Complete Sequence and Enhancer Function of the Homologous DNA Regions of Autographa californica Nuclear Polyhedrosis Virus

A germline transgenic silkworm that secretes recombinant proteins in the sericin layer of cocoon

A silk thread of the silkworm, Bombyx mori, is composed of the insoluble inner fibroin and the hydrophilic outer sericin layer, which are synthesized in the posterior and middle silk gland (MSG), respectively. This study aimed to develop a novel sericin 1 gene (ser1) promoter-driven recombinant expression system using transgenic silkworms, in which recombinant proteins are synthesized in MSG and secreted into the sericin layer. To obtain a high level of gene expression, we tested whether a baculovirus-derived enhancer, hr3, and a trans-regulator, IE1, are capable of stimulating the transcriptional activity of the ser1 promoter, using a transient gene expression system. The results showed that hr3 and IE1 cooperatively increased the ser1 promoter activity more than 30-fold. Then, transgenic silkworms were generated which expressed the EGFP with the signal peptide in MSG under the control of the hr3-linked ser1 promoter and IE1 gene. The silkworms exclusively secreted the EGFP into the sericin layer of cocoons as predicted. The expressed EGFP was extractable from cocoons through a simple procedure with neutral pH buffer solution. The expression system developed in this study enables us to produce recombinant proteins in bulk that can be easily extracted and purified.

No single homologous repeat region is essential for DNA replication of the baculovirus Autographa californica multiple nucleopolyhedrovirus

The presence of homologous repeat (hr) regions in multiple locations within baculovirus genomes has led to the hypothesis that they represent origins of DNA replication. This hypothesis has been supported by transient replication assays where plasmids carrying hrs replicated in the presence of virus DNA replication. This study investigated whether any specific hr region was essential for viral DNA replication in vivo, by generating a series of recombinant Autographa californica multiple nucleopolyhedrovirus where the lacZ gene replaced hr1, hr1a, hr2, hr3, hr4a or hr4b. In addition, a double-hr knockout virus was constructed where both hr2 and hr3 were deleted. The successful construction of these knockout viruses indicated that no specific region was essential for virus production. These recombinant viruses were characterized by titrations of budded virus, expression of a variety of virus-specific proteins and the synthesis of viral DNA at various times after infection. The results demonstrated that each hr was dispensable for all of these properties and that no single region was absolutely essential for virus replication in cell culture. The functional significance of multiple origin regions is still unclear.

Sequence analysis and organization of the Neodiprion abietis nucleopolyhedrovirus genome

Of 30 baculovirus genomes that have been sequenced to date, the only nonlepidopteran baculoviruses include the dipteran Culex nigripalpus nucleopolyhedrovirus and two hymenopteran nucleopolyhedroviruses that infect the sawflies Neodiprion lecontei (NeleNPV) and Neodiprion sertifer (NeseNPV). This study provides a complete sequence and genome analysis of the nucleopolyhedrovirus that infects the balsam fir sawfly Neodiprion abietis (Hymenoptera, Symphyta, Diprionidae). The N. abietis nucleopolyhedrovirus (NeabNPV) is 84,264 bp in size, with a G+C content of 33.5%, and contains 93 predicted open reading frames (ORFs). Eleven predicted ORFs are unique to this baculovirus, 10 ORFs have a putative sequence homologue in the NeleNPV genome but not the NeseNPV genome, and 1 ORF (neab53) has a putative sequence homologue in the NeseNPV genome but not the NeleNPV genome. Specific repeat sequences are coincident with major genome rearrangements that distinguish NeabNPV and NeleNPV. Genes associated with these repeat regions encode a common amino acid motif, suggesting that they are a family of repeated contiguous gene clusters. Lepidopteran baculoviruses, similarly, have a family of repeated genes called the bro gene family. However, there is no significant sequence similarity between the NeabNPV and bro genes. Homologues of early-expressed genes such as ie-1 and lef-3 were absent in NeabNPV, as they are in the previously sequenced hymenopteran baculoviruses. Analyses of ORF upstream sequences identified potential temporally distinct genes on the basis of putative promoter elements.

Genome sequence of an enhancin gene-rich nucleopolyhedrovirus (NPV) from Agrotis segetum: collinearity with Spodoptera exigua multiple NPV

The genome sequence of a Polish isolate of Agrotis segetum nucleopolyhedrovirus (AgseNPV-A) was determined and analysed. The circular genome is composed of 147,544 bp and has a G+C content of 45.7 mol%. It contains 153 putative, non-overlapping open reading frames (ORFs) encoding predicted proteins of more than 50 aa, together making up 89.8 % of the genome. The remaining 10.2 % of the DNA constitutes non-coding regions and homologous-repeat regions. One hundred and forty-three AgseNPV-A ORFs are homologues of previously reported baculovirus gene sequences. There are ten unique ORFs and they account for 3 % of the genome in total. All 62 lepidopteran baculovirus genes, including the 29 core baculovirus genes, were found in the AgseNPV-A genome. The gene content and gene order of AgseNPV-A are most similar to those of Spodoptera exigua (Se) multiple NPV and their shared homologous genes are 100 % collinear. Three putative enhancin genes were identified in the AgseNPV-A genome. In phylogenetic analysis, the AgseNPV-A enhancins form a cluster separated from enhancins of the Mamestra species NPVs.

Conserved molecular systems of the Baculoviridae

Although the Baculoviridae are a large and diverse family of viruses, they are united by a number of shared features that form the basis for their unique life cycle. These include the mechanism of cell entry, genome replication and processing, and late and very late gene transcription. In this review, the molecular systems that are conserved within the Baculoviridae and that are responsible these processes are described.

Stably Transformed Insect Cell Lines: Tools for Expression of Secreted and Membrane‐anchored Proteins and High‐throughput Screening Platforms for Drug and Insecticide Discovery

Insect cell-based expression systems are prominent amongst current expression platforms for their ability to express virtually all types of heterologous recombinant proteins. Stably transformed insect cell lines represent an attractive alternative to the baculovirus expression system, particularly for the production of secreted and membrane-anchored proteins. For this reason, transformed insect cell systems are receiving increased attention from the research community and the biotechnology industry. In this article, we review recent developments in the field of insect cell-based expression from two main perspectives, the production of secreted and membrane-anchored proteins and the establishment of novel methodological tools for the identification of bioactive compounds that can be used as research reagents and leads for new pharmaceuticals and insecticides.

Functional analysis of evolutionary conserved clustering of bZIP binding sites in the baculovirus homologous regions (hrs) suggests a cooperativity between host and viral transcription factors

The genome of the Autographa californica Multinucleocapsid Polyhedrosis Virus (AcMNPV) contains nine interspersed homologous regions (hrs) that function as potent enhancer sequences when linked in cis to either viral or heterologous RNA polymerase II-dependent promoters. Their activity is strongly increased by the binding of the major immediate early viral transregulator IE1 on 28-mer palindromic sites present in hrs. We show that hrs of AcMNPV additionally carry, in the interpalindromic sequences, a large number of cAMP response elements (CRE) and TPA response elements (TRE), known to bind ubiquitous cellular transcription factors of the bZIP family. Moreover, these clusters of CRE and TRE motifs are concentrated in hrs. Analysis of the 25 baculovirus genomes sequenced so far reveals that these motifs are evolutionary conserved in Lepidoptera NPVs, suggesting a functional role in the hr enhancer function. Consistently, EMSA experiments indicate that CRE and on a lesser extent TRE sites specifically bind insect host factors. Moreover, reporter assays reveal that these CRE sites have an additive stimulatory effect on RNAPol II-dependent transcription in Sf9 cells and are potentially able to synergize with the IE1-binding palindrome.

Focal distribution of baculovirus IE1 triggered by its binding to the hr DNA elements

In BmN cells infected with the baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV), IE1, a principal transcriptional activator, localizes to sites of viral DNA replication. IE1 initially displays focal distribution in BmNPV-infected cells prior to DNA synthesis, whereas the protein expressed by transfection with the ie1 gene is distributed throughout the nucleoplasm instead of localized to discrete subnuclear structures. To identify the inducer of focus formation for IE1, we conducted transfection experiments with an IE1-GFP construct and found that cotransfection with genomic DNA fragments bearing the homologous region (hr) sequences caused the formation of IE1-green fluorescent protein (GFP) foci. The transfection of insect cells with a single plasmid containing exclusively the hr3 sequence and the IE1-GFP gene was sufficient to form IE1-GFP foci. These results suggest that hr elements are a primary determinant of the focal distribution of IE1. An analysis of a series of hr3 deletion mutants showed that a single copy of the direct repeat could induce the formation of IE1 foci. Targeted mutagenesis within the hr-binding domain of IE1-GFP caused impairment of the hr-dependent IE1 localization, suggesting that binding of IE1 to the hr elements is essential for the onset of IE1 focus formation. The observation of BmNPV IE1 foci in non-BmNPV-susceptible cells suggests that no species-specific factors are required for hr-dependent IE1 focus formation.

Characterization of the replication of a baculovirus mutant lacking the DNA polymerase gene

In a previous study, the DNA polymerase gene (dnapol) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) was identified as one of six genes required for plasmid replication in a transient replication assay (M. Kool, C. Ahrens, R.W. Goldbach, G.F. Rohrmann, J.M. Vlak, Identification of genes involved in DNA replication of the Autographa californica, Proc. Natl. Acad. Sci. U.S.A. 91, (1994) 11212-11216); however, another study based on a similar approach reported that the virally encoded polymerase was only stimulatory (A. Lu, L.K. Miller, The roles of 18 baculovirus late expression factor genes in transcription and DNA replication, J. Virol. 69, (1995) 975-982). To reconcile the conflicting data and determine if the AcMNPV DNA polymerase is required for viral DNA replication during the course of an infection, a dnapol-null virus was generated using bacmid technology. To detect viral DNA replication, a highly sensitive assay was designed based on real-time PCR and SYBR green chemistry. Our results indicate that a bacmid in which the dnapol ORF was deleted is unable to replicate its DNA when transfected into Spodoptera frugiperda (Sf-9) cells, although when the dnapol ORF was introduced into the polyhedrin (polh) locus, this repaired virus could propagate at levels similar to the control virus. These results confirm that the AcMNPV-encoded DNA polymerase is required for viral DNA replication and the host DNA polymerases cannot substitute for the viral enzyme in this process.

Baculovirus proteins IE-1, LEF-3, and P143 interact with DNA in vivo: a formaldehyde cross-linking study

IE-1, LEF-3, and P143 are three of six proteins encoded by Autographa californica nucleopolyhedrovirus (AcMNPV) essential for baculovirus DNA replication in transient replication assays. IE-1 is the major baculovirus immediate early transcription regulator. LEF-3 is a single-stranded DNA binding protein (SSB) and P143 is a DNA helicase protein. To investigate their interactions in vivo, we treated AcMNPV-infected Spodoptera frugiperda cells with formaldehyde and separated soluble proteins from chromatin by cell fractionation and cesium chloride equilibrium centrifugation. Up to 70% of the total LEF-3 appeared in the fraction of soluble, probably nucleoplasmic proteins, while almost all P143 and IE-1 were associated with viral chromatin in the nucleus. This suggests that LEF-3 is produced in quantities that are higher than needed for the coverage of single stranded regions that arise during viral DNA replication and is consistent with the hypothesis that LEF-3 has other functions such as the localization of P143 to the nucleus. Using a chromatin immunoprecipitation procedure, we present the first direct evidence of LEF-3, P143, and IE-1 proteins binding to closely linked sites on viral chromatin in vivo, suggesting that they may form replication complexes on viral DNA in infected cells.

Sequence and organization of the Neodiprion lecontei nucleopolyhedrovirus genome

All fully sequenced baculovirus genomes, with the exception of the dipteran Culex nigripalpus nucleopolyhedrovirus (CuniNPV), have previously been from Lepidoptera. This study reports the sequencing and characterization of a hymenopteran baculovirus, Neodiprion lecontei nucleopolyhedrovirus (NeleNPV), from the redheaded pine sawfly. NeleNPV has the smallest genome so far published (81,755 bp) and has a GC content of only 33.3%. It contains 89 potential open reading frames, 43 with baculovirus homologues, 6 identified by conserved domains, and 1 with homology to a densovirus structural protein. Average amino acid identity of homologues ranged from 19.7% with CuniNPV to 24.9% with Spodoptera exigua nucleopolyhedrovirus. The conserved set of baculovirus genes has dropped to 29, since NeleNPV lacks an F protein homologue (ac23/ld130). NeleNPV contains 12 conserved lepidopteran baculovirus genes, including that for DNA binding protein, late expression factor 11 (lef-11), polyhedrin, occlusion derived virus envelope protein-18 (odv-e18), p40, and p45, but lacks 21 others, including lef-3, me53, immediate early gene-1, lef-6, pp31, odv-e66, few polyhedra 25k, odv-e25, protein kinase-1, fibroblast growth factor, and ubiquitin. The lack of identified baculovirus homologues may be due to difficulties in identification, differences in host-virus interactions, or other genes performing similar functions. Gene parity plots showed limited colinearity of NeleNPV with other baculoviruses, and phylogenetic analysis indicates that NeleNPV may have existed before the lepidopteran nucleopolyhedrovirus and granulovirus divergence. The creation of two new Baculoviridae genera to fit hymenopteran and dipteran baculoviruses may be necessary.

The gp64 locus of Anticarsia gemmatalis multicapsid nucleopolyhedrovirus contains a 3′ repair exonuclease homologue and lacks v-cath and ChiA genes

Anticarsia gemmatalis multicapsid nucleopolyhedrovirus (AgMNPV) is one of the most successful biological insecticides. In this study, we cloned and sequenced a 12.5 kbp BamHI-D restriction endonuclease fragment of the AgMNPV isolate 2D genome that includes the gp64 gene. We compared this highly conserved region with that of other baculoviruses. AgMNPV contained two genes, p22.2 and v-trex, in common with Choristoneura fumiferana MNPV (CfMNPV) that were not present in other baculoviruses. The v-trex gene has homology to a eukaryotic 3' repair exonuclease and appears to have been acquired from an invertebrate host. The v-trex gene product has the potential to be involved in virus recombination or UV-light tolerance. Multigene phylogenetic analysis suggested that AgMNPV is most closely related to Orgyia pseudotsugata MNPV (OpMNPV). AgMNPV differed from other group I NPVs in that ChiA and v-cath gene homologues were missing from the region downstream of the gp64 gene. Proteinase assays and genetic probes suggest the v-cath gene is absent from AgMNPV.

Genome sequence and organization of a nucleopolyhedrovirus isolated from the smaller tea tortrix, Adoxophyes honmai

Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV) has a distinctive pathology in A. honmai larvae, killing the host more slowly than other NPVs. To further understand the pathology of AdhoNPV, its genome was completely sequenced and compared with those of other baculoviruses. The AdhoNPV genome is 113,220 bp, with a G + C content of 35.6%. It contains 125 putative open reading frames (ORFs), of which 8 are unique to AdhoNPV, and 4 homologous regions. The other 117 ORFs display similarity to previously characterized baculovirus genes involved in early and late gene expression, DNA replication, and structural and auxiliary functions. The phylogenetic position of AdhoNPV, in relation to 15 other baculoviruses whose genomes have been completely sequenced, was assessed by three different analyses: gene sequence, gene order, and gene content. Although gene content analysis failed to support the group II NPVs, phylogenetic trees based on gene sequence and gene order showed AdhoNPV to be closely related to the group II NPVs.

Proteomic analysis of proteins that binds specifically to the homologous repeat regions of white spot syndrome virus

White spot syndrome virus (WSSV) is a major pathogen in the cultivated shrimp. Nine homologous repeat regions (hrrs) have been reported interspersed throughout the WSSV genome. In this investigation, the protein fraction that specifically bound to the hrrs was isolated by using DNA-affinity chromatography. A total of 9 (S1 to S9) and 5 (C1 to C5) proteins separated from the WSSV infected shrimp and the healthy shrimp, respectively, were detected by using two-dimensional polyacrylamide gel electrophoresis, and 6 proteins changed with WSSV infection were analyzed by mass spectrometry (MS). One (S4) of the 6 proteins examined was identified as WSSV ORF59 protein, and another (S3) was a shrimp arginine kinase. No homologous proteins were found with the remaining 4 proteins by searching in the WSSV ORF database and NCBI database. The specific binding site of the 6 proteins was then determined by gel mobility shift assay (GMSA). Temporal analysis revealed that ORF59 gene was transcribed at the early stage of the infection. The results we obtained provide important information to understand WSSV replication. The combination of DNA-affinity chromatography, 2D-PAGE and MS approaches should have general application to the identification of gene regulating proteins of WSSV. The results represent the first isolation of a set of proteins that bind to the hrrs, and, furthermore, lead us a new research direction for the prevention and the therapy of WSSV.

The homologous region sequence (hr1) of Autographa californica multinucleocapsid polyhedrosis virus can enhance transcription from non-baculoviral promoters in mammalian cells

The Autographa californica multinucleocapsid polyhedrosis virus homologous region sequence hr1 enhances transcription from the viral polyhedrin promoter in Spodoptera frugiperda insect cells and independently functions as an origin of replication (ori) sequence. The binding of the host nuclear protein, hr1-binding protein (hr1-BP), is crucial for the enhancer activity (Habib, S., Pandey, S., Chatterji, U., Burma, S., Ahmad, R., Jain, A., and Hasnain, S. E. (1996) DNA Cell Biol. 15, 737-747 and Habib, S., and Hasnain, S. E. (1996) J. Biol. Chem. 271, 28250-28258). We demonstrate that hr1 can also enhance transcription from non-baculoviral promoters like cytomegalovirus and hsp70 in mammalian cells but does not support ori activity in these cells. Unlike insect cells, hr1 can also function in mammalian cells as an enhancer when present in trans. hr1 DNA sequence binds with high affinity and specificity to nuclear factors in the mammalian cells. The insect hr1-BP- and the hr1-BP-like proteins from mammalian cells (mhr1-BP) have different properties with respect to ion requirements, DNA groove binding, and molecular size. When mammalian cells are infected with a recombinant baculovirus containing two promoters, the baculovirus polyhedrin and Drosophila hsp70 gene promoter, the hsp70 gene promoter alone is active in these cells, and this activity is further enhanced by the presence of an additional hr1 in the recombinant virus. hr1 may thus also have a role in baculovirus-mediated gene delivery in mammalian cells.

The complete sequence of the Adoxophyes orana granulovirus genome

The nucleotide sequence of the Adoxophyes orana granulovirus (AdorGV) DNA genome was determined and analysed. The genome contains 99,657 bp and has an A + T content of 65.5%. The analysis predicted 119 ORFs of 150 nucleotides or larger that showed minimal overlap. Of these putative genes, 104 (87%) were homologous to genes identified previously in other baculoviruses. The mean overall amino acid identity of AdorGV ORFs was highest with CpGV ORFs at 48%. Sixty-three ORFs were conserved among all lepidopteran baculoviruses and are considered to be common baculoviral genes. Several genes reported to have major roles in baculovirus biology were not found in the AdorGV genome. These included chitinase and cathepsin, which are involved in the liquefaction of the host, which explains why AdorGV-infected insects do not degrade in a typical manner. The AdorGV genome encoded two inhibitor of apoptosis (iap) genes iap-3 and iap-5. Among all of the granuloviruses genomes there was a very high level of gene collinearity. The genes shared by AdorGV and CpGV had exactly the same order along the genome with the exception of one gene, iap-3. The AdorGV genome did not contain typical homologous region (hr) sequences. However, it contained nine repetitive regions in the genome.

Developing baculovirus-insect cell expression systems for humanized recombinant glycoprotein production

The baculovirus-insect cell expression system is widely used to produce recombinant glycoproteins for many different biomedical applications. However, due to the fundamental nature of insect glycoprotein processing pathways, this system is typically unable to produce recombinant mammalian glycoproteins with authentic oligosaccharide side chains. This minireview summarizes our current understanding of insect protein glycosylation pathways and our recent efforts to address this problem. These efforts have yielded new insect cell lines and baculoviral vectors that can produce recombinant glycoproteins with humanized oligosaccharide side chains.

The highly conserved basic domain I of baculovirus IE1 is required for hr enhancer DNA binding and hr-dependent transactivation

The immediate-early protein IE1 is the principal transcriptional regulator of the baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV). Transactivation by IE1 is dramatically stimulated by cis linkage of the affected promoter to AcMNPV homologous region (hr) elements that contain palindromic 28-bp repeats (28-mers) with enhancer activity. This hr-dependent transcriptional enhancement requires binding of the 28-mer by dimeric IE1. Here, we have defined IE1 domains required for this DNA binding in order to investigate the mechanism of IE1 function. Analysis of a panel of IE1 insertion mutations indicated that disruption of a highly conserved domain (residues 152 to 161) consisting of mostly positive-charged residues (basic domain I) abolished hr-dependent transactivation. Targeted mutagenesis of basic residues within basic domain I caused loss of hr-dependent transactivation but had no effect on IE1 oligomerization, nuclear localization, or hr-independent transactivation of viral promoters. Alanine substitutions of K(152) and K(154) or K(160) and K(161) impaired IE1 binding to 28-mer DNA as a homodimer, indicating that these basic residues are required for enhancer binding. Consistent with a DNA-binding defect, 28-mer interaction was improved by heterodimerization with wild-type IE1 or by increasing mutated IE1 concentrations. DNA binding mediated by basic domain I was also required for IE1 transactivation that occurred through physically separated, unlinked hr elements. We concluded that basic domain I is the enhancer-binding domain for IE1. Our data also suggest that DNA binding activates IE1 for transcriptional enhancement, possibly through a conformational change involving basic domain I.

Use of an N-terminal half truncated IE1 as an antagonist of IE1, an essential regulatory protein in baculovirus

An immediate-early gene product of baculovirus, IE1, is essential for viral gene expression and for viral DNA replication. It has been demonstrated for Autographa californica nuclear polyhedrosis virus (AcNPV) that the C-terminal region of IE1 is required for dimerization. And the acidic N-terminal region of IE1 has been identified as the activation domain. We constructed an N-terminal 267 amino acid (a.a.) truncated mutant of Bombyx mori nuclear polyhedrosis virus (BmNPV) IE1, which was defective as a transactivator of a viral early gene (p35) promoter. We then examined possible IE1 antagonistic functions of this defective IE1, IE1TN, in BmNPV-infected cells. A transient expression experiment demonstrated that IE1TN strongly repressed the activation of the hr5-dependent p35 promoter derived from BmNPV infection. In addition, DpnI assay elucidated an inhibitory effect of IE1TN on the hr5-dependent replication of plasmid in BmN cells induced by NPV infection. A marked reduction in the production of virus was observed when the BmN cells were infected with BmNPV after transfection with IE1TN-expression plasmids. These results suggested that IE1TN could act as an IE1 antagonist in silkworm cells infected with BmNPV. We then analyzed the ability of IE1TN to inhibit the multiplication of BmNPV using transgenic silkworms. The BmNPV-resistance of the transgenic silkworms was very weak, suggesting insufficient expression of the transgene product, IE1TN.

Identification of six Autographa californica multicapsid nucleopolyhedrovirus early genes that mediate nuclear localization of G-actin

Nuclear filamentous actin (F-actin) is required for nucleopolyhedrovirus (NPV) progeny production in NPV-infected, cultured lepidopteran cells. We have determined that monomeric G-actin is localized within the nuclei of host cells during the early stage of infection by Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). With a library of cloned AcMNPV genomic fragments, along with a plasmid engineered to express enhanced green fluorescent protein-Bombyx mori G-actin in transient transfection experiments, we identified six AcMNPV early genes that mediate nuclear localization of G-actin in TN-368 cells: ie-1, pe38, he65, Ac004, Ac102, and Ac152. Within this subset, ie-1 and pe38 encode immediate-early transcriptional transactivators, he65 encodes a delayed-early product, and the products encoded by Ac004, Ac102, and Ac152 have not been characterized. We found that when driven by foreign promoters, ie-1, pe38, and Ac004 had to be expressed prior to Ac102 or he65 for nuclear G-actin to accumulate and that expression of Ac152 was no longer required. These results and others suggested that the product of Ac152 was a transactivator (directly or indirectly) of both Ac102 and he65 and that recruitment of G-actin to the nucleus was a temporally regulated process. Determining the functions of each of the six AcMNPV gene products with respect to our assay should provide valuable clues to basic cellular mechanisms of actin regulation and how AcMNPV infection affects them.

Binding of the baculovirus very late expression factor 1 (VLF-1) to different DNA structures

BACKGROUND

Baculovirus genomes encode a gene called very late expression factor 1 (VLF-1) that is a member of the integrase (Int) family of proteins. In this report we describe the binding properties of purified Autographa californica multiple capsid nucleopolyhedrovirus (AcMNPV) VLF-1 to a number of different DNA structures including homologous regions. In addition, its enzymatic activity was examined.

RESULTS

VLF-1 was expressed in a recombinant baculovirus as a fusion with both HA and HIS6 tags and its binding activity to different DNA structures was tested. No binding was evident to single and double strand structures, very low binding was observed to Y-forks, more binding was observed to three-way junctions, whereas cruciform structures showed high levels of binding. VLF-1 binding was affected by divalent cations; optimal binding to three-way junctions and cruciforms was 2 and 0 mM MgCl2, respectively. Homologous region (hr) sequences was also examined including oligomers designed to expose the hr palindrome as a hairpin, linear double strand, or H-shaped structure. Efficient binding was observed to the hairpin and H-shaped structure. No topoisomerase or endonuclease activity was detected. Sedimentation analysis indicated that *VLF-1 is present as a monomer.

CONCLUSIONS

An HA- and HIS-tagged version of AcMNPV VLF-1 showed structure-dependent binding to DNA substrates with the highest binding affinity to cruciform DNA. These results are consistent with the involvement of VLF-1 in the processing of branched DNA molecules at the late stages of viral genome replication. We were unable to detect enzymatic activity associated with these complexes.

Baculovirus transregulator IE1 requires a dimeric nuclear localization element for nuclear import and promoter activation

Immediate-early protein IE1 is a principal regulator of viral transcription and a contributor to origin-specific DNA replication of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). Since these viral functions involve interaction of dimeric IE1 with palindromic homologous region (hr) enhancer-origin elements of the AcMNPV genome within the nucleus, it is presumed that proper nuclear transport of IE1 is essential for productive infection. To investigate the mechanisms of IE1 nuclear import, we analyzed the effect of site-directed mutations on IE1 subcellular distribution. As demonstrated by fluorescence microscopy and biochemical fractionation of plasmid-transfected cells, wild-type IE1 localized predominantly to the nucleus. Substitution or deletion of amino acid residues within a positively charged domain (residues 534 to 538) adjacent to IE1's oligomerization motif impaired nuclear import and caused loss of transactivation. Moreover, upon coexpression, these import-defective mutations prevented nuclear entry of wild-type IE1. In contrast, double-mutated IE1 defective for both nuclear import and dimerization failed to block nuclear entry or transactivation by wild-type IE1. Thus, import-defective IE1 dominantly interfered with wild-type IE1 by direct interaction and cytosolic trapping. Collectively, our data indicate that the small basic domain encompassing residues R(537) and R(538) constitutes a novel nuclear localization element that functions only upon IE1 dimerization. These findings support a model wherein IE1 oligomerizes within the cytosol as a prerequisite for nuclear entry and subsequent high-affinity interaction with the symmetrical binding sites comprising AcMNPV hr enhancer-origin elements.

Comparative analysis of the complete genome sequences of Helicoverpa zea and Helicoverpa armigera single-nucleocapsid nucleopolyhedroviruses

The complete nucleotide sequence of Helicoverpa zea single-nucleocapsid nucleopolyhedrovirus (HzSNPV) has been determined (130869 bp) and compared to the nucleotide sequence of Helicoverpa armigera (Ha) SNPV. These two genomes are very similar in their nucleotide (97% identity) and amino acid (99% identity) sequences. The coding regions are much more conserved than the non-coding regions. In HzSNPV/HaSNPV, the 63 open reading frames (ORFs) present in all baculoviruses sequenced so far are much more conserved than other ORFs. HzSNPV has four additional small ORFs compared with HaSNPV, one of these (Hz42) being in a correct transcriptional context. The major differences between HzSNPV and HaSNPV are found in the sequence and organization of the homologous regions (hrs) and the baculovirus repeat ORFs (bro genes). The sequence identity between the HzSNPV and HaSNPV hrs ranges from 90% (hr1) to almost 100% (hr5) and the hrs differ in the presence/absence of one or more type A and/or B repeats. The three HzSNPV bro genes differ significantly from those in HaSNPV and may have been acquired independently in the ancestral past. The sequence data suggest strongly that HzSNPV and HaSNPV are variants of the same virus species, a conclusion that is supported by the physical and biological data.

Identification and molecular characterization of the baculovirus CfMNPV early genes: ie-1, ie-2 and pe38

Three early virus genes, ie-1, ie-2 and pe38, were identified and localized in the XbaI G region (91.2-98.6 m.u.) of the genome of Choristoneura fumiferana nucleopolyhedrovirus (CfMNPV), a baculovirus pathogenic to spruce budworm. Nucleotide sequence analysis indicated that these genes share varied sequence similarity with their homologues in other baculoviruses where they are involved in regulating virus gene expression and DNA replication. Sequence motifs characteristic of DNA binding and transactivation found in other baculovirus regulatory genes were conserved in the CfMNPV genes. Northern analysis demonstrated that all three CfMNPV genes were transcriptionally active in virus infected cells and followed the temporal expression pattern of immediate early baculovirus genes. Primer extension experiments revealed that typical baculovirus early transcription start sites (CAGT) were used for ie-1 and pe38 transcription initiation. Two regions of highly repetitive DNA were found in the odv-e56 to ie-2 and ie-2 and pe38 intergenic regions. These sequences are predicted to function as transcriptional enhancers and viral origins of DNA replication.

Novel baculovirus DNA elements strongly stimulate activities of exogenous and endogenous promoters

A DNA sequence upstream from the polyhedrin gene of baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV) was found to activate strongly the expression of full or minimal promoters derived from AcMNPV and other sources. Promoters tested included the minimal CMV (CMVm) promoter from human cytomegalovirus, the full heat shock 70 promoter from Drosophila, and the minimal p35 promoter from baculovirus. Deletion and mutagenesis analyses showed that this functional polyhedrin upstream (pu) activator sequence contains three open reading frames (ORFs), ORF4, ORF5, and lef2. In plasmid transfection assays, the pu sequence was able to confer high level luciferase expression driven by all of these full or minimal promoters in insect Sf21 cells. A known baculovirus enhancer, the homologous region (hr) of AcMNPV, further enhanced the expression of these promoters. Experiments showed that although multiple hr sequences function in an additive manner, pu and hr together function synergistically, resulting in as much as 18,000-fold promoter activation. Furthermore, a modified CMVm promoter containing pu and/or hr was inserted into the baculovirus genome to drive the luciferase coding region. The CMVm promoter expressed luciferase much earlier, and although it expressed a bit less than did the p10 promoter, the CMVm promoter gave rise to greater luciferase activity. Therefore, we have uncovered a cryptic viral sequence capable of activating a diverse group of promoters. Finally, these experiments demonstrate that synthetic sequences containing pu, hr, and different full or minimal promoters can generate a set of essentially unlimited novel promoters for weak to very strong expression of foreign proteins using baculovirus.

Improved glycosylation of a foreign protein by Tn-5B1-4 cells engineered to express mammalian glycosyltransferases

The major advantages of using the baculovirus-insect cell system for recombinant protein production are its ability to produce large amounts of recombinant proteins and its ability to provide eucaryotic modifications, such as glycosylation. However, the glycans linked to recombinant glycoproteins produced by this system typically differ from those found on native mammalian products. This is an important problem because glycans on mammalian glycoproteins can influence their functions in many different ways. The inability of baculovirus-infected insect cells to produce glycans identical to those found on native mammalian glycoproteins is due, in part, to the absence of functional levels of certain glycosyltransferases in insect cells. Thus, the purpose of this study was to engineer these activities into Tn-5B1-4, an established insect cell line that is widely used as a host for baculovirus-mediated protein production. Expression plasmids were constructed in which cDNAs encoding mammalian beta1,4-galactosyltransferase and alpha2,6-sialyltransferase were placed under the transcriptional control of a baculovirus immediate early promoter. These plasmids were then used to isolate two different transgenic Tn-5B1-4 derivatives and the biological and biochemical properties of these cell lines were examined. The results show that both of the engineered insect cell lines have improved glycoprotein-processing capabilities, relative to the parental cell line.

Mammalian glycosyltransferase expression allows sialoglycoprotein production by baculovirus-infected insect cells

The baculovirus-insect cell expression system is widely used to produce recombinant mammalian glycoproteins, but the glycosylated end products are rarely authentic. This is because insect cells are typically unable to produce glycoprotein glycans containing terminal sialic acid residues. In this study, we examined the influence of two mammalian glycosyltransferases on N-glycoprotein sialylation by the baculovirus-insect cell system. This was accomplished by using a novel baculovirus vector designed to express a mammalian alpha2,6-sialyltransferase early in infection and a new insect cell line stably transformed to constitutively express a mammalian beta1,4-galactosyltransferase. Various biochemical assays showed that a foreign glycoprotein was sialylated by this virus-host combination, but not by a control virus-host combination, which lacked the mammalian glycosyltransferase genes. Thus, this study demonstrates that the baculovirus-insect cell expression system can be metabolically engineered for N-glycoprotein sialylation by the addition of two mammalian glycosyltransferase genes.

Novel baculovirus expression vectors that provide sialylation of recombinant glycoproteins in lepidopteran insect cells

This report describes novel baculovirus vectors designed to express mammalian beta1,4-galactosyltransferase and alpha2,6-sialyltransferase genes at early times after infection. Sf9 cells infected with these viral vectors, unlike cells infected with a wild-type baculovirus, produced a sialylated viral glycoprotein during the late phase of infection. Thus, the two mammalian glycosyltransferases encoded by these viral vectors are necessary and sufficient for sialylation of a foreign glycoprotein in insect cells under the conditions used in this study. While some of the new baculovirus vectors described in this study produced less, one produced wild-type levels of infectious budded virus progeny.

Oligomerization mediated by a helix-loop-helix-like domain of baculovirus IE1 is required for early promoter transactivation

IE1 is a principal transcriptional regulator of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). Transactivation by IE1 is stimulated when early viral promoters are cis linked to homologous-region (hr) enhancer sequences of AcMNPV. This transcriptional enhancement is correlated with the binding of IE1 as a dimer to the 28-bp palindromic repeats comprising the hr enhancer. To define the role of homophilic interactions in IE1 transactivation, we have mapped the IE1 domains required for oligomerization. We report here that IE1 oligomerizes by a mechanism independent of enhancer binding, as demonstrated by in vitro pull-down assays using fusions of IE1 (582 residues) to the C terminus of glutathione S-transferase. In vivo oligomerization of IE1 was verified by immunoprecipitation of IE1 complexes from extracts of plasmid-transfected SF21 cells. Analyses of a series of site-directed IE1 insertion mutations indicated that a helix-loop-helix (HLH)-like domain extending from residue 543 to residue 568 is the primary determinant of oligomerization. Replacement of residues within the hydrophobic face of the putative dimerization domain disrupted IE1 homophilic interactions and caused loss of IE1 transactivation of hr-dependent promoters in plasmid transfection assays. Thus, oligomerization is required for IE1 transcriptional stimulation. HLH mutations also reduced IE1 stability and abrogated transactivation of non-hr-dependent promoters. These data support a model wherein IE1 oligomerizes prior to DNA binding to facilitate proper interaction with the symmetrical recognition sites within the hr enhancer and thereby promote the transcription of early viral genes.

The sequence of the Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus genome

The nucleotide sequence of the Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HaSNPV) DNA genome was determined and analysed. The circular genome encompasses 131,403 bp, has a G+C content of 39.1 mol% and contains five homologous regions with a unique pattern of repeats. Computer-assisted analysis revealed 135 putative ORFs of 150 nt or larger; 100 ORFs have homologues in Autographa californica multicapsid NPV (AcMNPV) and a further 15 ORFs have homologues in other baculoviruses such as Lymantria dispar MNPV (LdMNPV), Spodoptera exigua MNPV (SeMNPV) and Xestia c-nigrum granulovirus (XcGV). Twenty ORFs are unique to HaSNPV without homologues in GenBank. Among the six previously sequenced baculoviruses, AcMNPV, Bombyx mori NPV (BmNPV), Orgyia pseudotsugata MNPV (OpMNPV), SeMNPV, LdMNPV and XcGV, 65 ORFs are conserved and hence are considered as core baculovirus genes. The mean overall amino acid identity of HaSNPV ORFs was the highest with SeMNPV and LdMNPV homologues. Other than three 'baculovirus repeat ORFs' (bro) and two 'inhibitor of apoptosis' (iap) genes, no duplicated ORFs were found. A putative ORF showing similarity to poly(ADP-ribose) glycohydrolases (parg) was newly identified. The HaSNPV genome lacks a homologue of the major budded virus (BV) glycoprotein gene, gp64, of AcMNPV, BmNPV and OpMNPV. Instead, a homologue of SeMNPV ORF8, encoding the major BV envelope protein, has been identified. GeneParityPlot analysis suggests that HaSNPV, SeMNPV and LdMNPV (group II) have structural genomic features in common and are distinct from the group I NPVs and from the granuloviruses. Cluster alignment between group I and group II baculoviruses suggests that they have a common ancestor.

The Autographa californica nucleopolyhedrovirus IE-1 protein complex has two modes of specific DNA binding

Missing contact footprinting with formic acid as a modifying reagent was used to examine specific IE-1 binding contacts to double-stranded oligonucleotides that contained either a consensus hr repeat sequence or a sequence from the pe38 promoter, which is down regulated by IE-1. The hr repeat sequences contain two consensus IE-1 binding motifs (IBMs) flanking a central EcoRI site that are oriented in opposite directions with respect to each other. IE-1 was found to contact regions including both IBMs. The bases footprinted in the top strand included the left IBM (IBM-A), whereas bases in the bottom strand were footprinted in a region that included IBM-B and part of IBM-A. When substitution mutations were introduced into either IBM, bases on both strands of the remaining IBM were strongly footprinted. As with the hr IBM-mutant constructs, bases footprinted in the pe38 promoter construct included both strands of the single IBM.

Differential activity of two non-hr origins during replication of the baculovirus Autographa californica nuclear polyhedrosis virus genome

The identification of potential baculovirus origins of replication (ori) has involved the generation and characterization of defective interfering particles that contain major genomic deletions yet retain their capability to replicate by testing the replication ability of transiently transfected plasmids carrying viral sequences in infected cells. So far, there has not been any evidence to demonstrate the actual utilization of these putative origins in Autographa californica multinucleocapsid nuclear polyhedrosis virus (AcMNPV) replication. By using the method of origin mapping by competitive PCR, we have obtained quantitative data for the ori activity of the HindIII-K region and the ie-1 promoter sequence in AcMNPV. We also provide evidence for differential activity of the two ori in the context of the viral genome through the replication phase of viral infection. Comparison of the number of molecules representing the HindIII-K and ie-1 origins vis-à-vis the non-ori polH region in a size-selected nascent DNA preparation revealed that the HindIII-K ori is utilized approximately 14 times more efficiently than the ie-1 region during the late phase of infection. HindIII-K also remains the more active ori through the early and middle replication phases. Our results provide in vivo evidence in support of the view that AcMNPV replication involves multiple ori that are activated with vastly different efficiencies during the viral infection cycle.

Purification of Autographa californica nucleopolyhedrovirus DNA polymerase from infected insect cells

Autographa californica nucleopolyhedrovirus (AcMNPV) DNA polymerase was purified from virus-infected cells using conventional chromatographic methods. The enzymatic activity of fractions eluting from single-stranded agarose gels was found to exactly coincide with a single polypeptide with an apparent molecular mass of approximately 110,000 Da on denaturing polyacrylamide gels stained with Coomassie blue. This purification scheme resulted in a 228-fold purification of AcMNPV DNA polymerase with recovery of 3.5% of the initial activity. The specific activity of the most purified fraction of DNA polymerase was 5000 units/mg, which is sufficiently high to eliminate the possibility that contaminants significantly contribute to the polymerase activity. Preparations of purified DNA polymerase had 3'-5' exonuclease activity, but no 5'-3' exonuclease activity. The proofreading activity was apparently an intrinsic property of the enzyme as the ratio of nuclease activity to polymerase activity was constant throughout purification. Using a singly-primed M13 DNA template, RF-II DNA was detected within 3 min, indicating a polymerization rate of 40 nt/s. The effects of several DNA polymerase inhibitors on the enzymatic activity of purified DNA polymerase were also determined.

Characterization of the acidic domain of the IE1 regulatory protein from Orgyia pseudotsugata multicapsid nucleopolyhedrovirus

This study presents a detailed analysis of the acidic N-terminal region of the Orgyia pseudotsugata multicapsid nucleopolyhedrovirus (OpMNPV) transactivator IE1. The N-terminal region of IE1 is rich in acidic amino acids and has been hypothesized to be an acidic activation domain. Removal of the N-terminal 126 amino acids containing the acidic domain of IE1 resulted in complete loss of transactivation activity, indicating that this region is essential for transactivation. The OpMNPV acidic domain was replaced with the archetype acidic activation domain from VP16 and the acid-rich region of Autographa californica multicapsid NPV (AcMNPV) IE1. These chimeric constructs were fully capable of transactivation in transient assays. The chimeric OpMNPV IE1s containing the herpes simplex virus VP16 and AcMNPV IE1 acidic activation domains consistently transactivated a reporter gene to higher levels than the OpMNPV IE1 acidic activation domain. Transactivation by the chimeric constructs is enhanced synergistically when cotransfected with IE2 into Lymantria dispar and Spodoptera frugiperda cells. Both N- to C-terminal and C- to N-terminal deletions of the OpMNPV acidic activation domain were constructed to define functional domains within the OpMNPV IE1 acidic activation domain. At least two potential activation domains were identified. Within each of these domains, two core regions at amino acids 28-43 and amino acids 113-124 were identified that were similar to core regions of VP16 and GAL4, which contain predominately acidic and bulky hydrophobic amino acids.

The early baculovirus he65 promoter: On the mechanism of transcriptional activation by IE1

We have initiated studies on the mechanism of early transcriptional activation of the early he65 promoter during infection with Autographa californica multicapsid nuclear polyhedrosis virus. This analysis is based on a comparison of the sequences required for he65 promoter activation with those sequences that support specific protein binding. The he65 promoter is located immediately downstream of the homologous region (hr) 4a. The sequences of hr4a are characterized by two imperfect palindromes of 24 bp. The results of transient expression assays indicate promoter activation in the presence of both the proximal palindrome and the known viral trans-regulator IE1. The results of mobility shift assays and DNaseI footprinting analyses reveal differences in specific protein binding at and close to the proximal palindrome depending on whether the nuclear protein extracts are prepared from uninfected or infected cells. The analysis of the protein binding complex at the proximal inverted repeat with extracts from infected cells suggests the involvement of both IE1 and IE0 as oligomers. The minimal protein binding sequences include the left half-site of the 24 bp repeat with 9 additional bp of the flanking sequences. The right half-site of the repeat also directs binding although with lower affinity as confirmed by phenanthroline-copper footprinting assays. Both half-sites of the repeat are thus essential for he65 promoter activation, suggesting that IE1 acts via cooperative binding. We conclude that the proximal inverted repeat is able to interact with both IE1 and IE0 although IE1 is sufficient for activation at least in transient expression assays.

Genome organization of Xestia c-nigrum granulovirus

In order to characterize the genome organization of Xestia c-nigrum granulovirus (XcGV), mapping of putative XcGV genes was performed by construction of lambda and M13 phage libraries followed by Southern blot and nucleotide sequencing analyses. Mapping of the lambda (32 clones covering the entire XcGV genome) and M13 (133 clones made by random cloning) phage library clones was carried out by hybridization of the labeled lambda phage clone DNAs to 1) Southern blotted XcGV genomic DNA fragments cleaved with EcoRI, BamHI, or HindIII, and 2) dot blotted M13 clone DNAs. All 133 M13 clone DNAs were sequenced, and coding possibilities were investigated by computer-assisted homology search; in total, about 43 kb of the genome was sequenced. Amino acid sequence homology searches of 67 M13 clones suggested that these GV DNAs coded for previously characterized genes identified in nucleopolyhedroviruses (NPVs) and GVs. These 67 M13 clones were classified into 25 gene homolog groups (including 29 putative genes) based on their homologies to NPV and GV genes. The remaining M13 clones, except one that encoded a putative metalloproteinase, did not possess deduced amino acid sequences with significant homology to proteins in gene databases. Complete nucleotide sequences of the putative XcGV DNA polymerase and Ac144 homolog genes confirmed the reliability of our speculation of putative genes based on the M13 clones sequencing analysis. In a comparison of relative locations of putative XcGV genes with locations of their homologs in NPVs, most XcGV genes were mapped close to the corresponding locations in NPV genomes. These results suggested that XcGV, compared to NPVs, had relatively conserved gene arrangements, although about 22 kb of 43 kb of DNA sequenced randomly in the XcGV genome consisted of sequences/genes non-homologous to those of previously characterized NPVs.

A baculovirus (Bombyx mori nuclear polyhedrosis virus) repeat element functions as a powerful constitutive enhancer in transfected insect cells

It has been previously reported that baculovirus homologous regions, the regions of baculovirus genomes that contain the origins of DNA replication, can augment the expression of a small number of baculovirus genes in vitro. We are now reporting that a region of the genome of Bombyx mori nuclear polyhedrosis virus (BmNPV) containing the homologous region 3 (HR3) acts as an enhancer for the promoter of a nonviral gene, the cytoplasmic actin gene of the silkmoth B. mori. Incorporation of the HR3 sequences of BmNPV into an actin promoter-based expression cassette results in an augmentation of transgene expression in transfected cells by two orders of magnitude relative to the control recombinant expression cassette. This increase is due to a corresponding increase in the rate of transcription from the actin promoter and not to replication of the expression cassette and occurs only when the HR3 element is linked to the expression cassette in cis. A comparable degree of enhancement in the activity of the silkworm actin promoter occurs also in heterologous lepidopteran cells. Concomitant supplementation of transfected cells with the BmIE1 trans-activator, which was previously shown to be capable of functioning in vitro as a transcriptional co-activator of the cytoplasmic actin gene promoter, results in more than a 1,000-fold increase in the level of expression of recombinant proteins placed under the control of the actin gene promoter. These findings provide the foundation for the development of a nonlytic insect cell expression system for continuous high-level expression of recombinant proteins. Such a system should provide levels of expression of recombinant proteins comparable to those obtained from baculovirus expression systems and should also have the additional advantage of continuous production in a cellular environment that, in contrast to that generated by a baculovirus infection, supports continuously proper posttranslational modifications of recombinant proteins and the capability of expression of proteins from genomic as well as cDNA sequences.

DNA-dependent transregulation by IE1 of Autographa californica nuclear polyhedrosis virus: IE1 domains required for transactivation and DNA binding

IE1 is the principal early transregulator of Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV). The 582-residue protein stimulates viral transcription and binds as a dimer to 28-bp palindromic repeats (28-mers) comprising the AcMNPV homologous region (hr) transcription enhancers. To define IE1 domains responsible for hr-dependent transactivation, we first constructed a series of IE1 fusions to the DNA binding domain of the yeast GAL4 transactivator. In transfection assays, GAL4-IE1 fusions stimulated transcription from a TATA-containing AcMNPV promoter only upon cis linkage to GAL4 DNA binding sites. IE1 N-terminal residues 8 to 118 were sufficient for GAL4-binding-site-dependent transactivation. To identify IE1 residues required for hr interaction, we tested a series of IE1 mutations for 28-mer binding by using electrophoretic mobility shift assays. Deletion of IE1 residues other than the N-terminal transactivation domain eliminated 28-mer binding. Of 14 insertion mutations, only IE1(I425) and IE1(I553) failed to bind the 28-mer either as homodimers or as heterodimers with functional IE1. In contrast to insertion IE1(I425), IE1(I553) also failed to compete with wild-type IE1 for DNA binding and suggested a defect in oligomerization. Consistent with loss of oligomerization, substitutions within a hydrophobic repeat (residues 543 to 568) at the IE1 C terminus abolished 28-mer binding and demonstrated that this helix-loop-helix-like domain is required for DNA interaction. These data confirm that IE1 contains separable domains for transactivation and oligomerization-dependent DNA binding. Furthermore, they support a model wherein hr-mediated transactivation by IE1 involves sequence-specific DNA binding that contributes to transcriptional stimulation by interaction with components of the basal transcription complex.

DNA replication promotes high-frequency homologous recombination during Autographa californica multiple nuclear polyhedrosis virus infection

The relative ease with which foreign genes can be incorporated into the genome of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) indicates that a highly efficient recombinational process exists within infected cells. However, it is unclear whether this is due to marker transfer mediated by host cell enzymes or recombination events promoted by AcMNPV itself. To address the latter possibility, a pair of inverted repeat IS50 elements derived from the bacterial transposon Tn5 was inserted into the polyhedrin gene locus of the AcMNPV genome. Inversion of Tn5 sequences arising from recombination between its IS50 repeats could be readily detected in this virus, indicating that AcMNPV DNA undergoes high-frequency recombination during infection. To further characterize this process, a transient recombination assay was developed and used to identify the cis- and trans-acting requirements for Tn5 inversion in AcMNPV. A transfected Tn5-containing plasmid was found to undergo the same sequence inversion events seen in the viral genome, but only if it also contained a putative AcMNPV origin of replication (homologous region 2) in cis and was replicated by AcMNPV gene products supplied in trans. Taken together, these results indicated that recombination events which occur in infected cells were strictly dependent upon AcMNPV-mediated DNA replication. Direct support for this hypothesis was provided by the observation that the minimal set of AcMNPV genes that was essential for plasmid DNA replication also promoted recombination events leading to Tn5 inversion in the absence of any other viral function. Finally, using a panel of deletion mutants of the IS50 elements in Tn5, sequence inversion was shown to be the result of homologous rather than site-specific recombination, since it occurred independently of a discrete sequence within the transposon. These results demonstrate that the AcMNPV DNA replication machinery exhibits a strong propensity to promote homologous recombination events during infection and is likely to play a role in the high frequency of marker transfer observed in this virus.

A 38-kDa host factor interacts with functionally important motifs within the Autographa californica multinucleocapsid nuclear polyhedrosis virus homologous region (hr1) DNA sequence

We recently demonstrated that the Autographa californica multinucleocapsid nuclear polyhedrosis virus homologous region (hr1) enhances transcription from the viral polyhedrin promoter and also functions as a putative origin of replication (ori). Hr1, carrying five 28-base pair core palindrome units, has also been mapped with respect to its enhancer and ori functions (Habib, S., Pandey, S., Chatterji, U., Burma, S., Ahmad, R., Jain, A., and Hasnain, S. E. (1996) DNA Cell Biol. 15, 737-747). A 38-kDa host factor termed hr1-binding protein (hr1-BP) binds with high specificity and affinity (Kd approximately 6.5 x 10(-11) M) to functionally important motifs within hr1. The core palindrome as well as sequences immediately flanking it are required for this interaction. Divalent cations are not essential, and ionic interactions play only a minor role in complex formation. hr1-BP binds through the minor groove of the double helix to multiple sites within hr1, and binding occurs as a function of the number of modules within hr1. Phosphorylation of hr1-BP is important for host factor-hr1 interaction. Hr1-BP differs in several respects from the other host factor, polyhedrin promoter-binding protein, described previously (Burma, S., Mukherjee, B., Jain, A., Habib, S., and Hasnain, S. E. (1994) J. Biol. Chem. 269, 2750-2757). When hr1-BP was sequestered out, in vivo, by a plasmid carrying hr1 alone, the hr1-mediated enhancement of reporter expression was abolished, demonstrating that the binding of hr1-BP may be crucial for the enhancer activity of the dual function hr1 element.

Modifying the insect cell N-glycosylation pathway with immediate early baculovirus expression vectors

The baculovirus-insect cell expression system is well-suited for recombinant glycoprotein production because baculovirus vectors can provide high levels of expression and insect cells can modify newly synthesized proteins in eucaryotic fashion. However, the N-glycosylation pathway of baculovirus-infected insect cells differs from the pathway found in higher eucaryotes, as indicated by the fact that glycoproteins produced in the baculovirus system typically lack complex biantennary N-linked oligosaccharide side chains containing penultimate galactose and terminal sialic acid residues. We recently developed a new type of baculovirus vector that can express foreign genes immediately after infection under the control of the viral ie1 promoter. These immediate early baculovirus expression vectors can be used to modify the insect cell N-glycosylation pathway and produce a foreign glycoprotein with more extensively processed N-linked oligosaccharides. These vectors can also be used to study the influence of the late steps in N-linked oligosaccharide processing on glycoprotein function. Further development could lead to baculovirus-insect cell expression systems that can produce recombinant glycoproteins with complex biantennary N-linked oligosaccharides structurally identical to those produced by higher eucaryotes.

Bifunctionality of the AcMNPV homologous region sequence (hr1): enhancer and ori functions have different sequence requirements

The Autographa californica multinucleocapsid nuclear polyhedrosis virus (AcMNPV) homologous region sequence hr1 is a putative origin of replication (ori) sequence and can also function as a transcriptional enhancer for delayed-early genes. We demonstrate that this 750-bp sequence, carrying five 28-bp core palin-dromes, enhances expression from the very late polyhedrin promoter up to 11-fold in a classical enhancer fashion in transient expression assays. Enhancement is at the level of transcription, as evident from RNase protection assay analysis. It is mediated by an alpha-amanitin-insensitive RNA polymerase from the authentic polyhedrin promoter transcription start site and follows the temporal activation profile characteristic of the polyhedrin promoter. Three lines of evidence conclusively demonstrated that hr1 acts typically as an enhancer of polyhedrin gene transcription independent of its role as an ori: (i) linearized hr1-reporter plasmids, incapable of replicating in the host cell, could enhance transcription from the promoter; (ii) reporter plasmid copy number was not affected by the presence of aphidicolin during transfection; (iii) reporter plasmid DNA recovered from Sf9 cells was sensitive to Dpn I confirming its unreplicated state in the transfection regime followed by us. Molecular dissection of the hr1 sequence elements revealed that a core palindrome alone can function as an ori sequence whereas a palindrome along with flanking sequences is essential for the enhancer activity. Enhancement of luciferase expression from the polyhedrin promoter is a function of the number of core palindromes and flanking sequences. Our results demonstrate that hr1, which has several motifs for enhancer binding proteins and transcription factors, has a dual role associated with both DNA replication and transcriptional enhancement.

A variable region of Anticarsia gemmatalis nuclear polyhedrosis virus contains tandemly repeated DNA sequences

A variable region (PstI-T fragment) of two genotypic variants of the Anticarsia gemmatalis multiple nucleocapsid nuclear polyhedrosis virus (AgMNPV) was sequenced and compared. This region, which is known to have deletions and duplications in AgMNPV variants, was shown to be formed of units of a 127 bp tandemly repeated sequence containing two 30 bp imperfect palindromes. Southern blot experiments showed that the PstI-T fragment contained one of the four homologous regions mapped interspersed in the AgMNPV genome. The comparison of the nucleotide sequences of the two variants, AgMNPV-2D and AgMNPV-D7, showed that the difference between these two variants in one of these regions (hr4) was caused by the addition or elimination of 381 bp corresponding exactly to three of the 127 bp repeated sequences. An analysis of the sequence showed homology to the homologous regions (hr) of other baculoviruses. The sequence upstream of the repetitive region contained a sequence homologous to the N-terminal portion of the Autographa californica MNPV and Choristoneura fumiferana MNPV p74 gene.

Baculovirus--insect cell interactions

Baculovirus interactions with host cells range from the physical interactions that occur during viral binding and entry, to the complex and subtle mechanisms that regulate host gene expression and modify and regulate cellular and organismal physiology and defenses. Fundamental studies of baculovirus biochemistry and molecular biology have yielded many interesting and important discoveries on the mechanisms of these virus-host interactions. Information from such studies has also resulted in exciting new strategies for environmentally sound insect pest control, and in the development and improvement of a valuable eukaryotic expression vector system. In addition a number of important and valuable model biological systems have emerged from studies of baculoviruses. These include robust systems for studies of eukaryotic transcription, viral DNA replication, membrane fusion, and apoptosis. Because functions have been identified for only a small number of baculovirus genes, we can expect many exciting new discoveries in the future and an unfolding of the complex and intricate relationship between baculoviruses and insect cells.

Expression of PE38 and IE2, viral members of the C3HC4 finger family, during baculovirus infection: PE38 and IE2 localize to distinct nuclear regions

The pe38 gene of Autographa californica nuclear polyhedrosis virus represents one of the major early transcripts after viral infection. The function of the pe38 protein, which contains a C3HC4 zinc finger motif, is still not understood. We have raised polyclonal antiserum against the pe38 protein, PE38, produced in bacteria to investigate pe38 expression in the course of infection. A approximately 38-kDa polypeptide is first detectable at 2 h postinfection and decreases rapidly after 24 h. During the late phases of infection, a smaller protein of approximately 20 kDa which cross-reacts with the PE38-specific antiserum is visible at a constant level until 120 h postinfection. Since the pe38 gene shares a divergent promoter unit with the ie2 gene (formerly IEN), we have compared the expressions of the two genes. Polyclonal antibodies were raised against the bacterially expressed ie2 protein. The temporal expression pattern of the approximately 49-kDa ie2 protein is comparable to that of the approximately 38-kDa pe38 protein. Furthermore, both proteins are present in the nuclear fraction of A. californica nuclear polyhedrosis virus-infected Spodoptera frugiperda cells, but the approximately 38-kDa pe38 protein is also detectable in the cytoplasm while the smaller protein of approximately 20 kDa is exclusively present in the cytoplasmic fraction. Immunofluorescence analysis reveals that PE38 and IE2 localize to distinct regions within the nucleus mainly detected after transfection of pe38- and ie2-expressing constructs.

Transcriptional enhancer activity of hr5 requires dual-palindrome half sites that mediate binding of a dimeric form of the baculovirus transregulator IE1

The hr5 enhancer element stimulates early viral transcription and may function as an origin of DNA replication for Autographa californica nuclear polyhedrosis virus (AcMNPV). The smallest functional unit of hr5 is a 28-bp repeat consisting of an imperfect palindrome (28-mer). To identify essential sequences and examine the molecular basis of hr5 activity, the effects of site-directed mutations on transcriptional enhancement by the 28-mer and binding of the AcMNPV transregulator IE1 were investigated. In transfection assays and infections with AcMNPV recombinants, activation of a basal viral promoter required sequences within both halves of the 28-mer. Basal promoter activation also required a critical spacing between these half sites. Mobility shift assays indicated that hr5 probes containing a single 28-mer were bound by in vitro-synthesized IE1. Competition assays using DNA fragments that contained mutated 28-mers demonstrated that both half sites were required for optimal binding of IE1. Similar assays using mutated 28-mer DNAs and nuclear extracts indicated that the relative affinity with which AcMNPV infection-specific proteins bound to the 28-mer was similar to that of in vitro-synthesized IE1. By using a combination of DNA binding and antibody supershift assays, it was demonstrated that IE1 binds to the 28-mer as a dimer. Collectively, these findings support a model in which symmetrical IE1 binding and simultaneous interaction with each half site are required for IE1-mediated transcriptional enhancement by hr5. Thus, sequence-specific binding may be one of the mechanisms by which IE1 directly or indirectly transregulates baculovirus gene expression.

The baculovirus transactivator IE1 binds to viral enhancer elements in the absence of insect cell factors

The transregulatory IE1 protein of Autographa californica nuclear polyhedrosis virus binds to the viral enhancer element hr5. To test whether IE1 binds independently of host cell factors, IE1 was translated in rabbit reticulocyte extracts and tested for DNA binding activity by an electrophoretic mobility shift assay. Complexes with the hr5 probe were detected with translation reaction mixtures primed with ie1 RNA but not with control translation reaction mixtures. However, the DNA-protein complexes formed with IE1 translated in vitro migrated more slowly than complexes formed with IE1 that was transiently expressed in insect cells. Phosphatase treatment of the translation reactions resulted in an increase in the mobility of the DNA-protein complexes, suggesting that hyperphosphorylation was responsible for the altered migration. To further verify that IE1 was capable of binding DNA in the absence of host cell factors, an N-terminal truncation of IE1 was synthesized in vitro, and shown to interact with hr5. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of IE1 translated in vitro revealed that the mobility of the protein was heterogeneous. This pattern was altered by translation in the presence of an oligonucleotide corresponding to the IE1 specific binding site but was not affected by translation in the presence of a nonspecific DNA. These results suggest that binding of IE1 to DNA causes a conformational change in the protein that alters the accessibility of IE1 to protein kinases.

The lef-3 gene of Autographa californica nuclear polyhedrosis virus encodes a single-stranded DNA-binding protein

The Autographa californica nuclear polyhedrosis virus (AcNPV) replicates in the nuclei of infected cells and encodes several proteins required for viral DNA replication. As a first step in the functional characterization of viral replication proteins, we purified a single-stranded DNA-binding protein (SSB) from AcNPV-infected insect cells. Nuclear extracts were chromatographed on single-stranded DNA agarose columns. An abundant protein with an apparent molecular weight of 43,000 was eluted from the columns at 0.9 to 1.0 M NaCl. This protein was not evident in extracts prepared from control cells, suggesting that the SSB was encoded by the virus. SSB bound to single-stranded DNA in solution, and binding was nonspecific with respect to base sequence, as single-stranded vector DNA competed as efficiently as single-stranded DNA containing the AcNPV origin of DNA replication. Competition binding experiments indicated that SSB showed a preference for single-stranded DNA over double-stranded DNA. To determine whether SSB was encoded by the lef-3 gene of AcNPV, the lef-3 open reading frame was cloned under the control of the bacteriophage T7 promoter. Immunochemical analyses indicated that LEF-3 produced in bacteria or in rabbit reticulocyte lysates specifically reacted with antiserum produced by immunization with purified SSB. Immunoblot analyses of infected cell extracts revealed that SSB/LEF-3 was detected by 4 h postinfection and accumulated through 48 h postinfection.

The roles of eighteen baculovirus late expression factor genes in transcription and DNA replication

A set of 18 plasmid subclones of the Autographa californica nuclear polyhedrosis virus genome supports expression from a late viral promoter in transient expression assays (J. W. Todd, A. L. Passarelli, and L. K. Miller, J. Virol. 69:968-974, 1995). Using this set of plasmids, we have assigned a role for each of the 18 genes required for optimal late gene expression with respect to its involvement at the levels of transcription, translation, and/or DNA replication. RNase protection analyses demonstrated that all of the known late expression factor genes (lefs) affected the steady-state level of reporter gene RNA. Thus, none of the lefs appeared to be specifically involved in translation. A subset of the lefs supported plasmid replication; ie-1, lef-1, lef-2, lef-3, p143, and p35 were essential for plasmid replication, while ie-n, lef-7, and dnapol had stimulatory effects. The predicted sequence of lef-7 suggests that it is a homolog of herpesvirus single-stranded DNA-binding protein (UL29). The role of p35 in plasmid replication appears to be suppression of apoptosis, because p35 could be functionally replaced in the replication assay by either Cp-iap or Op-iap, two heterologous baculovirus genes which suppress apoptosis by a mechanism which appears to differ from that of p35. Thus, one or more of the replication-related lefs or the process of plasmid replication appears to induce cellular apoptosis. Our results indicate that the remaining lefs, lefs 4 through 11, p47, and 39K (pp31), function either at the level of transcription or at that of mRNA stabilization.

Identification of genes involved in DNA replication of the Autographa californica baculovirus

By use of a transient replication assay, nine genes involved in DNA replication were identified in the genome of the Autographa californica baculovirus. Six genes encoding helicase, DNA polymerase, IE-1, LEF-1, LEF-2, and LEF-3 are essential for DNA replication while three genes encoding P35, IE-2, and PE38 stimulate DNA replication. No stimulation by the AcMNPV pcna gene, encoding a protein with sequence homology to proliferating-cell nuclear antigen, was observed. A pattern of amino acids found in a number of single-stranded-DNA-binding proteins was identified in the carboxyl-terminal region of IE-1.