Novel regulatory properties of the IE1 and IE0 transactivators encoded by the baculovirus Autographa californica multicapsid nuclear polyhedrosis virus

A Tale of Two Transcriptomic Responses in Agricultural Pests via Host Defenses and Viral Replication

Autographa californica Multiple Nucleopolyhedrovirus (AcMNPV) is a baculovirus that causes systemic infections in many arthropod pests. The specific molecular processes underlying the biocidal activity of AcMNPV on its insect hosts are largely unknown. We describe the transcriptional responses in two major pests, Spodoptera frugiperda (fall armyworm) and Trichoplusia ni (cabbage looper), to determine the host-pathogen responses during systemic infection, concurrently with the viral response to the host. We assembled species-specific transcriptomes of the hemolymph to identify host transcriptional responses during systemic infection and assessed the viral transcript abundance in infected hemolymph from both species. We found transcriptional suppression of chitin metabolism and tracheal development in infected hosts. Synergistic transcriptional support was observed to suggest suppression of immune responses and induction of oxidative stress indicating disease progression in the host. The entire AcMNPV core genome was expressed in the infected host hemolymph with a proportional high abundance detected for viral transcripts associated with replication, structure, and movement. Interestingly, several of the host genes that were targeted by AcMNPV as revealed by our study are also targets of chemical insecticides currently used commercially to control arthropod pests. Our results reveal an extensive overlap between biological processes represented by transcriptional responses in both hosts, as well as convergence on highly abundant viral genes expressed in the two hosts, providing an overview of the host-pathogen transcriptomic landscape during systemic infection.

Transcriptome of Cydia pomonella granulovirus in susceptible and type I resistant codling moth larvae

The baculovirus Cydia pomonella granulovirus (CpGV) is a biocontrol agent used worldwide against the codling moth (CM), Cydia pomonella L., a severe pest in organic and integrated pome fruit production. Its successful application is increasingly challenged by the occurrence of CM populations resistant to commercial CpGV products. Whereas three types (I-III) of CpGV resistance have been identified, type I resistance compromising the efficacy of CpGV-M, the so-called Mexican isolate of CpGV, is assumed to be the most widely distributed resistance type in Central Europe. Despite the wide use of CpGV products as biocontrol agents, little information is available on gene-expression levels in CM larvae. In this study, the in vivo transcriptome of CpGV-M infecting susceptible (CpS) and resistant (CpRR1) CM larvae was analysed at 24, 48, 72, 96 and 120 hours post infection in the midgut and fat body tissue by using a newly developed microarray covering all ORFs of the CpGV genome. According to their transcript abundance, the CpGV genes were grouped into four temporal clusters to which groups of known and unknown function could be assigned. In addition, sets of genes differentially expressed in the midgut and fat body were found in infected susceptible CpS larvae. For the resistant CpRR1 larvae treated with CpGV-M, viral entry in midgut cells could be confirmed from onset but a significantly reduced gene expression, indicating that type I resistance is associated with a block of viral gene transcription and replication.

Effective production of oligomeric membrane proteins by EarlyBac-insect cell system

Despite major advances in methodologies for membrane protein production over the last two decades, there remain challenging protein complexes that are technically difficult to yield by conventional recombinant expression methods. A large number of these proteins are multimeric membrane proteins from eukaryotic species, which are required to pass through stringent quality control mechanisms of host cells for proper folding and complex assembly. Here, we describe the development procedure to improve the production efficiency of multi-oligomeric membrane protein complexes in insect cells and recombinant baculovirus, which involves screening of promoters, enhancers, and untranslated regions for expression levels, using calcium homeostasis modulator (CALHM) and N-methyl-d-aspartate receptor (NMDAR) proteins as examples. We demonstrate that our insect cell expression strategy is effective in expression of both multi-homomeric CALHM proteins and multi-heteromeric NMDARs.

The interaction between baculoviruses and their insect hosts

Baculoviruses are double-stranded circular DNA viruses that infect arthropods via the midgut. Because of their superiority as eukaryotic expression systems and their importance as biopesticides, extensive research on the functions of baculovirus genes as well as on the host response to baculovirus infection has been carried out, including transcriptomic and proteomic analyses of the midgut. The morphological and cellular changes caused by baculovirus infection are also important to better understand the infection pathway. Thanks to these previous studies, we now have a clearer picture of the mechanisms of action of the virus and of host immunity. In this paper, we systematically reviewed studies on the interaction between baculoviruses and their insect hosts. By better understanding these interactions, baculoviruses can be developed for use as more efficient biopesticides to improve agricultural development in the future.

Screening and optimization of an efficient Bombyx mori nucleopolyhedrovirus inducible promoter

Pathogen-inducible promoters have been studied extensively and widely used in resistance breeding and gene therapy. However, few reports have been published that explore the efficacy of Bombyx mori nucleopolyhedrovirus (BmNPV)-inducible promoters in antiviral research in the Bombyx mori (Lepidoptera). Here, we screened BmNPV promoters (VP1054, P33, Bm21, Bm122, 39K, P143, and P6.9) and found that the 39K promoter had the highest BmNPV-induced transcriptional activity by dual-luciferase reporter assays system. By 5' truncation analysis, two regions of 39K promoter were critical for optimal virus-inducible activity, indicated that they could serve as a candidate to produce synthetic pathogen-induced promoters. Furthermore, we enhanced the virus-inducible activity of BmNPV 39K promoter using a hybrid enhancer comprising hr3 and polh-up (designated as HP39K). Finally, we showed that RNAi regulated by HP39K promoter could significantly inhibit the proliferation of BmNPV in silkworm cells. Taken together, our results have practical value in antiviral research of silkworm and baculovirus expression system.

Introduction to Baculovirus Molecular Biology

The development of baculovirus expression vector systems has accompanied a rapid expansion of our knowledge about the genes, their function and regulation in insect cells. Classification of these viruses has also been refined as we learn more about differences in gene content between isolates, how this affects virus structure and their replication in insect larvae. Baculovirus gene expression occurs in an ordered cascade, regulated by early, late and very late gene promoters. There is now a detailed knowledge of these promoter elements and how they interact first with host cell-encoded RNA polymerases and later with virus-encoded enzymes. The composition of this virus RNA polymerase is known. The virus replication process culminates in the very high level expression of both polyhedrin and p10 gene products in the latter stages of infection. It has also been realized that the insect host cell has innate defenses against baculoviruses in the form of an apoptotic response to virus invasion. Baculoviruses counter this by encoding apoptotic-suppressors, which also appear to have a role in determining the host range of the virus. Also of importance to our understanding of baculovirus expression systems is how the virus can accumulate mutations within genes that affect recombinant protein yield in cell culture. The summary in this chapter is not exhaustive, but should provide a good preparation to those wishing to use this highly successful gene expression system.

Tightly regulated expression of Autographa californica multicapsid nucleopolyhedrovirus immediate early genes emerges from their interactions and possible collective behaviors

To infect their hosts, DNA viruses must successfully initiate the expression of viral genes that control subsequent viral gene expression and manipulate the host environment. Viral genes that are immediately expressed upon infection play critical roles in the early infection process. In this study, we investigated the expression and regulation of five canonical regulatory immediate-early (IE) genes of Autographa californica multicapsid nucleopolyhedrovirus: ie0, ie1, ie2, me53, and pe38. A systematic transient gene-expression analysis revealed that these IE genes are generally transactivators, suggesting the existence of a highly interactive regulatory network. A genetic analysis using gene knockout viruses demonstrated that the expression of these IE genes was tolerant to the single deletions of activator IE genes in the early stage of infection. A network graph analysis on the regulatory relationships observed in the transient expression analysis suggested that the robustness of IE gene expression is due to the organization of the IE gene regulatory network and how each IE gene is activated. However, some regulatory relationships detected by the genetic analysis were contradictory to those observed in the transient expression analysis, especially for IE0-mediated regulation. Statistical modeling, combined with genetic analysis using knockout alleles for ie0 and ie1, showed that the repressor function of ie0 was due to the interaction between ie0 and ie1, not ie0 itself. Taken together, these systematic approaches provided insight into the topology and nature of the IE gene regulatory network.

The promoter of the white spot syndrome virus immediate-early gene WSSV108 is activated by the cellular KLF transcription factor

A series of deletion and mutation assays of the white spot syndrome virus (WSSV) immediate-early gene WSSV108 promoter showed that a Krüppel-like factor (KLF) binding site located from -504 to -495 (relative to the transcription start site) is important for the overall level of WSSV108 promoter activity. Electrophoretic mobility shift assays further showed that overexpressed recombinant Penaeus monodon KLF (rPmKLF) formed a specific protein-DNA complex with the (32)P-labeled KLF binding site of the WSSV108 promoter, and that higher levels of Litopenaeus vannamei KLF (LvKLF) were expressed in WSSV-infected shrimp. A transactivation assay indicated that the WSSV108 promoter was strongly activated by rPmKLF in a dose-dependent manner. Lastly, we found that specific silencing of LvKLF expression in vivo by dsRNA injection dramatically reduced both WSSV108 expression and WSSV replication. We conclude that shrimp KLF is important for WSSV genome replication and gene expression, and that it binds to the WSSV108 promoter to enhance the expression of this immediate-early gene.

Baculovirus resistance in codling moth is virus isolate-dependent and the consequence of a mutation in viral gene pe38

The baculovirus Cydia pomonella granulovirus (CpGV) is widely applied as a biocontrol agent of codling moth. After field resistance of codling moth populations had been observed against the commercially used Mexican (M) isolate of CpGV, infection experiments of larvae of the resistant codling moth strain CpRR1 showed that several other naturally occurring CpGV isolates (I12, S, E2, and I07) from different geographic origins are still infectious to resistant CpRR1. Whole-genome sequencing and phylogenetic analyses of these geographic CpGV variants revealed that their genomes share only a single common difference from that of CpGV-M, which is a mutation coding for a repeat of 24 nucleotides within the gene pe38; this mutation results in an additional repeat of eight amino acids that appears to be inserted to PE38 of CpGV-M only. Deletion of pe38 from CpGV-M totally abolished virus infection in codling moth cells and larvae, demonstrating that it is an essential gene. When the CpGV-M deletion mutant was repaired with pe38 from isolate CpGV-S, which originated from the commercial product Virosoft and is infectious for the resistant codling moth strain CpRR1, the repaired CpGV-M mutant was found to be fully infectious for CpRR1. Repair using pe38 from CpGV-M restored infectivity for the virus in sensitive codling moth strains, but not in CpRR1. Therefore, we conclude that CpGV resistance of codling moth is directed to CpGV-M but not to other virus isolates. The viral gene pe38 is not only essential for the infectivity of CpGV but it is also the key factor in overcoming CpGV resistance in codling moth.

A Baculovirus immediate-early gene, ie1, promoter drives efficient expression of a transgene in both Drosophila melanogaster and Bombyx mori

Many promoters have been used to drive expression of heterologous transgenes in insects. One major obstacle in the study of non-model insects is the dearth of useful promoters for analysis of gene function. Here, we investigated whether the promoter of the immediate-early gene, ie1, from the Bombyx mori nucleopolyhedrovirus (BmNPV) could be used to drive efficient transgene expression in a wide variety of insects. We used a piggyBac-based vector with a 3xP3-DsRed transformation marker to generate a reporter construct; this construct was used to determine the expression patterns driven by the BmNPV ie1 promoter; we performed a detailed investigation of the promoter in transgene expression pattern in Drosophila melanogaster and in B. mori. Drosophila and Bombyx belong to different insect orders (Diptera and Lepidoptera, respectively); however, and to our surprise, ie1 promoter-driven expression was evident in several tissues (e.g., prothoracic gland, midgut, and tracheole) in both insects. Furthermore, in both species, the ie1 promoter drove expression of the reporter gene from a relatively early embryonic stage, and strong ubiquitous ie1 promoter-driven expression continued throughout the larval, pupal, and adult stages by surface observation. Therefore, we suggest that the ie1 promoter can be used as an efficient expression driver in a diverse range of insect species.

Use of bacterial artificial chromosomes in baculovirus research and recombinant protein expression: current trends and future perspectives

The baculovirus expression system is one of the most successful and widely used eukaryotic protein expression methods. This short review will summarise the role of bacterial artificial chromosomes (BACS) as an enabling technology for the modification of the virus genome. For many years baculovirus genomes have been maintained in E. coli as bacterial artificial chromosomes, and foreign genes have been inserted using a transposition-based system. However, with recent advances in molecular biology techniques, particularly targeting reverse engineering of the baculovirus genome by recombineering, new frontiers in protein expression are being addressed. In particular, BACs have facilitated the propagation of disabled virus genomes that allow high throughput protein expression. Furthermore, improvement in the selection of recombinant viral genomes inserted into BACS has enabled the expression of multiprotein complexes by iterative recombineering of the baculovirus genome.

Conserved structural motifs at the C-terminus of baculovirus protein IE0 are important for its functions in transactivation and supporting hr5-mediated DNA replication

IE0 and IE1 are transactivator proteins of the most studied baculovirus, the Autographa californica multiple nucleopolyhedrovirus (AcMNPV). IE0 is a 72.6 kDa protein identical to IE1 with the exception of its 54 N-terminal amino acid residues. To gain some insight about important structural motifs of IE0, we expressed the protein and C‑terminal mutants of it under the control of the Drosophila heat shock promoter and studied the transactivation and replication functions of the transiently expressed proteins. IE0 was able to promote replication of a plasmid bearing the hr5 origin of replication of AcMNPV in transient transfections with a battery of eight plasmids expressing the AcMNPV genes dnapol, helicase, lef-1, lef-2, lef-3, p35, ie-2 and lef-7. IE0 transactivated expression of the baculovirus 39K promoter. Both functions of replication and transactivation were lost after introduction of selected mutations at the basic domain II and helix-loop-helix conserved structural motifs in the C-terminus of the protein. These IE0 mutants were unable to translocate to the cell nucleus. Our results point out the important role of some structural conserved motifs to the proper functioning of IE0.

The protamine-like DNA-binding protein P6.9 epigenetically up-regulates Autographa californica multiple nucleopolyhedrovirus gene transcription in the late infection phase

Protamines are a group of highly basic proteins first discovered in spermatozoon that allow for denser packaging of DNA than histones and will result in down-regulation of gene transcription[1]. It is well recognized that the Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) encodes P6.9, a protamine-like protein that forms the viral subnucleosome through binding to the viral genome[29]. Previous research demonstrates that P6.9 is essential for viral nucleocapsid assembly, while it has no influence on viral genome replication[31]. In the present study, the role of P6.9 in viral gene transcription regulation is characterized. In contrast to protamines or other protamine-like proteins that usually down-regulate gene transcription, P6.9 appears to up-regulate viral gene transcription at 12-24 hours post infection (hpi), whereas it is non-essential for the basal level of viral gene transcription. Fluorescence microscopy reveals the P6.9's co-localization with DNA is temporally and spatially synchronized with P6.9's impact on viral gene transcription, indicating the P6.9-DNA association contributes to transcription regulation. Chromatin fractionation assay further reveals an unexpected co-existence of P6.9 and host RNA polymerase II in the same transcriptionally active chromatin fraction at 24 hpi, which may probably contribute to viral gene transcription up-regulation in the late infection phase.

Localization and functional analysis of SeMNPV IE1 in mammalian cells

In this paper, the function of the ie1 gene from baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV), belonging to group II nucleopolyhedrovirus, was studied in mammalian cells. We amplified the SeMNPV ie1 gene and expressed it by fusing to the C terminal of enhanced GFP protein in HEK 293 cells. Confocal microscopy revealed that the IE1-GFP fusion protein was localized in the nucleus of the mammalian cells. The promoter sequences of AcMNPV gp64, SeMNPV F protein and Drosophila hsp70 were also analyzed, to further study the function of SeMNPV IE1. The results showed that, in the absence of the hr sequence, IE1 improved the expression of the F promoter but didn't influence the gp64 promoter significantly, but IE1 moderately stimulated the hsp70 promoter.

Comparative transient expression assay analysis of hycu-hr6- and IE1-dependent regulation of baculovirus gp64 early promoters in three insect cell lines

We previously demonstrated that the Hyphantria cunea multicapsid nucleopolyhedrovirus (HycuMNPV) gp64 gene (hycu-gp64) is uniquely localized on the viral genome with a large homologous region of 1582bp, hycu-hr6, immediately upstream of the hycu-gp64 gene. In the present study, we compared the regulation of gp64 early promoters from HycuMNPV, Autographa californica multicapsid NPV (AcMNPV) and Bombyx mori NPV (BmNPV) by cis-acting hycu-hr6 and trans-acting IE1s in three cell lines (Spodoptera frugiperda Sf9, Bombyx mori BM-N and Spilosoma imparilis SpIm). A transient expression assay with plasmids harboring a reporter luciferase gene demonstrated that the gp64 early promoters are positively regulated by hycu-hr6, independent of virus and cell types. In contrast, gp64 early promoters were regulated positively or negatively by trans-acting IE1s, in a cell- and virus-type dependent manner, indicating that cellular factors, as well as viral factors, are responsible for IE1-dependent regulation of gp64 early promoters. However, hycu-gp64 early promoter activity was consistently suppressed by HycuMNPV IE1 (Hycu-IE1), irrespective of the cell lines used. Analysis of the hycu-gp64 early promoter region revealed two novel sequence elements that were involved in Hycu-IE1-dependent negative regulation of the hycu-gp64 early promoter. These two novel regulatory sequence elements could compensate for each other but could not be substituted with AcMNPV IE1 binding motif (Ac-IBM). These results suggest that IE1 regulates gp64 early promoters to produce the proper amount of GP64 protein, depending upon NPV-insect cell systems.

Baculoviruses deficient in ie1 gene function abrogate viral gene expression in transduced mammalian cells

One of the newest niches for baculoviruses-based technologies is their use as vectors for mammalian cell transduction and gene therapy applications. However, an outstanding safety issue related to such use is the residual expression of viral genes in infected mammalian cells. Here we show that infectious baculoviruses lacking the major transcriptional regulator, IE1, can be produced in insect host cells stably transformed with IE1 expression constructs lacking targets of homologous recombination that could promote the generation of wt-like revertants. Such ie1-deficient baculoviruses are unable to direct viral gene transcription to any appreciable degree and do not replicate in normal insect host cells. Most importantly, the residual viral gene expression, which occurs in mammalian cells infected with wt baculoviruses is reduced 10 to 100 fold in cells infected with ie1-deficient baculoviruses. Thus, ie1-deficient baculoviruses offer enhanced safety features to baculovirus-based vector systems destined for use in gene therapy applications.

Changes in gene expression in the permissive larval host lightbrown apple moth (Epiphyas postvittana, Tortricidae) in response to EppoNPV (Baculoviridae) infection

Host cell and virus gene expression were measured five days after per os inoculation of 3rd instar lightbrown apple moth (LBAM) larvae with the Epiphyas postvittana nucleopolyhedrovirus (EppoNPV). Microarray analysis identified 84 insect genes that were up-regulated and 18 genes that were down-regulated in virus-infected larvae compared with uninfected larvae. From the 134 viral open reading frames represented on the microarray, 81 genes showed strong expression. Of the 38 functionally identifiable regulated insect genes, 23 coded for proteins that have roles in one of five processes; regulation of transcription and translation, induction of apoptosis, and maintenance of both juvenility and actin cytoskeletal integrity. Of the 34 functionally identifiable viral genes that were most strongly expressed, 12 had functions associated with these five processes, as did a further seven viral genes which were expressed at slightly lower levels. A survey of the LBAM-expressed sequence tag library identified further genes involved in these processes. In total, 135 insect genes and 38 viral genes were analysed by quantitative polymerase chain reaction. Twenty-one insect genes were strongly up-regulated and 31 genes strongly down-regulated. All 38 viral genes examined were highly expressed. These data suggest that induction of apoptosis and regulation of juvenility are the major 'battlegrounds' between virus and insect, with the majority of changes observed representing viral control of insect gene expression. Transcription and translational effects seem to be exerted largely through modulation of mRNA and protein degradation. Examples of attempts by the insect to repel the infection via changes in gene expression within these same processes were, however, also noted. The data also showed the extent to which viral transcription dominated in the infected insects at five days post inoculation.

Analysis of promoter activity of selected Cotesia plutellae bracovirus genes

In a previous study, we cloned 27 discrete genome segments of Cotesia plutellae bracovirus (CpBV) and provided the complete nucleotide sequences and annotation. Seven putative coding regions were predicted from one of the largest segments, CpBV-S30. The activity of promoters associated with six predicted ORFs from this segment were investigated using both transient and baculovirus expression assays with enhanced green fluorescent protein as a reporter gene. CpBV promoters showed activity earlier than the polyhedrin promoter and the activity of some of these promoters was superior to that of the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ie-1 promoter in the baculovirus expression assays. The promoter of ORF3004 showed the highest level of activity in insect cells, exhibiting 24 % of the activity obtained with the AcMNPV polyhedrin promoter in Sf9 cells. In Spodoptera exigua larvae, the ORF3006 promoter showed the highest activity, with about 35 % of the activity measured with the polyhedrin promoter. In addition, analysis of the ORF3006 promoter revealed that the region between -382 and -422 from the translation start point was critical for activity of this promoter. These results suggest that the CpBV-S30 promoters characterized here could be useful tools in a variety of biotechnological applications, such as gene expression analyses and insecticide development.

AcMNPV AC16 (DA26, BV/ODV-E26) regulates the levels of IE0 and IE1 and binds to both proteins via a domain located within the acidic transcriptional activation domain

IE0 and IE1 are the primary viral regulatory proteins of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) involved in the transactivation of early genes, stimulation of late gene expression, and viral DNA replication. The protein interactions required for IE0 or IE1 to achieve these varied roles are not well defined, so to identify proteins that interact with IE0 and IE1, tandem affinity purification (TAP) and LC-MS/MS was used. Analysis of purified proteins identified AC16 (DA26, BV/ODV-E26) from TAP tagged IE0 virus infected Sf9 cells. Co-immunoprecipitation confirmed that AC16 interacts with both IE0 and IE1 and yeast 2-hybrid analysis mapped the domain required for interaction with AC16. Mutation of the AC16 binding domain enhanced BV production by viruses expressing only IE0 but had no effect if only IE1 is expressed. An ac16 deletion virus was constructed and was shown not to affect the temporal expression of IE0 and IE1; however the relative level of IE0 to IE1 was significantly increased.

Transactivator IE1 is required for baculovirus early replication events that trigger apoptosis in permissive and nonpermissive cells

Immediate early viral protein IE1 is a potent transcriptional activator encoded by baculoviruses. Although the requirement of IE1 for multiplication of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) is well established, the functional roles of IE1 during infection are unclear. Here, we used RNA interference to ablate IE1, plus its splice variant IE0, and thereby define in vivo activities of these early proteins, including gene-specific regulation and induction of host cell apoptosis. Confirming an essential replicative role, simultaneous ablation of IE1 and IE0 by gene-specific double-stranded RNAs inhibited AcMNPV late gene expression, reduced yields of budded virus by more than 1,000-fold, and blocked production of occluded virus particles. Depletion of IE1 and IE0 had no effect on early expression of the envelope fusion protein gene gp64 but abolished early expression of the caspase inhibitor gene p35, which is required for prevention of virus-induced apoptosis. Thus, IE1 is a positive, gene-specific transactivator. Whereas an AcMNPV p35 deletion mutant caused widespread apoptosis in permissive Spodoptera frugiperda cells, ablation of IE1 and IE0 prevented this apoptosis. Silencing of ie-1 also prevented AcMNPV-induced apoptosis in nonpermissive Drosophila melanogaster cells. Thus, de novo synthesis of IE1 is required for virus-induced apoptosis. We concluded that IE1 causes apoptosis directly or contributes indirectly by promoting virus replication events that subsequently trigger cell death. This study reveals that IE1 is a gene-selective transcriptional activator which is required not only for expedition of virus multiplication but also for blocking of its own proapoptotic activity by upregulation of baculovirus apoptotic suppressors.

Serine/threonine kinase (pk-1) is a component of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) very late gene transcription complex and it phosphorylates a 102 kDa polypeptide of the complex

The baculovirus gene, protein kinase-I (pk-1) encodes a serine/threonine kinase that is essential for very late gene expression. Late and very late genes of the baculoviruses are transcribed by an alpha-amanitin resistant RNA polymerase. The very late gene promoter transcription initiation complex was isolated from nuclei of Autographa californica multiple nucleopolyhedrovirus (AcMNPV)-infected Sf9 cells by DNA affinity purification and found to contain 4 major polypeptides of sizes approximately 102, 38, 32, and 18 kDa. The 32 kDa polypeptide was immunoreactive to AcMNPV anti-pk-1 antibody and phosphorylated the 102 kDa polypeptide, earlier reported as late gene expression factor LEF-8. Electrophoretic mobility shift assays with anti-pk-1 antibody indicated the binding of promoter DNA with recombinant AcMNPV-pk-1 and transcription initiation complex proteins. All these results suggested AcMNPV-pk-1 to be a component of the viral very late gene transcription initiation complex.

Ecdysone response element in a baculovirus immediate-early gene, ie1, promoter

A computer-assisted analysis identified tentative target sequences for regulatory proteins including ecdysone-inducible factors such as BmFTZ-F1 and Broad-Complex Z4 (BR-C Z4) in the ie1 promoter of BmNPV. A transient expression experiment using BmN cells and a series of truncated ie1 promoter constructs demonstrated that the activity of the ie1 promoter responded to alpha-ecdysone and 20-hydroxyecdysone, which required a tridecameric nucleotide stretch (ie1EcRE, 5'-GTGTTATCGACCT-3') homologous to the ecdysone response element reported for Drosophila (DmEcRE). RT-PCR demonstrated the expression of BmEcR and BmUSP, which are required as ecdysone-specific activators for EcRE-mediated activation, in BmN cells. Furthermore, the ie1 EcRE-mediated response was confirmed by using a recombinant BmNPV possessing a luciferase gene under the control of the ie1 promoter with or without ie1 EcRE. This is the first report of an ecdysone response element in a baculoviral gene promoter. These results also suggested that the regulation of the ie1 by ecdysone may militate viral replication at least under certain conditions during natural infections in vivo.

Introduction to baculovirus molecular biology

The development of baculovirus expression vector systems has accompanied a rapid expansion of our knowledge about the genes, their function, and regulation in insect cells. Classification of these viruses has also been refined as we learn more about differences in gene content between isolates, how this affects virus structure, and their replication in insect larvae. Baculovirus gene expression occurs in an ordered cascade, regulated by early, late, and very late gene promoters. There is now a detailed knowledge of these promoter elements and how they interact first with host cell-encoded RNA polymerases and later with virus-encoded enzymes. The composition of this virus RNA polymerase is known. The virus replication process culminates in the very high level expression of both polyhedrin and p10 gene products in the latter stages of infection. It has also been realized that the insect host cell has innate defenses against baculoviruses in the form of an apoptotic response to virus invasion. Baculoviruses counter this by encoding apoptotic-suppressors, which also appear to have a role in determining the host range of the virus. Also of importance to our understanding of baculovirus expression systems is how the virus can accumulate mutations within genes that affect recombinant protein yield in cell culture. The summary in this chapter is not exhaustive, but should provide a good preparation to those wishing to use this highly successful gene expression system.

Functional and structural characterisation of AgMNPV ie1

We have located and cloned the Anticarsia gemmatalis multicapsid nucleopolyhedrovirus isolate 2D (AgMNPV-2D) genomic DNA fragment containing the immediate early 1 ORF and its flanking regions. Computer assisted analysis of the complete ie1 locus nucleotide sequence information was used to locate regulatory signals in the upstream region and conserved nucleotide and amino acid sequences. Comparative studies led to the identification of several characteristic protein motifs and to the conclusion that AgMNPV-2D is more closely related to Choristoneura fumiferana defective NPV than to other Group I nucleopolyhedrovirus. We have also shown that the AgMNPV IE1 protein was able to transactivate an early Autographa californica MNPV promoter and its own promoter in transient expression assays. In order to investigate the biological functionality of the ie1 promoter, the ie1 upstream activating region (UAR) was molecularly dissected and cloned upstream of the E. coli lacZ ORF. The results obtained, after transfection of UFL-AG-286 insect cells, leading us to find that the -492 and -357 versions contains sequence motifs important for the level of the lacZ reporter gene expression.

Serine/Threonine kinase dependent transcription from the polyhedrin promoter of SpltNPV-I

Polyhedrin (polh) and p10 are the two hyper-expressed very late genes of nucleopolyhedroviruses. Alpha amanitin resistant transcription from Spodoptera litura nucleopolyhedrovirus (SpltNPV-I) polyhedrin promoter was observed with virus infected nuclear extract of NIV-HA-197 cells but not with that from uninfected nuclear extract. Anti-protein kinase-1 (pk1) antibody inhibited the transcription and the inhibition reversed on addition of pk1, however, pk1 mutant protein, K50M having no phosphorylation activity did not overcome the transcription inhibition. Chromatin immuno-precipitation assays with viral anti-pk1 antibody showed the interaction of pk1 with the polh while electrophoretic mobility shift assays indicated the strong binding affinity (K(d) approximately 5.5x10(-11)) of purified pk1 with the polh promoter. These results suggested that the viral coded pk1 acts as a transcription factor in transcribing baculovirus very late genes.

The AcMNPV pp31 gene is not essential for productive AcMNPV replication or late gene transcription but appears to increase levels of most viral transcripts

The pp31 gene of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) encodes a phosphorylated DNA binding protein that associates with virogenic stroma in the nuclei of infected cells. Prior studies of pp31 by transient late expression assays suggested that pp31 may play an important role in transcription of AcMNPV late genes [Todd, J. W., Passarelli, A. L., and Miller, L. K. (1995). Eighteen baculovirus genes, including lef-11, p35, 39K, and p47, support late gene expression. J. Virol. 69, 968-974] although genetic studies of the closely related BmNPV pp31 gene suggested that pp31 may be dispensable [Gomi, S., Zhou, C. E., Yih, W., Majima, K., and Maeda, S. (1997). Deletion analysis of four of eighteen late gene expression factor gene homologues of the baculovirus, BmNPV. Virology 230 (1), 35-47]. In the current study, we examined the role of the pp31 gene in the context of the AcMNPV genome during infection. We used a BACmid-based system to generate a pp31 knockout in the AcMNPV genome. The pp31 knockout was subsequently rescued by reinserting the pp31 gene into the polyhedrin locus of the same virus genome. We found that pp31 was not essential for viral replication although the absence of pp31 resulted in a lower viral titer. Analysis of viral DNA replication in the absence of pp31 showed that the kinetics of viral DNA replication were unaffected. An AcMNPV oligonucleotide microarray was used to compare gene expression from all AcMNPV genes in the presence or absence of pp31. In the absence of pp31, a modest reduction in transcripts was detected for many viral genes (99 genes) while no substantial increase or decrease was observed for 43 genes. Transcripts from 6 genes (p6.9, ORF 97, ORF 60, ORF 98, ORF 102 and chitinase) were reduced by 66% or more compared to the levels detected from the control virus. Microarray results were further examined by qPCR analysis of selected genes. In combination, these data show that deletion of the pp31 gene was not lethal and did not appear to affect viral DNA replication but resulted in an apparent modest down-regulation of a subset of AcMNPV genes that included both early and late genes.

The baculoviruses occlusion-derived virus: virion structure and function

Baculoviruses play an important ecological role regulating the size of insect populations. For many years, baculoviruses have been applied as targeted biocontrol agents against forestry and agriculture pests. Baculovirus insecticides are effective against insect pests such as velvetbean caterpillar (Anticarsia gemmatalis ), cotton bollworm (Helicoverpa zea ), and gypsy moth (Lymantria dispar ). Baculoviruses are transmitted to insects by the oral route mediated by the occlusion-derived virus (ODV). The ODV is also specialized to exploit the insect midgut that is one of the most extreme biological environments where the viruses are subject to caustic pH and digestive proteases. The molecular biology of the ODV reveals new frontiers in protein chemistry. Finally, ODVs establishes infection in insect gut tissues that are virtually nonsupportive to virus replication and which are continuously sloughed away. ODVs carry with them a battery of proteins that enable them to rapidly exploit and harness these unstable cells for virus replication.

Baculovirus-mediated gene delivery into Mammalian cells does not alter their transcriptional and differentiating potential but is accompanied by early viral gene expression

Gene delivery to neural cells is central to the development of transplantation therapies for neurological diseases. In this study, we used a baculovirus derived from the domesticated silk moth, Bombyx mori, as vector for transducing a human cell line (HEK293) and primary cultures of rat Schwann cells. Under optimal conditions of infection with a recombinant baculovirus containing the reporter green fluorescent protein gene under mammalian promoter control, the infected cells express the transgene with high efficiency. Toxicity assays and transcriptome analyses suggest that baculovirus infection is not cytotoxic and does not induce differential transcriptional responses in HEK293 cells. Infected Schwann cells retain their characteristic morphological and molecular phenotype as determined by immunocytochemistry for the marker proteins S-100, glial fibrillary acidic protein, and p75 nerve growth factor receptor. Moreover, baculovirus-infected Schwann cells are capable of differentiating in vitro and express the P0 myelination marker. However, transcripts for several immediate-early viral genes also accumulate in readily detectable levels in the transduced cells. This transcriptional activity raises concerns regarding the long-term safety of baculovirus vectors for gene therapy applications. Potential approaches for overcoming the identified problem are discussed.

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.

The immediate-early protein IE0 of the Autographa californica nucleopolyhedrovirus is not essential for viral replication

The role of the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) immediate-early protein IE0 in the baculoviral infection is not clear. In this study, we constructed the recombinant virus vAcDeltaie0 null for ie0 expression by targeted mutagenesis replacing exon0 with the cat gene. We found that vAcDeltaie0 replicated efficiently in Spodoptera littoralis SL2 cells, which are poorly permissive for AcMNPV. In contrast, in Spodoptera frugiperda SF9 cells, which are fully permissive for AcMNPV, vAcDeltaie0 DNA replication and budded virus production were delayed. These results and recently published data (X. Dai et al., J. Virol. 78:9633-9644, 2004) indicate that ie0 is not essential for AcMNPV replication but enhances it in permissive SF9 cells.

The Autographa californica multiple nucleopolyhedrovirus ie0-ie1 gene complex is essential for wild-type virus replication, but either IE0 or IE1 can support virus growth

The immediate-early ie0-ie1 gene complex expresses the only baculovirus spliced gene that produces an alternate protein product. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) IE1 is a potent transcriptional transactivator that is essential for viral replication in transient assays. IE1 contains 582 amino acids that are arranged into different domains, including an acidic activation domain at the N terminus, a DNA binding domain, and an oligomerization domain at the C terminus. IE0 is a 52-amino-acid N-terminally elongated form of IE1. We investigated the functions of IE0 and IE1 in virus-infected cells by constructing the first ie1 open reading frame knockout virus. An infectious AcMNPV bacmid was used to generate the ie1 knockout, and the resulting virus, AcBacIE1KO, effectively deletes both ie0 and ie1. AcBacIE1KO does not infect Spodoptera frugiperda cells, showing that the ie0-ie1 gene complex is essential for viral infection. Rescue viruses of AcBacIE1KO were constructed that express only IE1, IE1 and IE0, or only IE0. Our results show that both IE0 and IE1 can function independently, but not equivalently, to support replication, producing infectious virus. Viruses expressing predominately, or only, IE0 produced significantly fewer cells with polyhedra than either the IE1 counterpart or wild-type virus. In addition, DNA replication was prolonged and budded virus and late gene expression were delayed. Viruses expressing only IE1 also produced fewer polyhedra, but replication was slightly faster and achieved higher levels than that of the wild-type virus. Both IE0 and IE1 are therefore required and must be expressed in the correct quantitative ratios to achieve a wild-type infection.

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.

Analysis of baculovirus IE1 in living cells: dynamics and spatial relationships to viral structural proteins

IE1, a principal transcriptional activator of the baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV), is an essential factor for viral DNA replication. During viral infection, IE1 accumulates in discrete subnuclear structures where viral DNA replication occurs. To analyse the dynamic properties of IE1, we monitored green fluorescent protein-tagged IE1 (IE1-GFP) in BmNPV-infected B. mori cells by live-cell microscopy. Time-lapse imaging showed that IE1-associated structures gradually expanded and occasionally fused with one another, while photobleaching experiments revealed that IE1-GFP was relatively immobile inside the IE1-associated structures. To investigate the spatial relationships between IE1 and viral structural proteins in infected cells, three GFP-tagged viral components were expressed together with DsRed-tagged IE1. Two structural proteins that constitute the occlusion-derived virus (ODV), P91-GFP and GFP-ODV-E25, localized to the periphery of the IE1-associated structures. While local accumulations of these proteins were often in contact with the IE1-associated structures, they did not extend beyond the boundaries of the structures. In contrast, the major capsid protein VP39-GFP predominantly accumulated within the IE1-associated structures. These data indicated, in conjunction with the finding of a high DNA content in the structures, that IE1 localizes to the virogenic stroma and therefore support the prediction previously proposed that the virogenic stroma is a site for viral DNA replication as well as for the assembly of nucleocapsids.

Identification and characterization of a major early-transcribed gene of Trichoplusia ni single nucleocapsid nucleopolyhedrovirus using the baculovirus expression system

An early transcribed gene (me-53) of a South Africa strain of Trichoplusia ni single nucleocapsid nucleopolyhedrovirus (TnSNPV) was sequenced and identified. It has an open reading frame of 1146 nucleotides that encodes a protein of 382 amino acids with a molecular mass of 45.2 kDa. The deduced protein sequence alignment of 13 baculovirus ME-53s indicated that the TnSNPV ME-53 shares the highest homologies with NPV subgroup II-A Spodoptera exigua multiple and Mamestra configurata (Maco) nucleopolyhedrovirus ME-53s. The zinc finger-like motifs at the C-termini of ME-53s are highly conserved with similar patterns of cysteine positions. Upon introduction of the gene and a green fluorescent protein reporter gene into the baculovirus expression vector system, the transcriptional analysis of me-53 in two cell lines infected with the Autographa californica nucleopolyhedrovirus (AcMNPV) recombinant revealed that an early TnSNPV me-53 transcript can be detected by 1 h postinfection (hpi) until 12 hpi and a late one from 18 hpi up to 48 hpi, while early and late transcripts of the AcMNPV me-53 of the recombinant can be detected at 3 and 24 hpi, respectively. This suggested that the early and late promoters of both AcMNPV and TnSNPV me-53s were recognized in recombinant virus-infected cells. The regulatory elements of the TnSNPV me-53 promoter were also analyzed.

Autographa californica multiple nucleopolyhedrovirus exon0 (orf141), which encodes a RING finger protein, is required for efficient production of budded virus

exon0 (orf141) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a highly conserved baculovirus gene that codes for a predicted 261-amino-acid protein. Located in the C-terminal region of EXON0 are a predicted leucine-rich coiled-coil domain and a RING finger motif. The 5' 114 nucleotides of exon0 form part of ie0, which is a spliced gene expressed at very early times postinfection, but transcriptional analysis revealed that exon0 is transcribed as a late gene. To determine the role of exon0 in the baculovirus life cycle, we used AcMNPV bacmids and generated exon0 knockout viruses (Ac-exon0-KO) by recombination in Escherichia coli. Ac-exon0-KO progressed through the very late phases in Sf9 cells, as evidenced by the development of occlusion bodies in the nuclei of the transfected or infected cells. However, production of budded virus (BV) in Ac-exon0-KO-infected cells was reduced at least 3 orders of magnitude in comparison to that in wild-type virus infection. Microscopy revealed that Ac-exon0-KO was restricted primarily to the cells initially infected, exhibiting a single-cell infection phenotype. Slot blot assays and Western blot analysis indicated that exon0 deletion did not affect the onset or levels of viral DNA replication or the expression of IE1, IE0, and GP64 prior to BV release. These results demonstrate that exon0 is required for efficient production of BV in the AcMNPV life cycle but does not affect late occlusion-derived virus.

Role of AcMNPV IE0 in baculovirus very late gene activation

IE0 is the only known baculovirus protein that is produced by splicing. In this study, we have explored the role of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) IE0 and its interaction with IE1 in the activation of very late gene expression from the polyhedrin promoter using transient assays. IE0 is co-expressed with IE1 throughout infection up to late times post-infection (p.i.) but shows peak levels of expression at early times. Significant changes in the ratios of the relative levels of IE0 to IE1 were observed throughout the course of infection. To study IE0 in the absence of IE1, we constructed a plasmid pAc-IE0(M-->A) that expressed only IE0. This was due to a mutation of the internal AUG that prevented translation of IE1 from the ie0 mRNA. Both IE0 and IE0(M-->A) were able to replace IE1 in transient assays, showing that IE0 is functional for very late gene activation and should be considered the 20th late gene expression factor (lef). In transient assays, IE0 showed that maximum very late gene expression is achieved at very low relative levels of protein. In contrast, IE1 requires higher levels of protein to obtain maximum very late gene expression. Furthermore, when the levels of IE0 become too high, very late gene expression rapidly declines. Interestingly, co-expression of IE0 and IE1 results in a mutually antagonistic affect on very late gene expression.

High-level gene expression in Aedes albopictus cells using a baculovirus Hr3 enhancer and IE1 transactivator

BACKGROUND

Aedes aegypti is the key vector of both the Yellow Fever and Dengue Fever viruses throughout many parts of the world. Low and variable transgene expression levels due to position effect and position effect variegation are problematic to efforts to create transgenic laboratory strains refractory to these viruses. Transformation efficiencies are also less than optimal, likely due to failure to detect expression from all integrated transgenes and potentially due to limited expression of the transposase required for transgene integration.

RESULTS

Expression plasmids utilizing three heterologous promoters and three heterologous enhancers, in all possible combinations, were tested. The Hr3/IE1 enhancer-transactivator in combination with each of the constitutive heterologous promoters tested increased reporter gene expression significantly in transiently transfected Aedes albopictus C7-10 cells.

CONCLUSIONS

The addition of the Hr3 enhancer to expression cassettes and concomitant expression of the IE1 transactivator gene product is a potential method for increasing the level of transgene expression in insect systems. This mechanism could also potentially be used to increase the level of transiently-expressed transposase in order to increase the number of integration events in transposon-mediated transformation experiments.

The acidic activation domains of the baculovirus transactivators IE1 and IE0 are functional for transcriptional activation in both insect and mammalian cells

The acidic activation domains (AADs) of the baculovirus transactivators IE1 and IE0 are essential for transcriptional transactivation. To compare the relative transcriptional activation potentials of IE1 and IE0 AADs of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and Orgyia pseudotsugata MNPV (OpMNPV), we constructed two ecdysone receptor (EcR)-based inducible expression systems to analyse six baculovirus AADs in two insect cell lines (Ld652Y and Sf9) and two mammalian cell lines (NIH-3T3 and CHO). For insect cell expression, the AADs were fused to the C, D, E and F domains of the spruce budworm Choristoneura fumiferana EcR. For mammalian cell expression the AADs were fused to the E and F domains of mammalian Mus musculus retinoid X receptor. In Ld652Y and Sf9 cells, chimeric proteins containing the AcMNPV AADs activated gene expression to higher levels than those containing the OpMNPV AADs. In NIH-3T3 cells, chimeras containing AcMNPV IE1 and IE0 AADs consistently activated gene expression to higher levels than the archetypal mammalian herpesvirus VP16 AAD. In contrast, OpMNPV AADs only activated expression by 5-15 % relative to the VP16 AAD. In CHO cells, both AcMNPV and OpMNPV AADs exhibited intermediate transactivation levels relative to VP16 AAD. These results show that the baculovirus AADs are functional for transcriptional activation in mammalian cells and that AcMNPV AADs generally appear to be more potent than OpMNPV AADs in both insect and mammalian cells.

Deletion of pe38 attenuates AcMNPV genome replication, budded virus production, and virulence in heliothis virescens

The pe38 gene product of Autographa californica M nucleopolyhedrovirus (AcMNPV) has been shown to be involved in transcriptionally transactivating viral genes and augmenting viral DNA replication in transient assays. To assess the role of pe38 during infection, we generated a knockout virus, Delta pe38-E9/E9, in which the pe38 open reading frame was replaced with that of the green fluorescent protein. We compared mutant and wild-type (WT) viral replication in insect cell culture and virulence in Heliothis virescens larvae. Compared to WT, Delta pe38-E9/E9 budded virus (BV) production was delayed by at least 3 h, and BV yields were reduced over 99%. Similarly, Delta pe38-E9/E9 DNA synthesis levels were greatly reduced relative to those of WT, but onset of DNA replication was the same for both viruses. In bioassays, nearly sevenfold more Delta pe38-E9/E9 virus than WT virus was required to achieve an LD(50) when administered orally, but not hemocoelically. These results support the hypothesis that the kinetics of AcMNPV BV production greatly impact virulence in larvae infected orally (the natural route of infection) and that PE38 is an important, but not essential, factor in viral DNA synthesis and BV production.

Using double-stranded RNA to prevent in vitro and in vivo viral infections by recombinant baculovirus

Introduction of double-stranded RNA (dsRNA) into a wide variety of cells and organisms results in post-transcriptional depletion of the homologue endogenous mRNA. This well-preserved phenomenon known as RNA interference (RNAi) is present in evolutionarily diverse organisms such as plants, fungi, insects, metazoans, and mammals. Because the identification of the targeted mRNA by the RNAi machinery depends upon Watson-Crick base-pairing interactions, RNAi can be exquisitely specific. We took advantage of this powerful and flexible technique to demonstrate that selective silencing of genes essential for viral propagation prevents in vitro and in vivo viral infection. Using the baculovirus Autographa californica, a rapidly replicating and highly cytolytic double-stranded DNA virus that infects many different insect species, we show for the first time that introduction of dsRNA from gp64 and ie1, two genes essential for baculovirus propagation, results in prevention of viral infection in vitro and in vivo. This is the first report demonstrating the use of RNAi to inhibit a viral infection in animals. This inhibition was specific, because dsRNA from the polyhedrin promoter (used as control) or unrelated dsRNAs did not affect the time course of viral infection. The most relevant consequences from the present study are: 1) RNAi offers a rapid and efficient way to interfere with viral genes to assess the role of specific proteins in viral function and 2) using RNAi to interfere with viral genes essential for cell infection may provide a powerful therapeutic tool for the treatment of viral infections.

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.

Reduced expression of the immediate-early protein IE0 enables efficient replication of Autographa californica multiple nucleopolyhedrovirus in poorly permissive Spodoptera littoralis cells

Infection of Spodoptera littoralis SL2 cells with the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) results in apoptosis and low yields of viral progeny, in contrast to infection with S. littoralis nucleopolyhedrovirus (SlNPV). By cotransfecting SL2 cells with AcMNPV genomic DNA and a cosmid library representing the complete SlNPV genome, we were able to rescue AcMNPV replication and to isolate recombinant virus vAcSL2, which replicated efficiently in SL2 cells. Moreover, vAcSL2 showed enhanced infectivity for S. littoralis larvae compared to AcMNPV. The genome of vAcSL2 carried a 519-bp insert fragment that increased the distance between the TATA element and the transcriptional initiation site (CAGT) of immediate-early gene ie0. This finding correlated with low steady-state levels of IE0 and higher steady-state levels of IE1 (the product of the ie1 gene, a major AcMNPV transactivator, and a multifunctional protein) than of IE0. Mutagenesis of the ie0 promoter locus by insertion of the chloramphenical acetyltransferase (cat) gene yielded a new recombinant AcMNPV with replication properties identical to those of vAcSL2. Thus, the analysis indicated that increasing the steady-state levels of IE1 relative to IE0 should enable AcMNPV replication in SL2 cells. This suggestion was confirmed by constructing a recombinant AcMNPV bearing an extra copy of the ie1 gene under the control of the Drosophila hsp70 promoter. These results suggest that IE0 plays a role in the regulation of AcMNPV infection and show, for the first time, that significant improvement in the ability of AcMNPV to replicate in a poorly permissive cell line and organism can be achieved by increasing the expression of the main multiple functional protein, IE1.

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.

Identification, sequence analysis, and phylogeny of the immediate early gene 1 of the Trichoplusia ni single nucleocapsid polyhedrosis virus

Substantial research has been conducted on the immediate early I (ie-1) genes from the prototype baculovirus Auographa californica multicapsid nuclear polyhedrosis virus (AcMNPV) and the Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus (OpMNPV). In both cases ie-1 gene products have been implicated in transcriptional activation and repression. In this study an ie-1 homolog was identified from Trichoplusia ni single nucleocapsid polyhedrosis virus (TniSNPV). Nucleotide sequence analysis indicated that the TniSNPV ie-1 gene consists of a 2,217 nucleotide open reading frame (ORF), encoding a protein with a molecular mass of 84.464 kDa. This represents the largest baculovirus ie-1 gene characterised to date. Of the seven ie-1 homologs identified to date, the TniSNPV ie-1 shared most sequence similarity with the ie-1 gene of Spodoptera exigua MNPV (SeMNPV) (41%). At the nucleotide level, expected TATA and CAGT motifs were found to precede each ie-1 ORE. At the protein level, it was confirmed that the N-termini are poorly conserved, but share the characteristic of having a high proportion of acidic amino acids. In addition it was found that N-terminal regions significantly matched the SET domain in the Swiss-Prot prosite database. The C-terminal regions of the deduced IE-1 sequences were found to be substantially more conserved than the N-termini. Several conserved motifs were identified in the C-terminal sequences. A phylogenetic tree of nine baculovirus IE-1 proteins was constructed using maximum parsimony analysis. The phylogenetic estimation of the ie-1 genes shows that TniSNPV is a member of the previously described lepidopteran NPV group II and it is most closely related to SeMNPV.

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.

Negative regulatory regions of the PAT1 promoter of Hz-1 virus contain GATA elements which associate with cellular factors and regulate promoter activity

The persistence-associated transcript 1 (PAT1) is actively expressed during persistent infection with Hz-1 virus, while transcription of the rest of the viral genes is shut down. Previously, results of a series deletion of the PAT1 promoter suggested that the regions from nucleotides -312 to -212 and nucleotides -158 to -90 negatively regulate the promoter activity. Here, the negative regulatory effect of the -312/-90 fragment was confirmed using a heterologous IE0 promoter of Autographa californica multiple nucleopolyhedrovirus. Further, the negative regulation of the -312 to -212 region was orientation-independent. The results of electrophoresis mobility shift assays showed that cellular protein(s) bind specifically to DNA fragments -312/-212 and -158/-90. In each of these fragments, a GATA element was identified by computer-assisted analysis. Mutating both GATA elements in the -312/-90 fragment completely eliminated its negative effect on IE0 promoter activity, while mutating only one of these elements had little or no effect. Together, these results suggest that the GATA element has a negative regulatory role on the IE0 and PAT1 promoters.

Sequence analysis of the Plutella xylostella granulovirus genome

The Plutella xylostella granulovirus (PxGV) genome DNA was sequenced and the predicted open reading frames (ORFs) were compared to genes of the first-sequenced GV, Xestia c-nigrum GV (XcGV), and those from other baculoviruses and organisms. PxGV DNA has a size of 100,999 bp with a G + C content of 40.7%. The analysis predicted 120 ORFs with a size of 150 nucleotides or larger that showed minimal overlap. Blast searches followed by a comparison of ORF arrangement with those of completely sequenced baculovirus genomes showed the presence of 102 homologs to other genes in the database. Among them, 74 and 100 were homologous to genes of Autographa californica NPV (AcMNPV) and XcGV, respectively. A striking feature of the relationship between the genomes of PxGV and XcGV was the conservation of the order and orientation of homologous genes. Even though the XcGV genome is much larger than that of PxGV (178 vs 101 kb) and had many more predicted ORFs (181 vs 120) with an average amino acid sequence relatedness of 42%, the order and orientation of almost all homologous genes was conserved. The PxGV genome contained four homologous regions (hrs), each with 10 to 23 repeated sequences of 101 to 105 nucleotides containing a 15-bp imperfect palindrome in the center of the repeats.