in silkworms.

Baculovirus Transfer Vectors

The production of a recombinant baculovirus expression vector normally involves mixing infectious virus DNA with a plasmid-based transfer vector and then co-transfecting insect cells to initiate virus infection. The aim of this chapter is to provide an update on the range of baculovirus transfer vectors currently available. Some of the original transfer vectors developed are now difficult to obtain but generally have been replaced by superior reagents. We focus on those that are available commercially and should be easy to locate. These vectors permit the insertion of single or multiple genes for expression, or the production of proteins with specific peptide tags that aid subsequent protein purification. Others have signal peptide coding regions permitting protein secretion or plasma membrane localization. A table listing the transfer vectors also includes information on the parental virus that should be used with each one. Methods are described for the direct insertion of a recombinant gene into the virus genome without the requirement for a transfer vector. The information provided should enable new users of the system to choose those reagents most suitable for their purposes.

A bi-cistronic baculovirus expression vector for improved recombinant protein production

Baculoviruses are one of the most studied insect viruses both in basic virology research and in biotechnology applications. Incorporating an internal ribosome entry site (IRES) into the baculovirus genome generates bi-cistronic baculoviruses expression vectors that produce two genes of interest. The bi-cistronic baculoviruses also facilitate recombinant virus isolation and titer determination when the green fluorescent protein was co-expressed. Furthermore, when the secretion proteins were co-expressed with the cytosolic green fluorescent protein, the cell lysis and cytosolic protein released into the culture medium could be monitored by the green fluorescence, thus facilitating purification of the secreted proteins.

Baculovirus transfer vectors

The production of a recombinant baculovirus expression vector normally involves mixing infectious virus DNA with a plasmid-based transfer vector and then cotransfecting insect cells to initiate virus infection. The aim of this chapter is to provide an update on the range of baculovirus transfer vectors currently available. It is impractical to list every transfer vector that has ever been used. Instead, we focus on those that are available commercially and should be easy to locate. These vectors permit the insertion of single or multiple genes for expression, or the production of proteins with specific peptide tags that aid subsequent protein purification. A table listing the transfer vectors also included information on the parental virus that should be used with each one. Recent developments in recombinant baculovirus production are also described. Some of these permit the direct insertion of a recombinant gene into the virus genome without the requirement for a transfer vector. The information provided should enable new users of the system to choose those reagents most suitable for their purposes.

A suite of parallel vectors for baculovirus expression

The expression of proteins using recombinant baculoviruses is a mature and widely used technology. However, some aspects of the technology continue to detract from high throughput use and the basis of the final observed expression level is poorly understood. Here, we describe the design and use of a set of vectors developed around a unified cloning strategy that allow parallel expression of target proteins in the baculovirus system as N-terminal or C-terminal fusions. Using several protein kinases as tests we found that amino-terminal fusion to maltose binding protein rescued expression of the poorly expressed human kinase Cot but had only a marginal effect on expression of a well-expressed kinase IKK-2. In addition, MBP fusion proteins were found to be secreted from the expressing cell. Use of a carboxyl-terminal GFP tagging vector showed that fluorescence measurement paralleled expression level and was a convenient readout in the context of insect cell expression, an observation that was further supported with additional non-kinase targets. The expression of the target proteins using the same vectors in vitro showed that differences in expression level were wholly dependent on the environment of the expressing cell and an investigation of the time course of expression showed it could affect substantially the observed expression level for poorly but not well-expressed proteins. Our vector suite approach shows that rapid expression survey can be achieved within the baculovirus system and in addition, goes some way to identifying the underlying basis of the expression level obtained.

Development of a bi-cistronic baculovirus expression vector by the Rhopalosiphum padi virus 5' internal ribosome entry site

A bi-cistronic baculovirus transfer vector was constructed based on the 5'UTR internal ribosome entry site (IRES) of the Rhopalosiphum padi virus (RhPV). Recombinant baculoviruses containing the red fluorescent protein gene and green fluorescent protein gene flanking the RhPV 5'UTR IRES can simultaneously produce dual fluorescence in recombinant virus-infected Spodoptera frugiperda 21 cells (Sf21) under the control of a polyhedrin promoter. Quantization by fluorescence spectrophotometry of the fluorescent proteins produced in Sf21 cells indicated that the translational efficacy of the RhPV 5'UTR IRES was about 3-fold weaker than cap-dependent translation. We also demonstrated that recombinant baculoviruses containing the human interferon-gamma gene (IFN-gamma) and green fluorescent protein gene flanking the RhPV 5'UTR IRES can produce IFN-gamma proteins as well as green fluorescent proteins. These results suggest that the RhPV IRES can be used in the development of bi-cistronic baculovirus expression vectors for production of heterologous multiprotein complexes or can be combined with selection markers to facilitate applications of baculovirus expression systems.

Drag&Drop cloning in yeast

We have developed a set of vectors that have enhanced capabilities for efficiently constructing and expressing differentially tagged fusion proteins using Drag&Drop cloning in the yeast Saccharomyces cerevisiae. The pGREG vectors are based on the pRS series with an additional general kanR selection marker. In vivo homologous recombination is used to introduce genes of interest into galactose-inducible expression vectors (pGREGs), permitting the formation of amino-terminal fusions. The vectors all contain common regions for recombination that flank the stuffer fragment. Introduction of common recombination sequences at the end of PCR fragments will permit the cloning of genes without the need for specific restriction sites. In this process, the selectable stuffer HIS3 gene is replaced by successful gene integration, and a screen for loss of the selection marker identifies potential recombinants. Due to the modular structure of the vectors, genes introduced into one vector can be readily transferred by in vivo recombination to all other members of the vector system, thus permitting rapid and easy Drag&Drop construction of a series of tagged proteins. The pGREG series combines features for expression, tagging, integration, localization and library construction with the advantage of obtaining immediate results from sub-sequent experiments. This Drag&Drop system also allows efficient cloning and expression of heterologous genes in large-scale experiments.

An efficient method for precise gene substitution in the AcMNPV genome by homologous recombination in E. coli

The RecA-mediated homologous recombination method was improved and used to direct gene replacement in baculoviruses. With this method, the p74 gene in the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) genome was substituted precisely by the p74 gene of Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV). In the recombinant bacmid, the AcMNPV p74 gene promoter controlled directly the expression of SpltMNPV p74 gene. Results of RT-PCR showed transcription of SpltMNPV p74 gene in the recombinant, implying the potential use of this easy and efficient method in baculovirus gene function research.

Improved generation of recombinant baculovirus genomes in Escherichia coli

An improved method for the generation of recombinant baculoviruses by Tn7-mediated transposition is described. The method is based on the modified donor vector (pBVboost) and an improved selection scheme of the baculovirus bacmids in Escherichia coli with a mutated SacB gene. Recombinant bacmids can be generated at a frequency of approximately 10(7)/microg of donor vector with a negligible background. This easy-to-use and efficient pBVboost system provides the basis for a high-throughput generation of recombinant baculoviruses as well as a more convenient way to produce single viruses. The introduced selection scheme is also useful for the construction of other vectors by transposition in E.coli.

Improving baculovirus recombination

Recombinant baculoviruses have established themselves as a favoured technology for the high-level expression of recombinant proteins. The construction of recombinant viruses, however, is a time consuming step that restricts consideration of the technology for high throughput developments. Here we use a targeted gene knockout technology to inactivate an essential viral gene that lies adjacent to the locus used for recombination. Viral DNA prepared from the knockout fails to initiate an infection unless rescued by recombination with a baculovirus transfer vector. Modified viral DNA allows 100% recombinant virus formation, obviates the need for further virus purification and offers an efficient means of mass parallel recombinant formation.

Secretion and purification of recombinant beta1-4 galactosyltransferase from insect cells using pFmel-protA, a novel transposition-based baculovirus transfer vector

The palette of transfer vectors available for generation of recombinant baculoviruses based on transposition-mediated recombination has been enlarged by constructing the pFmel-protA vector. The pFmel-protA plasmid includes the honeybee melittin secretion signal and a Staphylococcus aureus protein A fusion protein tag, which allows the secretion and purification of recombinant proteins. Using this system, the human beta1-4 galactosyltransferase-I protein was expressed in Sf9 insect cells at a level ranging from 22 to 28 U (4.8 to 6.0 mg)/L. The protein A tag enabled a simple monitoring of recombinant protein expression by enzyme-linked immunosorbent assay and Western blotting. Single step purification was achieved by immunoglobulin G affinity chromatography achieving a recovery yield of 28% and a specific activity of 1.9 U per mg of recombinant protein.

A novel expression system based on host-range expansion of baculovirus

A host range expanded recombinant Autographa californica multiple-nucleocapsid nucleopolyhedrosis virus AcMNPV/r2 was obtained by cotransfection of the bacmid DNA from Escherichia coli DH10Bac along with a plasmid pBmH-M containing HindIII M fragment of Bombyx mori nuclear polyhedrosis virus (BmNPV) genomic DNA. A recombinant transposon vector carrying a mutant green fluorescent protein gene (GFP) and a polyhedrin gene was constructed. Transposition was carried out in both E. coli DH10Bac and E. coli DH10BmH, which contains AcMNPV/r2 and a helper plasmid. Recombinant DNAs were transfected into Sf-9 cells to generate recombinant virus AcMNPV/r3 and AcMNPV/r4 respectively. Viral stock of AcMNPV/r4 was then infected into Bombyx mori cells (BmN) and Bombyx mori larvae (silkworm). Analysis shows that GFP was highly expressed in Bombyx mori larvae. This expression system, is practicable therefore for mass production of foreign gene products.

p300 and ATF-2 are components of the DRF complex, which regulates retinoic acid- and E1A-mediated transcription of the c-jun gene in F9 cells

Transcriptional activation of the c-jun gene is a critical event in the differentiation of F9 cells. In our previous studies we characterized an element [differentiation response element (DRE)] in the c-jun promoter that is both necessary and sufficient to confer the capacity for differentiation-dependent up-regulation. This element binds the differentiation regulatory factor (DRF) complex, of which one component is the adenovirus E1A-associated protein p300. We have now identified activation transcription factor-2 (ATF-2) as a DNA-binding subunit of the DRF complex. p300 and ATF-2 interact with each other in vivo and in vitro. The bromodomain and the C/H2 domain of p300 mediate the binding to ATF-2, which in turn requires a proline-rich region between amino acids 112 and 350 for its interaction with p300. The phosphorylation of the serine residue at position 121 of ATF-2 appears to be induced by protein kinase C alpha (PKC alpha) after treatment of cells with retinoic acid (RA) or induction with E1A. In cotransfection assays, wild-type ATF-2 enhanced the transcription of an E2/tk-luciferase construct, in conjunction with p300-E2. However, a mutant form of ATF-2 with a mutation at position 121 (pCMVATF-2(Ser121-Ala)) did not. These results suggest that ATF-2 and p300 cooperate in the control of transcription by forming a protein complex that is responsive to differentiation-inducing signals, such as RA or E1A, and moreover, that the phosphorylation of ATF-2 by PKC alpha is probably a signaling event in the pathway that leads to the transactivation of the c-jun gene in F9 cells.

Manipulation of baculovirus vectors

Baculovirus expression vectors provide an excellent system for the synthesis of recombinant proteins in insect cells. This article presents sufficient background information to allow the nonspecialist to understand the basic principles of the technology and the development of baculovirus expression vectors. A summary of the most commonly used plasmids and viruses is presented. Detailed techniques are described to enable recombinant baculoviruses to be constructed. These methods include the protocols required for propagating insect cells in culture and their subsequent infection with viruses.

Generation of a p10-based baculovirus expression vector in yeast with infectivity for insect larvae and insect cells

A new, versatile baculovirus vector was developed for the generation of recombinants in the yeast Saccharomyces cerevisiae and for the expression of foreign proteins in both insect larvae and in insect cells. This vector is based on Autographa californica multiple nucleocapsid nucleopolyhedrovirus (AcMNPV) and exploits the 10-kDa protein promoter (p10) for the expression of the foreign gene. The p10 locus was used for the insertion of a yeast-selectable marker system (ARS-URA-URA3) and of a gene for screening and titration of recombinants in insect cells (beta-galactosidase). The polyhedron-positive phenotype of this vector is maintained allowing its use in insect larvae, by feeding polyhedra, and in insect cells, by infecting with budded virus. The generation of this baculovirus vector requires a single recombination step in yeast prior to infection of insect cells, but has the advantage over the vector designed previously (Patel et al., A new method for the isolation of recombinant baculovirus, Nucleic Acids Research 20 (1992) 97-104) that these vectors can also be used in insects.

Characterization of delta, kappa, and mu human opioid receptors overexpressed in baculovirus-infected insect cells

The cDNAs encoding human delta (hDOR), kappa (hKOR) and micro (hMOR) opioid receptors were cloned in the baculovirus Autographa californica (AcMNPV) under the control of the polyhedrin promoter with or without an amino-terminal hexahistidine tag. Expression levels were optimized in Spodoptera frugiperda (Sf9) cells and were in the following order hMOR > hDOR > hKOR. The receptors bound antagonists with affinity values similar to those published previously for the receptors expressed in mammalian cells. They also retained selectivity toward specific antagonists. The three receptors bound peptidic agonists with low affinity, suggesting that they might not be functionally coupled to intracellular effectors. Introduction of an amino-terminal hexahistidine tag decreased the levels of expression markedly. Only hMOR-his was expressed at a level allowing binding study, but no difference could be detected in the affinities of both agonists and antagonists compared with the nontagged protein. hMOR expression was also optimized in High Five cells leading to a further increase in protein production. The pharmacological profile was similar to the one obtained when the receptor was expressed in Sf9 cells. Our results show that the baculovirus expression system is suitable for large scale production of human opioid receptors.

A new vector for efficient generation of p10-single-late-promoter recombinant baculoviruses

A new baculovirus (BacTen) was constructed in order to generate p10 recombinant expression vectors at high frequency. This virus is an AcMNPV derivative, with the polyhedrin gene deleted and thus exhibiting p10 promoter as a single strong late promoter. The polyhedrin coding sequence was re-inserted subsequently under the control of the p10 promoter, in place of the p10 coding sequence. Two flanking Bsu36I restriction sites were inserted together with the polyhedrin coding region. BacTen can, therefore, be efficiently restricted at the p10 locus and used in co-transfection experiments along with p10 transfer vectors carrying the foreign gene to be expressed. It is shown with three independent transfer vectors, that the proportion of recombinants in the viral progeny can be as high as 80% The BacTen baculovirus represents a new powerful tool for the generation of p10 promoter based expression vectors. in a system without the background of considerable production of very late proteins.

Baculovirus vectors for expression in insect cells

Recombinant baculoviruses now represent a mature technology in which vector development, particularly for the control of expression level, has reached a plateau. However, other aspects of expression, such as the production of multiple proteins, improved product purification or maximizing protein processing, remain areas for novel vector and host cell development. This year has seen these topics come to the fore in descriptions of new expression systems.

A recombination-efficient baculovirus vector for simultaneous expression of multiple genes

The baculovirus system is an extremely powerful tool for expression of heterologous genes in eukaryotic environment. A multiple expression vector, pBacUCmP3, was constructed which harbored two copies of the Autographa californica nuclear polyhedrosis virus very late gene promoter and the Drosophila melanogaster 70-kDa heat-shock protein (hsp70) promoter with downstream unique restriction sites for cloning of three independent foreign genes. Co-transfection of pBacUCmP3 with Bsu36I-linearized viral DNA yields recombinant progeny viruses at very high frequencies. The utility of this multiple expression transfer vector was demonstrated using three heterologous reporter genes encoding the beta-subunit of the human chorionic gonadotropin hormone, firefly luciferase and the bacterial beta-galactosidase (beta Gal) enzyme. The expression of reporter genes, monitored at various times post-infection, confirmed that while beta-Gal synthesis was under the transcriptional control of the hsp70 promoter, the beta hCG and Luc proteins were synthesized as a function of polyhedrin promoter activation profile. This vector will be useful for multiple synthesis of proteins at different time points.

Green fluorescent protein as a tool for screening recombinant baculoviruses

The gene encoding the green fluorescent protein (GFP) from the jellyfish Aequorea victoria, ligated to the honeybee melittin signal peptide-encoding sequence, was inserted under transcriptional control of the polyhedrin promoter of the Autographa californica nuclear polyhedrosis virus and expressed in the Spodoptera frugiperda insect cell line Sf9 during viral infection. The recombinant green fluorescent protein was identified by SDS-PAGE gel electrophoresis followed by Coomassie blue staining of lysates from the recombinant baculovirus infected insect cells. Emission and excitation scanning of the recombinant baculovirus infected insect cells gave an emission maximum of 509 nm and excitation maximum of 398 nm. The GFP protein expressed was also detected in infected insect cells by a flow cytometer analysis.

PAK1, a gene that can regulate p53 activity in yeast

The ability of p53 protein to activate transcription is central to its tumor-suppressor function. We describe a genetic selection in Saccharomyces cerevisiae which was used to isolate a mutant strain defective in p53-mediated transcriptional activation. The defect was partially corrected by overexpression of a yeast gene named PAK1 (p53 activating kinase), which localizes to the left arm of chromosome IX. PAK1 is predicted to encode an 810-aa protein with regions of strong similarity to previously described Ser/Thr-specific protein kinases. PAK1 sequences upstream of the coding region are characteristic of those regulating genes involved in cell cycle control. Expression of PAK1 was associated with an increased specific activity of p53 in DNA-binding assays accompanied by a corresponding increase in transactivation. Thus, PAK1 is the prototype for a class of genes that can regulate the activity of p53 in vivo, and the system described here should be useful in identifying other genes in this class.

ATF-2 contains a phosphorylation-dependent transcriptional activation domain

The ATF-2 transcription factor can mediate adenovirus E1A-inducible transcriptional activation. Deletion analysis has indicated that the N-terminal region of ATF-2 is essential for this response. Furthermore, the N-terminus can activate transcription in the absence of E1A when fused to a heterologous DNA binding domain. However, in the intact protein this activation domain is masked. In this report we show that residues in the N-terminus required for activation are also required for mediating E1A stimulation. In particular two threonine residues at positions 69 and 71 are essential. These residues are phosphorylated in vivo and can be efficiently phosphorylated in vitro by the JNK/SAPK subgroup of the MAPK family. ATF-2 can bind to a UV-inducible kinase through a region in the N-terminus that is distinct from the sites of phosphorylation; this binding region is both necessary for phosphorylation by JNK/SAPK in vitro and for transcriptional activation in vivo. The activity of the N-terminus is stimulated by UV irradiation which stimulates the signalling pathway leading to JNK/SAPK. Finally, although ATF-2 binds to the E1A protein, the N-terminal activation domain is not required for this interaction. The results show that ATF-2, like other members of the ATF/CREB family of DNA binding proteins is regulated by specific signalling pathways.

ATF-2 is preferentially activated by stress-activated protein kinases to mediate c-jun induction in response to genotoxic agents

The major regulators of the c-jun promoter are ATF-2 and c-Jun. They act as pre-bound heterodimers on two 'AP-1-like' sites, and are preferentially addressed by different types of extracellular signals. The transactivating potential of ATF-2 is stimulated to a higher extent than that of c-Jun by a broad group of agents causing DNA damage and other types of cellular stress, such as short-wavelength UV, or the alkylating compounds N-methyl-N'-nitro-N-nitroso-guanidine (MNNG) or methylmethanesulphonate (MMS). In contrast, treatment with the phorbol ester TPA preferentially enhances c-Jun-dependent transactivation but does not affect ATF-2. Accordingly, UV and MMS but not TPA induce c-jun transcription in F9 cells, which express ATF-2, but not c-Jun. Stimulation of ATF-2-dependent transactivation by genotoxic agents requires the presence of threonines 69 and 71 located in the N-terminal transactivation domain. These sites are the target of p54 and p46 stress-activated protein kinases (SAPKs) which bind to, and phosphorylate ATF-2 in vitro. However, p46 and p54 kinase activity is not increased by phorbol ester, which strongly suggests that the protein kinase phosphorylating c-Jun in response to TPA is distinct from SAPKs and does not act on ATF-2. Our data demonstrate that distinct signal transduction pathways converge at c-Jun/ATF-2, whereby each subunit is individually addressed by a specific class of protein kinases. This allows fine tuned modulation of c-jun expression by a large spectrum of extracellular signals.

Direct cloning into the Autographa californica nuclear polyhedrosis virus for generation of recombinant baculoviruses

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Current methods for manipulating baculoviruses

Recombinant baculoviruses have become popular expression vectors for heterologous proteins. Recently, several novel strategies for manipulating the baculovirus genome have been developed. These include linearizing the viral genome at the point of insertion of the foreign gene to be expressed, reconstituting a replicating baculovirus genome in S. cerevisiae and E. coli, and in vitro enzymatically mediated recombination into the genome. Each of these techniques constitutes a distinct approach to the problem of manipulating this complex DNA molecule, and each has distinct advantages for the various purposes to which the recombinant viruses may be applied.

Baculovirus systems for the expression of human gene products

Significant advances in basic and applied biology have resulted from the use of baculovirus vectors for the expression of heterologous proteins in cultured insect cells and in insect larvae. The development of improved vectors has greatly facilitated the construction of recombinant baculoviruses, both by increasing the efficiency of identifying recombinant viruses and by reducing or eliminating the tedious steps used to purify the desired recombinant virus from its non-recombinant parent virus.

Efficient generation of infectious recombinant baculoviruses by site-specific transposon-mediated insertion of foreign genes into a baculovirus genome propagated in Escherichia coli

The construction and purification of recombinant baculovirus vectors for the expression of foreign genes in insect cells by standard transfection and plaque assay methods can take as long as 4 to 6 weeks. This period can be reduced to several days by using a novel baculovirus shuttle vector (bacmid) that can replicate in Escherichia coli as a plasmid and can infect susceptible lepidopteran insect cells. The bacmid is a recombinant virus that contains a mini-F replicon, a kanamycin resistance marker, and attTn7, the target site for the bacterial transposon Tn7. Expression cassettes comprising a baculovirus promoter driving expression of a foreign gene that is flanked by the left and right ends of Tn7 can transpose to the target bacmid in E. coli when Tn7 transposition functions are provided in trans by a helper plasmid. The foreign gene is expressed when the resulting composite bacmid is introduced into insect cells.

The 35-kilodalton protein gene (p35) of Autographa californica nuclear polyhedrosis virus and the neomycin resistance gene provide dominant selection of recombinant baculoviruses

Autographa californica nuclear polyhedrosis virus (AcMNPV) recombinants were constructed to test the effectiveness of the AcMNPV 35-kilodalton protein gene (35K gene) and the bacterial neomycin resistance gene (neo) as dominant selectable markers for baculoviruses. Insertion of the AcMNPV apoptosis suppressor gene (p35) into the genome of p35-deletion mutants inhibited premature host cell death and increased virus yields up to 1200-fold at low multiplicities in Spodoptera frugiperda (SF21) cell cultures. When placed under control of an early virus promoter, the bacterial neomycin resistance gene (neo) restored multiplication of AcMNPV in the same cells treated with concentrations of the antibiotic G418 that inhibited wild-type virus growth greater than 1000-fold. The selectivity of these dominant markers was compared by serial passage of recombinant virus mixtures. After four passages, the proportion of p35-containing virus increased as much as 2,000,000-fold relative to deletion mutants, whereas the proportion of neo-containing viruses increased 500-fold relative to wild-type virus under G418 selection. The strength and utility of p35 as a selectable marker was further demonstrated by the construction of AcMNPV expression vectors using polyhedrin-based transfer plasmids that contain p35. Recombinant viruses with foreign gene insertions at the polyhedrin locus accounted for 15 to 30% of the transfection progeny. The proportion of desired viruses was increased to greater than 90% by linearizing the parental virus DNA at the intended site of recombination prior to transfection. These results indicate that p35 and neo facilitate the selection of baculovirus recombinants and that p35, in particular, is an effective marker for the generation of AcMNPV expression vectors.

The SRF accessory protein Elk-1 contains a growth factor-regulated transcriptional activation domain

The Elk-1 and SRF transcription factors form a ternary complex at the c-fos serum response element (SRE). Growth factor stimulation rapidly induces a reversible change in the electrophoretic mobility of the ternary complex, accompanied by increased phosphorylation of the Elk-1 C-terminal region and by the activation of a 42 kd cellular Elk-1 kinase. Phosphorylation of Elk-1 in vitro by partially purified p42/p44 MAP kinase induces a similar reduction in ternary complex mobility but has little effect on the efficiency of its formation. In vitro, MAP kinase phosphorylates the Elk-1 C-terminal region at multiple sites, which are also phosphorylated following growth factor stimulation in vivo. The Elk-1 C-terminal region functions as a regulated transcriptional activation domain whose activity in vivo is dependent on the integrity of the MAP kinase sites. These findings directly link transcriptional activation by the SRE to the growth factor-regulated phosphorylation of an SRE-binding protein.

Baculoviruses: high-level expression in insect cells

Baculoviruses continue to serve as workhorse vectors for the high-level expression of eukaryotic genes in insect cells; however, numerous researchers are also finding novel uses for these vectors by taking advantage of the unique nature of the viruses and their host cells. Concurrently, the technology involved in constructing and utilizing these vectors is being improved so that the time and effort required to construct expression vectors are reduced.