A baculovirus gp64 early promoter is activated by host transcription factor binding to CACGTG and GATA elements

Enhancing NA immunogenicity through novel VLP designs

Current influenza virus vaccines poorly display key neuraminidase (NA) epitopes and do not robustly induce NA-reactive antibodies; instead, they focus on the induction of hemagglutinin (HA)-reactive antibodies. Next-generation influenza vaccines should be optimized in order to activate NA-reactive B cells and to induce a broadly cross-reactive and protective antibody response. We aimed at enhancing the immunogenicity of the NA on vaccines by two strategies: (i) modifying the HA:NA ratio of the vaccine preparation and (ii) exposing epitopes on the lateral surface or beneath the head of the NA by extending the NA stalk. The H1N1 glycoproteins from the influenza virus A/California/04/2009 strain were displayed on human immunodeficiency virus 1 (HIV-1) gag-based virus-like particles (VLP). Using the baculovirus insect cell expression system, we biased the quantity of surface glycoproteins employing two different promoters, the very late baculovirus p10 promoter and the early and late gp64 promoter. This led to a 1:1 to 2:1 HA:NA ratio, which was approximately double or triple the amount of NA as present on the wild-type influenza A virus (HA:NA ratio 3:1 to 5:1). Furthermore, by insertion of 15 amino acids from the A-New York/61/2012 strain (NY12) which prolongates the NA stalk (NA long stalk; NA-LS), we intended to improve the accessibility of the NA. Six different types of VLPs were produced and purified using a platform downstream process based on Capto-Core 700™ followed by Capto-Heparin™ affinity chromatography combined with ultracentrifugation. These VLPs were then tested in a mouse model. Robust titers of antibodies that inhibit the neuraminidase activity were elicited even after vaccination with two low doses (0.3 μg) of the H1N1 VLPs without compromising the anti-HA responses. In conclusion, our results demonstrate the feasibility of the two developed strategies to retain HA immunogenicity and improve NA immunogenicity as a future influenza vaccine candidate.

Entomopathogenic Viruses in the Neotropics: Current Status and Recently Discovered Species

The market for biological control of insect pests in the world and in Brazil has grown in recent years due to the unwanted ecological and human health impacts of chemical insecticides. Therefore, research on biological control agents for pest management has also increased. For instance, insect viruses have been used to protect crops and forests around the world for decades. Among insect viruses, the baculoviruses are the most studied and used viral biocontrol agent. More than 700 species of insects have been found to be naturally infected by baculoviruses, with 90% isolated from lepidopteran insects. In this review, some basic aspects of baculovirus infection in vivo and in vitro infection, gene content, viral replication will be discussed. Furthermore, we provide examples of the use of insect viruses for biological pest control and recently characterized baculoviruses in Brazil.

Host AAA+ ATPase TER94 Plays Critical Roles in Building the Baculovirus Viral Replication Factory and Virion Morphogenesis

TER94 is a multifunctional AAA+ ATPase crucial for diverse cellular processes, especially protein quality control and chromatin dynamics in eukaryotic organisms. Many viruses, including coronavirus, herpesvirus, and retrovirus, coopt host cellular TER94 for optimal viral invasion and replication. Previous proteomics analysis identified the association of TER94 with the budded virions (BVs) of baculovirus, an enveloped insect large DNA virus. Here, the role of TER94 in the prototypic baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) life cycle was investigated. In virus-infected cells, TER94 accumulated in virogenic stroma (VS) at the early stage of infection and subsequently partially rearranged in the ring zone region. In the virions, TER94 was associated with the nucleocapsids of both BV and occlusion-derived virus (ODV). Inhibition of TER94 ATPase activity significantly reduced viral DNA replication and BV production. Electron/immunoelectron microscopy revealed that inhibition of TER94 resulted in the trapping of nucleocapsids within cytoplasmic vacuoles at the nuclear periphery for BV formation and blockage of ODV envelopment at a premature stage within infected nuclei, which appeared highly consistent with its pivotal function in membrane biogenesis. Further analyses showed that TER94 was recruited to the VS or subnuclear structures through interaction with viral early proteins LEF3 and helicase, whereas inhibition of TER94 activity blocked the proper localization of replication-related viral proteins and morphogenesis of VS, providing an explanation for its role in viral DNA replication. Taken together, these data indicated the crucial functions of TER94 at multiple steps of the baculovirus life cycle, including genome replication, BV formation, and ODV morphogenesis.IMPORTANCE TER94 constitutes an important AAA+ ATPase that associates with diverse cellular processes, including protein quality control, membrane fusion of the Golgi apparatus and endoplasmic reticulum network, nuclear envelope reformation, and DNA replication. To date, little is known regarding the role(s) of TER94 in the baculovirus life cycle. In this study, TER94 was found to play a crucial role in multiple steps of baculovirus infection, including viral DNA replication and BV and ODV formation. Further evidence showed that the membrane fission/fusion function of TER94 is likely to be exploited by baculovirus for virion morphogenesis. Moreover, TER94 could interact with the viral early proteins LEF3 and helicase to transport and further recruit viral replication-related proteins to establish viral replication factories. This study highlights the critical roles of TER94 as an energy-supplying chaperon in the baculovirus life cycle and enriches our knowledge regarding the biological function of this important host factor.

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.

Real-Time Expression Analysis of Selected Anticarsia gemmatalis multiple nucleopolyhedrovirus Gene Promoters during Infection of Permissive, Semipermissive and Nonpermissive Cell Lines

Baculovirus infection follows a transcriptionally controlled sequence of gene expression that occurs by activation of different viral gene promoter sequences during infection. This sequence of promoter activation may be disrupted by cellular defenses against viral infection, which might interfere with viral progeny formation. In this work, the activity of the ie1, gp64, lef-1, vp39, p6.9 and polh promoters of the Anticarsia gemmatalis multiple nucleopolyhedrovirus was assessed during infection of permissive, semipermissive and nonpermissive cell lines by a novel methodology that detects reporter protein luminescence in real-time. This technique allowed us to characterize in rich detail the AgMNPV promoters in permissive cell lines and revealed differential profiles of expression in cells with limited permissivity that correlate well with limitations in viral DNA replication. Semipermissive and nonpermissive cell lines presented delays and restrictions in late and very late promoter expression. Cells undergoing apoptosis did not inhibit late gene expression; however, viral progeny formation is severely affected. This work demonstrates the application of the real-time luminescence detection methodology and how the promoter expression profile may be used to diagnose cellular permissivity to baculovirus infection.

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.

A betabaculovirus encoding a gp64 homolog

Background

A betabaculovirus (DisaGV) was isolated from Diatraea saccharalis (Lepidoptera: Crambidae), one of the most important insect pests of the sugarcane and other monocot cultures in Brazil.

Results

The complete genome sequence of DisaGV was determined using the 454-pyrosequencing method. The genome was 98,392 bp long, which makes it the smallest lepidopteran-infecting baculovirus sequenced to date. It had a G + C content of 29.7 % encoding 125 putative open reading frames (ORF). All the 37 baculovirus core genes and a set of 19 betabaculovirus-specific genes were found. A group of 13 putative genes was not found in any other baculovirus genome sequenced so far. A phylogenetic analysis indicated that DisaGV is a member of Betabaculovirus genus and that it is a sister group to a cluster formed by ChocGV, ErelGV, PiraGV isolates, ClanGV, CaLGV, CpGV, CrleGV, AdorGV, PhopGV and EpapGV. Surprisingly, we found in the DisaGV genome a G protein-coupled receptor related to lepidopteran and other insect virus genes and a gp64 homolog, which is likely a product of horizontal gene transfer from Group 1 alphabaculoviruses.

Conclusion

DisaGV represents a distinct lineage of the genus Betabaculovirus. It is closely related to the CpGV-related group and presents the smallest genome in size so far. Remarkably, we found a homolog of gp64, which was reported solely in group 1 alphabaculovirus genomes so far.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2408-9) contains supplementary material, which is available to authorized users.

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.

Autographa californica multiple nucleopolyhedrovirus odv-e25 (Ac94) is required for budded virus infectivity and occlusion-derived virus formation

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) odv-e25 is a core gene found in all lepidopteran baculoviruses, but its function is unknown. In this study, we generated an odv-e25-knockout AcMNPV and investigated the roles of ODV-E25 in the baculovirus life cycle. The odv-e25 knockout was subsequently rescued by reinserting the odv-e25 gene into the same virus genome. Fluorescence microscopy showed that transfection with the odv-e25-null bacmid vAcBac(KO) was insufficient for propagation in cell culture, whereas the 'repair' virus vAcBac(RE) was able to function in a manner similar to that of the control vAcBac. We found that odv-e25 was not essential for the release of budded viruses (BVs) into culture medium, although the absence of odv-e25 resulted in a 100-fold lower viral titer at 24 h post-transfection (p.t.). Analysis of viral DNA replication in the absence of odv-e25 showed that viral DNA replication was unaffected in the first 24 h p.t. Furthermore, electron microscopy revealed that polyhedra were found in the nucleus, while mature occlusion-derived viruses (ODVs) were not found in the nucleus or polyhedra in odv-e25 null transfected cells, which indicated that ODV-E25 was required for the formation of ODV.

Characterization of a late gene, ORF75 from Bombyx mori nucleopolyhedrovirus

Open reading frame 75 (Bm-p33) of Bombyx mori nucleopolyhedrovirus (BmNPV) is a homologue of Autographa californica multiple nucleopolyhedrovirus ORF92. The gene is conserved among all baculoviruses that have been completely sequenced to date and is considered to be a baculovirus core set gene. No amino acid mutation was found in Bm-p33 sequences among six BmNPV strains differing in geography, phenotype, or host. The Bm-p33 transcript can be detected as early as 12 h post infection (h p.i.) and remains detectable until 96 h p.i. The Bm-p33 protein was detected in cell lysates from 18 h p.i. through 96 h p.i., and no positive band could be detected in budded viruses (BVs) and occlusion-derived viruses (ODVs) by western blot using anti-Bm-p33 serum. Immunofluorescence microscopy indicated that Bm-p33 accumulated in the nuclear membrane and the intranuclear region, especially near the nuclear membrane of the virus-infected cells. Bm75 RNAi significantly decreased the mRNA level. However, no obvious effects on ODV formation and BV production in BmNPV-infected cells could be detected. Bm-p33 is a BmNPV late gene encoding a nonstructural protein which may function mainly in the nucleus of the infected cells.

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.

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.

Cloning and sequence analysis of the Antheraea pernyi nucleopolyhedrovirus gp64 gene

Frequent outbreaks of the purulence disease of Chinese oak silkworm are reported in Middle and Northeast China. The disease is produced by the pathogen Antheraea pernyi nucleopolyhedrovirus (AnpeNPV). To obtain molecular information of the virus, the polyhedra of AnpeNPV were purified and characterized. The genomic DNA of AnpeNPV was extracted and digested with HindIII. The genome size of AnpeNPV is estimated at 128 kb. Based on the analysis of DNA fragments digested with HindIII, 23 fragments were bigger than 564 bp. A genomic library was generated using HindIII and the positive clones were sequenced and analysed. The gp64 gene, encoding the baculovirus envelope protein GP64, was found in an insert. The nucleotide sequence analysis indicated that the AnpeNPV gp64 gene consists of a 1,530 nucleotide open reading frame (ORF), encoding a protein of 509 amino acids. Of the eight gp64 homologues, the AnpeNPV gp64 ORF shared the most sequence similarity with the gp64 gene of Anticarsia gemmatalis NPV, but not Bombyx mori NPV. The upstream region of the AnpeNPV gp64 ORF encoded the conserved transcriptional elements for early and late stage of the viral infection cycle. These results indicated that AnpeNPV belongs to group I NPV and was far removed in molecular phylogeny from the BmNPV.

Identification and characterization of odv-e25 of Spodoptera litura multicapsid nucleopolyhedrovirus

Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) odv-e25 is 684 bp long, potentially encoding 227 amino acids with a predicted molecular weight of 24.9 kDa. Homology analysis indicated that SpltMNPV ODV-E25 has 35-65% amino acid identity with that of other known baculoviruses. RT-PCR results revealed that the odv-e25 is transcribed actively at the late stage of infection and the mRNA start site was mapped within a consensus baculovirus late promoter sequence (TTAAG). Western blot analysis of odv-e25 expression with an antiserum made against 6 x His tagged ODV-E25 expressed in Escherichia coli indicated that it was present as a doublet of approximately 27 kDa from 24 h through 96 h in SpltMNPV-infected Spli-221 cells. Similar results were seen on Western blots of Spodoptera exigua (Se)MNPV-infected Se301 cells. Immunofluorescence analysis showed that ODV-E25 was predominantly present in the cytoplasm of SpltMNPV-infected cells and localized to the envelopes of occlusion-derived virus.

Characterization of Spodoptera litura multicapsid nucleopolyhedrovirus 38.7 k protein, which contains a conserved BRO domain

Homology analysis revealed that Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) 38.7 k protein has 22-83% amino acid identities with Ecotropis obliqua NPV, Mamestra configurata MNPV, Helicoverpa armigera SNPV, H. zea SNPV, S. exigua MNPV and S. littoralis MNPV 38.7 k proteins. Analysis of the relationship of these 38.7 k proteins indicated that they contain a conserved BRO-N domain, and SpltMNPV and SpliMNPV 38.7 k proteins also contain a motif found in all known viral and prokaryotic single-strand DNA binding proteins. RT-PCR results showed that SpltMNPV 38.7 k gene is transcribed actively at the late stage of infection and the mRNA start site was mapped within a consensus baculovirus late promoter motif (ATAAG). Western blot analysis revealed that the 38.7 k was expressed in infected S. litura cells as a 41 kDa form and this protein distributed in the nucleus of infected cells. Using a histone extraction protocol, SpltMNPV 38.7 k could be detected in the histone H1 fraction. Micrococcal nuclease treatment released SpltMNPV 38.7 k protein from the chromatin fraction, suggesting that its involvement in nucleosome structures. Furthermore, column chromatography using DNA-cellulose showed that SpltMNPV 38.7 k protein interacted with nucleic acids. It was proposed that SpltMNPV 38.7 k might function as a DNA-binding protein.

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.

Characterization of p24 Gene of Spodoptera litura Multicapsid Nucleopolyhedrovirus

Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) p24 gene is 753 bp long, potentially encoding 244 amino acids with a predicted molecular weight of 27.3 kDa. Homology analysis indicated that SpltMNPV P24 has 20-36% amino acid identity with that of other known baculoviruses. RT-PCR results showed that the p24 gene is transcribed actively at the late stage of infection and the mRNA start site was mapped within a consensus baculovirus late promoter sequence (ATAAG). Western blot analysis of extracts from SpltMNPV-infected S. litura cells detected a specific 28 kDa protein, and this protein was not N-glycosylated. Structural localization revealed that SpltMNPV P24 was associated with the nucleocapsid of occlusion-derived virus (ODV) as a complex form of 83 kDa.

Microarray and RT-PCR screening for white spot syndrome virus immediate-early genes in cycloheximide-treated shrimp

Here, we report for the first time the successful use of cycloheximide (CHX) as an inhibitor to block de novo viral protein synthesis during WSSV (white spot syndrome virus) infection. Sixty candidate IE (immediate-early) genes were identified using a global analysis microarray technique. RT-PCR showed that the genes corresponding to ORF126, ORF242 and ORF418 in the Taiwan isolate were consistently CHX-insensitive, and these genes were designated ie1, ie2 and ie3, respectively. The sequences for these IE genes also appear in the two other WSSV isolates that have been sequenced. Three corresponding ORFs were identified in the China WSSV isolate, but only an ORF corresponding to ie1 was predicted in the Thailand isolate. In a promoter activity assay in Sf9 insect cells using EGFP (enhanced green fluorescence protein) as a reporter, ie1 showed very strong promoter activity, producing higher EGFP signals than the insect Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus (OpMNPV) ie2 promoter.

Identification and characterization of a DNA photolyase-containing baculovirus from Chrysodeixis chalcites

A hitherto unknown single nucleocapsid nucleopolyhedrovirus (SNPV) with a unique property was isolated from larvae of the looper Chrysodeixis chalcites (Lepidoptera, Noctuidae, Plusiinae). Polyhedrin, lef-8, and pif-2 gene sequences were obtained by PCR with degenerate primers and used for phylogenetic analysis. ChchNPV belonged to class II NPVs and its polyhedrin sequence was most similar to that of class II NPVs of other members of the subfamily Plusiinae. Further genetic characterization involved the random cloning of HindIII fragments into a plasmid vector and analysis by end-in sequencing. A gene so far unique to baculoviruses was identified, which encodes a putative DNA repair enzyme: cyclobutane pyrimidine dimer (CPD) DNA photolyase (dpl). The transcriptional activity of this gene was demonstrated in both ChchNPV-infected C. chalcites larvae and infected Trichoplusia ni High Five cells by RT-PCR and 5' and 3' RACE analysis. The possible role of this gene in the biology of the virus is discussed.

Anticarsia gemmatalis multicapsid nucleopolyhedrovirus v-trex gene encodes a functional 3' to 5' exonuclease

The viral three-prime repair exonuclease (v-trex) gene of the Anticarsia gemmatalis multicapsid nucleopolyhedrovirus (AgMNPV) is the first baculovirus gene to be described with significant homology to a 3' exonuclease. v-trex is an early gene that is expressed by AgMNPV from 3 h post-infection. In the present study, the AgMNPV v-trex ORF was cloned into the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) under the control of a polyhedrin promoter. The resulting virus produced an abundant, soluble protein that migrated with an apparent size of 23.7 kDa. The 3' to 5' exonuclease activity associated with this v-trex-expressing recombinant AcMNPV was 2000-fold above that of wild-type AcMNPV. This exonuclease activity was inhibited by EDTA and was activated in the presence of Mg2+ and, to a lesser extent, Mn2+. From these results, the AgMNPV v-trex gene is concluded to encode an independently active 3' to 5' exonuclease.

Sequence analysis of the genome of the Neodiprion sertifer nucleopolyhedrovirus

The genome of the Neodiprion sertifer nucleopolyhedrovirus (NeseNPV), which infects the European pine sawfly, N. sertifer (Hymenoptera: Diprionidae), was sequenced and analyzed. The genome was 86,462 bp in size. The C+G content of 34% was lower than that of the majority of baculoviruses. A total of 90 methionine-initiated open reading frames (ORFs) with more than 50 amino acids and minimal overlapping were found. From those, 43 ORFs were homologous to other baculovirus ORFs, and 29 of these were from the 30 conserved core genes among all baculoviruses. A NeseNPV homolog to the ld130 gene, which is present in all other baculovirus genomes sequenced to date, could not be identified. Six NeseNPV ORFs were similar to non-baculovirus-related genes, one of which was a trypsin-like gene. Only one iap gene, containing a single BIR motif and a RING finger, was found in NeseNPV. Two NeseNPV ORFs (nese18 and nese19) were duplicates transcribed in opposite orientations from each other. NeseNPV did not have an AcMNPV ORF 2 homolog characterized as the baculovirus repeat ORF (bro). Six homologous regions (hrs) were located within the NeseNPV genome, each containing small palindromes embedded within direct repeats. A phylogenetic analysis was done to root the tree based upon the sequences of DNA polymerase genes of NeseNPV, 23 other baculoviruses, and other phyla. Baculovirus phylogeny was then constructed with 29 conserved genes from 24 baculovirus genomes. Culex nigripalpus nucleopolyhedrovirus (CuniNPV) was the most distantly related baculovirus, branching to the hymenopteran NeseNPV and the lepidopteran nucleopolyhedroviruses and granuloviruses.

Identification of a novel protein associated with envelope of occlusion-derived virus in Spodoptera litura multicapsid nucleopolyhedrovirus

Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) ORF137 (Splt137) is one of 29 unique SpltMNPV ORFs. Splt137 has the potential to code for a polypeptide of 231 amino acid residues with predicted molecular weight of 27.5 kDa. Computer-assisted analysis of the predicted amino acid sequences of Splt137 protein showed 1 N-glycosylation site and 11 phosphorylation sites. For identification of Spit137, antibody was prepared by immunization of rabbits with purified Splt137 protein produced in Escherichia coli. This antibody was used to analyse Splt137 protein using Western blot. A 36-kDa protein was found both in the infected cells and envelope fractions of occlusion-derived virus (ODV) but could not be detected in the budded virus (BV). Tunicamycin treatment of SpltMNPV infected cells suggested that the 36-kDa protein had undergone N-glycosylation. Our data suggested that Splt137 protein was a novel envelope protein of ODV and might exist as a more complex form of 79-kDa protein in intact ODV. Further, transcriptional analysis with RT-PCR and 5' RACE analysis suggested that Splt137 might perform functions early and late in infection.

Molecular cloning and sequence analysis of the Anticarsia gemmatalis multicapsid nuclear polyhedrosis virus GP64 glycoprotein

The gp64 locus of Anticarsia gemmatalis multicapsid nucleopolyhedrovirus isolate Santa Fe (AgMNPV-SF) was characterised molecularly in our laboratory. To this end, we have located and cloned a AgMNPV-SF genomic DNA fragment containing the gp64 gene and sequenced the complete gp64 locus. Nucleotide sequence analysis indicated that the AgMNPV gp64 gene consists of a 1500 nucleotide open reading frame (ORF), encoding a protein of 499 amino acids. Of the seven gp64 homologues identified to date, the AgMNPV gp64 ORF shared most sequence similarity with the gp64 gene of Orgyia pseudotsugata MNPV. The GP64 from AgMNPV is the smallest baculoviral envelope glycoprotein found to date, differing in 10 or more residues from the other group I nucleopolyhedroviruses. The biological activity of AgMNPV GP64 protein was assessed by cell fusion assays in UFL-AG-286 cells using the obtained recombinant plasmids. In the upstream and downstream regions, relative to the gp64 ORF, we found different conserved transcriptional and post-transcriptional regulatory elements, respectively.

Characterization of Spodoptera exigua multicapsid nucleopolyhedrovirus ORF17/18, a homologue of Xestia c-nigrum granulovirus ORF129

Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) contains a number of genes with a homologue found so far only in a distantly related baculovirus. One of these, SeMNPV ORF17/18 (Se17/18) shares 55% amino acid similarity to ORF129 of Xestia c-nigrum granulovirus (XcGV). Se17/18 was transcribed in cultured S. exigua 301 cells, as a polyadenylated transcript of 1.1 kb. 5'-RACE analysis demonstrated that Se17/18 transcripts started at 134, 131 and 126 nt upstream of the putative translational start codon. These sites overlap with a baculovirus consensus early promoter motif. Se17/18 transcripts were detected by Northern blot analysis and RT-PCR with increasing abundance from 8 h to 24 h post infection (p.i.) and still present until 72 h p.i. A C-terminal GFP-fusion protein of Se17/18 was primarily localized in the cytoplasm of Se301 and Sf21 cells. A chicken polyclonal antiserum was raised that reacted specifically to Se17/18 protein produced in E. coli. However, no immunoreactive protein was detected in SeMNPV-infected Se301 cells and S. exigua larvae, neither in concentrated BV and ODV preparations. These observations and the inability to detect a C-terminal GFP-fusion protein of Se17/18 in Se301 cells using a GFP antibody suggest that Se17/18 protein is present, if at all, in spurious amounts. Based on the low homology of the Se17/18 protein to (methyl) transferases its possible involvement in transcription regulation is discussed.

Identification of a novel occlusion derived virus-specific protein in Spodoptera exigua multicapsid nucleopolyhedrovirus

Understanding the molecular basis of the distinct biological properties of Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV), such as its narrow host range and high virulence, requires detailed information on the temporal expression and subcellular localization of SeMNPV gene products. The expression of two unique SeMNPV ORFs, 116 (Se116) and 117 (Se117), which show 45% amino acid similarity, was analyzed. Se116 and Se117 were expressed both in cultured cells and in larvae of S. exigua as polyadenylated transcripts of 0.80 and 0.75 kb, respectively. These transcripts initiated from ATCA(G/T)T promoter motifs, commonly found for baculovirus early genes. Se116 transcripts were detected with increasing abundance from 8 to 48 h p.i., whereas Se117 transcripts were present from 4 h p.i. and most abundantly at 24 h p.i. Western blot analysis of infected Se301 cells revealed 27- and 23-kDa proteins for Se116 and Se117, respectively. C-terminal GFP-fusion proteins of Se116 and Se117 were primarily localized in the nucleus of Se301 cells. When Se301 cells were infected with SeMNPV, both GFP-fusion proteins were localized in the virogenic stroma of the nucleus. While the function of the Se116 protein is still enigmatic, the Se117 protein appeared to be a structural protein associated with nucleocapsids of occlusion-derived SeMNPV virions but not of budded virus.

Expression in Spodoptera frugiperda (Sf21) insect cells of BT-R(1), a cadherin-related receptor from Manduca sexta for Bacillus thuringiensis Cry1Ab toxin

The cadherin-related receptor of Manduca sexta, BT-R(1), for the Cry1A family of Bacillus thuringiensis insecticidal toxins, was expressed in cultured Spodoptera frugiperda (Sf21) insect cells utilizing the expression vector deltaOp-gp64. Recombinant BT-R(1) was released by the Sf21 cells in soluble form into the culture medium and represents approximately 58% of total BT-R(1) produced by the cells. The soluble protein was purified by affinity chromatography using Cry1Ab toxin coupled to Sepharose 4B. The apparent molecular mass of purified soluble recombinant BT-R(1) is 195 kDa. Radiolabeled toxin bound to purified soluble BT-R(1) with a K(d) value of 1.1 nM, which is similar to that of both membrane-bound BT-R(1) in Sf21 cells and natural BT-R(1) from M. sexta larval midgut tissue. Binding of radiolabeled toxin to soluble BT-R(1) was competitively inhibited by unlabeled Cry1Ab toxin but not by other Cry toxins as was observed also for membrane-bound BT-R(1). The recombinant soluble protein was stable in culture medium for at least 3 days at 27 degrees C and for 7 days at 4 degrees C and exhibited toxin-binding properties similar to the natural protein. Apparently, neither membrane association nor the extent of glycosylation influences the binding affinity and specificity of BT-R(1). Approximately 1 mg of purified BT-R(1) was obtained per liter of insect cell culture supernatant, representing approximately 2 x 10(9) Sf21 cells.

Analysis of sequences involved in IE2 transactivation of a baculovirus immediate-early gene promoter and identification of a new regulatory motif

Opep-2 is a unique baculovirus early gene that has only been identified in the Orgyia pseudotsugata multiple capsid nucleopolyhedrovirus (OpMNPV). Previous analyses have shown this gene is expressed at very early times post-infection (p.i.) but is shut down by 36-48 h p.i. The promoter of opep-2 therefore, represents a class of early genes that is temporally regulated. In this study, a detailed analysis of the opep-2 promoter is performed to analyze the role individual motifs play in early gene expression. A new 13 base pair regulatory element was identified and shown to be essential in controlling high-level expression of this gene. In addition, mutational analysis revealed that GATA and CACGTG motifs, which have been shown to bind cellular factors in Sf9 and Ld652Y cells, played minor roles in influencing opep-2 expression in the absence of other viral factors. The OpMNPV transactivator IE2 causes a significant activation of the opep-2 promoter. Cotransfection of an extensive number of promoter deletions and mutations did not show any sequence specificity for IE2 transactivation. This is the first detailed analysis of the sequence requirements for IE2 transactivation, and these results suggest that IE2 does not bind directly to specific elements in the opep-2 promoter.

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.

A discrete stage of baculovirus GP64-mediated membrane fusion

Viral fusion protein trimers can play a critical role in limiting lipids in membrane fusion. Because the trimeric oligomer of many viral fusion proteins is often stabilized by hydrophobic 4-3 heptad repeats, higher-order oligomers might be stabilized by similar sequences. There is a hydrophobic 4-3 heptad repeat contiguous to a putative oligomerization domain of Autographa californica multicapsid nucleopolyhedrovirus envelope glycoprotein GP64. We performed mutagenesis and peptide inhibition studies to determine if this sequence might play a role in catalysis of membrane fusion. First, leucine-to-alanine mutants within and flanking the amino terminus of the hydrophobic 4-3 heptad repeat motif that oligomerize into trimers and traffic to insect Sf9 cell surfaces were identified. These mutants retained their wild-type conformation at neutral pH and changed conformation in acidic conditions, as judged by the reactivity of a conformationally sensitive mAb. These mutants, however, were defective for membrane fusion. Second, a peptide encoding the portion flanking the GP64 hydrophobic 4-3 heptad repeat was synthesized. Adding peptide led to inhibition of membrane fusion, which occurred only when the peptide was present during low pH application. The presence of peptide during low pH application did not prevent low pH-induced conformational changes, as determined by the loss of a conformationally sensitive epitope. In control experiments, a peptide of identical composition but different sequence, or a peptide encoding a portion of the Ebola GP heptad motif, had no effect on GP64-mediated fusion. Furthermore, when the hemagglutinin (X31 strain) fusion protein of influenza was functionally expressed in Sf9 cells, no effect on hemagglutinin-mediated fusion was observed, suggesting that the peptide does not exert nonspecific effects on other fusion proteins or cell membranes. Collectively, these studies suggest that the specific peptide sequences of GP64 that are adjacent to and include portions of the hydrophobic 4-3 heptad repeat play a dynamic role in membrane fusion at a stage that is downstream of the initiation of protein conformational changes but upstream of lipid mixing.

Modulation of translational efficiency by contextual nucleotides flanking a baculovirus initiator AUG codon

In a previous study of translational regulation of a baculovirus gene, we observed that translation initiated at an unexpectedly high efficiency from an AUG codon found in what was believed to be a poor context (M.-J. Chang and G. W. Blissard, 1997, J. Virol. 71, 7448-7460). In the current study, we examined the roles of nucleotides flanking a baculovirus AUG initiator codon in modulating translation initiation in lepidopteran insect cells. The roles of nucleotides flanking the AcMNPV gp64 initiator codon were examined by site-directed mutagenesis and functional assays in transfected Sf9 cells. To eliminate potential cis-acting sequences and effects, the gp64 initiator context was cloned in-frame with a chloramphenicol acetyl transferase reporter gene and under the control of a heterologous promoter. All possible single-nucleotide substitutions were generated in positions -6 to -1 and +4 to +6, relative to the A of the initiator AUG codon, which was designated +1. Constructs were transfected into lepidopteran cells and translation products were quantified by an enzyme-linked immunosorbent assay procedure. Substitutions of pyrimidines or other nucleotides at the -3 position resulted in little or no detectable effect on translation efficiency. In contrast, specific substitutions at the +4 and +5 positions resulted in approximately 2- to 3-fold increases in translation. Substitution of A in the +4 position resulted in an approximately 3-fold increase in translation, and substitution of any nucleotide for T in the +5 position resulted in approximately 1.9- to 2.8-fold increases. Substitutions at other positions (-6 to -1 and +6) resulted in no detectable increase or decrease in translation efficiency. These experimental results suggest an optimal initiator context of 5'-N N N N N N A U G A a/c/g N-3' for efficient translation initiation in lepidopteran cells. Consensus translation initiation contexts were generated from baculovirus genes and lepidopteran genes, then compared with the experimental results from the gp64 initiator context.

The LEF-4 subunit of baculovirus RNA polymerase has RNA 5'-triphosphatase and ATPase activities

The baculovirus Autographa californica nuclear polyhedrosis virus encodes a DNA-dependent RNA polymerase that is required for transcription of viral late genes. This polymerase is composed of four equimolar subunits, LEF-8, LEF-4, LEF-9, and p47. The LEF-4 subunit has guanylyltransferase activity, suggesting that baculoviruses may encode a full complement of capping enzymes. Here we show that LEF-4 is a bifunctional enzyme that hydrolyzes the gamma phosphates of triphosphate-terminated RNA and also hydrolyzes ATP and GTP to the respective diphosphate forms. Alanine substitution of five residues previously shown to be essential for vaccinia virus RNA triphosphatase activity inactivated the triphosphatase component of LEF-4 but not the guanylyltransferase domain. Conversely, mutation of the invariant lysine in the guanylyltransferase domain abolished the guanylyltransferase activity without affecting triphosphatase function. We also investigated the effects of substituting phenylalanine for leucine at position 105, a mutation that results in a virus that is temperature sensitive for late gene expression. We found that this mutation had no significant effect on the ATPase or guanylyltransferase activity of LEF-4 but resulted in a modest decrease in RNA triphosphatase activity.

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.

The p95 gene of Bombyx mori nuclear polyhedrosis virus: temporal expression and functional properties

As part of our effort to identify baculovirus proteins acting as transcriptional regulators, we have characterized a gene, p95, of Bombyx mori nuclear polyhedrosis virus (BmNPV) that encompasses an open reading frame for a putative 95-kDa polypeptide (P95). The N-terminal half of the conceptually translated P95 contains two zinc finger-type DNA-binding motifs, and its C terminus contains a proline-rich region reminiscent of transcriptional activation regions. Northern blot analysis indicates that two mRNA species, 3.5 and 1.7 kb in size, are transcribed from the p95 gene at different times postinfection. These two mRNA species are produced by differential polyadenylation site usage. While the longer transcript can encode the P95 protein, the shorter one may encode a prematurely terminated version of the P95 polypeptide produced by ribosome frameshifting occurring at heptanucleotide "slippage" sites located near the relevant polyadenylation site. Transcription of the p95 gene is initiated at a proximal site located 70 nucleotides upstream of the translation start codon of P95, a middle site located 170 nucleotides from the start codon, and a set of three closely spaced distal sites located 385, 390, and 409 nucleotides from the translation start codon. The middle and distant initiation sites are utilized before and after BmNPV DNA replication, while transcripts initiated at the proximal site occur largely during the late and very late stages of viral infection. Transient-expression assays indicate that P95 can stimulate gene expression driven by the promoter of its own gene and the promoter of the cytoplasmic actin gene of B. mori. The P95-mediated trans activation can be further augmented by BmIE1, an immediate-early gene product of BmNPV. In contrast to the case with the actin promoter, however, the promoter of the p95 gene can be trans activated by the product of its own gene only in the presence of BmIE1. Our data suggest that proteins P95 and BmIE1 of BmNPV and, by analogy, those of other baculoviruses may interact with each other and synergize to potentiate transcription.

Temporal expression of the AcMNPV lef-4 gene and subcellular localization of the protein

Autographa californica nucleopolyhedrovirus (AcMNPV) lef-4 gene [ORF 90; Ayres et al. (1994) Virology 202, 586-605] is involved in both late and very late gene expression [Passarelli and Miller (1993) Virology 197, 704-714]. The transcriptional properties of this gene have been analyzed. It is transcribed as a single 1.6-kb mRNA and transcripts were first detected 3 h postinfection (pi). The extremities of the transcript have been mapped by primer extension and 3' RACE-PCR to positions -56 from the translation start codon and +96 downstream of the stop codon. A rabbit polyclonal antiserum has been raised against an internal polypeptide of LEF-4. A 55-kDa protein was observed by Western blot analysis from 5 h pi. LEF-4 localizes preferentially in the nucleus of infected cells and is associated with the virogenic stroma.

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.

Identification of two independent transcriptional activation domains in the Autographa californica multicapsid nuclear polyhedrosis virus IE1 protein

The Autographa californica multicapsid nuclear polyhedrosis virus immediate-early protein, IE1, is a 582-amino-acid phosphoprotein that regulates the transcription of early viral genes. Deletion of N-terminal regions of IE1 in previous studies (G. R. Kovacs, J. Choi, L. A. Guarino, and M. D. Summers, J. Virol. 66:7429-7437, 1992) resulted in the loss of transcriptional activation, suggesting that this region may contain an acidic activation domain. To identify independently functional transcriptional activation domains, we developed a heterologous system in which potential regulatory domains were fused with a modified Escherichia coli Lac repressor protein that contains a nuclear localization signal (NLacR). Transcriptional activation by the resulting NLacR-IE1 chimeras was measured with a basal baculovirus early promoter containing optimized Lac repressor binding sites (lac operators). Chimeras containing IE1 peptides dramatically activated transcription of the basal promoter only when lac operator sequences were present. In addition, transcriptional activation by NLacR-IE1 chimeras was allosterically regulated by the lactose analog, isopropyl-beta-D-thiogalactopyranoside (IPTG). For a more detailed analysis of IE1 regulatory domains, the M1 to T266 N-terminal portion of IE1 was subdivided (on the basis of average amino acid charge) into five smaller regions which were fused in various combinations to NLacR. Regions M1 to N125 and A168 to G222 were identified as independent transcriptional activation domains. Some NLacR-IE1 chimeras exhibited retarded migration in sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels. As with wild-type IE1, this aberrant gel mobility was associated with phosphorylation. Mapping studies with the NLacR-IE1 chimeras indicate that the M1 to A168 region of IE1 is necessary for this phosphorylation-associated effect.

Baculovirus gp64 gene expression: negative regulation by a minicistron

Small upstream open reading frames (ORFs) or minicistrons located in the 5' leader of eukaryotic mRNAs have been shown to play a role in translational regulation of some eukaryotic genes, particularly mammalian proto-oncogenes. A survey of the baculovirus Autographa californica multicapsid nuclear polyhedrosis virus genome suggests that at least 10 transcripts from late genes contain potential minicistrons, and at least three of these minicistrons appear to be conserved in homologous genes of the related Orygia pseudotsugata MNPV. The position of the minicistron from one of these genes, gp64, is also conserved in gp64 genes from several baculoviruses, suggesting a potential regulatory function. To identify the potential role of the gp64 minicistron in regulating translation from gp64 late mRNAs, we generated a series of recombinant viruses containing the gp64 promoter and minicistron in combination with a chloramphenicol acetyltransferase reporter gene (cat) inserted into the polyhedrin locus. We first fused a cat reporter in frame with the minicistron coding region to demonstrate that the minicistron initiator ATG was in a context suitable for translational initiation. In subsequent experiments, a cat reporter was fused in frame to the downstream gp64 ORF, and various constructs containing point mutations that inactivated the minicistron were examined. Translational efficiency in the presence and absence of the minicistron was measured by quantitative analysis of gp64-cat RNA and the GP64-CAT protein. In the absence of a functional minicistron, translational efficiency from the downstream gp64-cat reporter ORF increased. Surprisingly, single-point mutations that inactivated the minicistron initiator ATG also resulted in utilization of an upstream in-frame ATG that is found within the minicistron coding region and that is in a poor translational initiation context. Double-point mutation constructs that inactivated both the minicistron initiator ATG and the upstream in-frame ATG also resulted in increased translational efficiency from the downstream gp64-cat ORF. Thus, the gp64 minicistron serves as a negative regulatory element that decreases translation of the gp64 ORF on late mRNAs.

Ligand specificity and affinity of BT-R1, the Bacillus thuringiensis Cry1A toxin receptor from Manduca sexta, expressed in mammalian and insect cell cultures

The Manduca sexta receptor for the Bacillus thuringiensis Cry1Aa, Cry1Ab, and Cry1Ac toxins, BT-R1, has been expressed in heterologous cell culture, and its ligand binding characteristics have been determined. When transfected with the BT-R1 cDNA, insect and mammalian cell cultures produce a binding protein of approximately 195 kDa, in contrast to natural BT-R1 from M. sexia, which has an apparent molecular weight of 210 kDa. Transfection of cultured Spodoptera frugiperda cells with the BT-R1 cDNA imparts Cry1A-specific high-affinity binding activity typical of membranes prepared from larval M. sexta midguts. Competition assays with BT-R1 prepared from larval M. sexta midguts and transiently expressed in cell culture reveal virtually identical affinities for the Cry1Aa, Cry1Ab, and Cry1Ac toxins, clearly demonstrating the absolute specificity of the receptor for toxins of the lepidopteran-specific Cry1A family. BT-R1 therefore remains the only M. sexta Cry1A binding protein to be purified, cloned, and functionally expressed in heterologous cell culture, and for the first time, we are able to correlate the Cry1Aa, Cry1Ab, and Cry1Ac toxin sensitivities of M. sexta to the identity and ligand binding characteristics of a single midgut receptor molecule.

Involvement of host factors in transcription from baculovirus very late promoters -- a review

The baculovirus expression vector system has emerged as the system of choice for the expression of a number of heterologous genes of both prokaryotic and eukaryotic origin. This system utilizes the baculovirus very late, hyperactive polyhedrin and p10 promoters to drive the transcription of foreign genes. Regulation of transcription from these promoters is presently not well understood even though a number of viral gene products that may be important for transcription have been identified. Fresh insight into host-virus interactions during baculovirus pathogenesis is now offered by the identification of insect host factors that interact with transcriptionally essential motifs of these promoters as well as cis-acting enhancer-like elements upstream from the promoter.

The GP64 envelope fusion protein is an essential baculovirus protein required for cell-to-cell transmission of infection

To demonstrate the essential nature of the baculovirus GP64 envelope fusion protein (GP64 EFP) and to further examine the role of this protein in infection, we inactivated the gp64 efp gene of Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV) and examined the biological properties of this virus in vivo. To provide GP64 EFP during construction of the recombinant GP64 EFP-null AcMNPV baculovirus, we first generated a stably transfected insect cell line (SfpOP64-6) that constitutively expressed the GP64 EFP of Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus (OpMNPV). The AcMNPV gp64 efp gene was inactivated by inserting the bacterial lacZ gene in frame after codon 131 of the gp64 efp gene. The inactivated gp64 gene was cloned into the AcMNPV viral genome by replacement of the wild-type gp64 efp locus. When propagated in the stably transfected insect cells (Sf9OP64-6 cells), budded virions produced by the recombinant AcMNPV GP64 EFP-null virus (vAc64z) contained OpMNPV GP64 EFP supplied by the Sf9OP64-6 cells. Virions propagated in Sf9OP64-6 cells were capable of infecting wild-type Sf9 cells, and cells infected by vAc64z exhibited a blue phenotype in the presence of X-Gal (5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside). Using cytochemical staining to detect vAc64z infected cells, we demonstrated that this GP64 EFP-null virus is defective in cell-to-cell propagation in cell culture. Although defective in cell-to-cell propagation, vAc64z produces occlusion bodies and infectious occlusion-derived virions within the nucleus. Occlusion bodies collected from cells infected by vAc64z were infectious to midgut epithelial cells of Trichoplusia ni larvae. However, in contrast to infection by a control virus, infection by vAc64z did not proceed into the hemocoel. Analysis of vAc64z occlusion bodies in a standard neonate droplet feeding assay showed no virus-induced mortality, indicating that occluded virions produced from vAc64z could not initiate a productive (lethal) infection in neonate larvae. Thus, GP64 EFP is an essential virion structural protein that is required for propagation of the budded virus from cell to cell and for systemic infection of the host insect.

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.

Nucleotide sequence of a 1446 base pair SalI fragment and structure of a novel early gene of Leucania separata nuclear polyhedrosis virus

A 1446 bp SalI fragment of LsNPV was sequenced by the silver staining method, and two large open reading frames (ORFs, ORF1 and ORF2) were found, both contain typical characteristics of the 5' regulatory elements of baculovirus early genes. ORF1 is 345 bp long with the capacity to encode a putative protein of 114 amino acid residues with MW about 13 kDa and was designated p13 gene, ORF2 comprises 248 bp from the 3' end of the fragment. In the untranslated region (UTR) of ORF1, a 33 bp mini cistron (ORF3), a core recognition sequence (CGTCG) for many bHLHzip transcription factors and a late promoter sequence TTAAG are present. In the UTR of ORF2, two host transcription factor binding elements (CACGTG and GATA motif) and two CGT motifs were found. Some regular leucine zipper-like structures, designated leucine trans-conformation structure and LVT repeat, were found near the N-terminus and the middle of p13 protein. The leucine trans-conformation structure that is near the N-terminus consists of 4 leucines and 7 other amino acids between every two leucines, and every leucine is located at a conformation shift point of the predicted secondary structure of the p13 protein. In LVT repeat, L-6aa-V-6aa-T-6aa is repeated once. The functions of those structures remain unclear, and the two ORFs, not found in the genome of Autographa californica nuclear polyhedrosis virus, are possibly two new genes.

The CAGT motif functions as an initiator element during early transcription of the baculovirus transregulator ie-1

The highly conserved tetranucleotide CAGT is located at the RNA start site of the transregulator gene ie-1 of Autographa californica nuclear polyhedrosis virus (AcMNPV). The presence of this motif within numerous baculovirus early promoters and its similarity to transcriptional initiators suggested a fundamental role in viral transcription regulation. To determine the function of the CAGT motif, site-specific mutations were introduced within the ie-1 promoter fused to a reporter gene within AcMNPV recombinants. In previous studies, deletion of the CAGT motif (nucleotides -1 to +3) and the adjacent downstream activating region (nucleotides +11 to +24) abolished ie-1 transcription. Here, we show that nucleotide replacements within the CAGT motif reduced steady-state levels of ie-1 RNAs from the proper start site (+1), both early and late in infection. These CAGT mutations caused comparable reductions in the yield of ie-1 runoff RNAs from in vitro transcription reactions using nuclear extracts from AcMNPV-infected cells; the CA dinucleotide was most sensitive to substitution. Thus, the CAGT motif affects the rate of ie-1 transcription. Deletions upstream and downstream from the ie-1 RNA start site demonstrated that nucleotides -6 to +11 encompassing the CAGT motif were sufficient for proper transcription in a TATA-independent manner. Nonetheless, additional regulatory elements, which included the ie-1 TATA element, the ie-1 downstream activating region, and a heterologous upstream activating region, stimulated transcription from the motif. Thus, by all criteria examined, the ie-1 CAGT motif functions as a transcriptional initiator by its capacity to determine the position of the RNA start site and to regulate the rate of transcription. These findings suggest that by stimulating early transcription through the recruitment of host factors, the CAGT initiator accelerates expression of viral genes, such as ie-1, that are critical to establishing a productive infection.

Differential transcription of baculovirus late and very late promoters: fractionation of nuclear extracts by phosphocellulose chromatography

An in vitro transcription system based on cytidine-free cassette was developed for the late 39k gene and the very late polyhedrin gene of Autographa californica nuclear polyhedrosis virus (AcNPV). Optimization of transcription conditions revealed that a preincubation step was not required for transcription of late and very late promoters, although preincubation was essential for efficient transcription from an early promoter. The 39k and polyhedrin constructs were actively transcribed by nuclear extracts prepared from AcNPV-infected Spodoptera frugiperda cells at 12 or 36 h postinfection but not by nuclear extracts prepared from uninfected or infected cells harvested during the early phase of infection. Transcription from the very late polyhedrin promoter was fivefold higher than that from the 39k late promoter with the nuclear extract prepared at 36 h postinfection. The 36-h extract was fractionated by phosphocellulose chromatography. Transcription activity eluted in two fractions, at 0.3 and 0.5 M KCl. Both the 39k and polyhedrin constructs were transcribed by these fractions; however, the patterns of late and very late transcription were distinctly different. With the 0.3 M fraction, incorporation into the 39k transcript was approximately 10-fold higher than incorporation into the polyhedrin transcript. Alternatively, with the 0.5 M fraction, transcription of the polyhedrin construct was twofold higher than transcription of the 39k construct. These results indicate that this in vitro system will be useful for purification and identification of factors that discriminate between late and very late promoters.

Overlapping TATA-dependent and TATA-independent early promoter activities in the baculovirus gp64 envelope fusion protein gene

In previous studies to characterize basal and activated transcription from the early promoter of the gp64 envelope fusion protein (efp) gene of the Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus, the TATA box was identified as a functional element, essential for basal transcription from a minimal promoter construct. In the current study, we used discrete deletions and multiple point mutations that removed the functional TATA box from larger promoter constructs of the gp64 efp gene to reveal an overlapping TATA-independent transcriptional activity. TATA-independent transcriptional activity was inhibited in vitro by alpha-amanitin but not by tagetitoxin, demonstrating that like the TATA-dependent activity, the TATA-independent activity is mediated by RNA polymerase II. Using constructs in which the TATA box (TATATAA) was destroyed by substitution mutations, we identified four elements that are required for the TATA-independent activity. Two of the required elements, GATA (at -114) and CACGTG (at -104), were previously shown to specifically bind host transcription factors and activate transcription from the TATA-dependent wild-type gp64 efp promoter. The role of the early start site consensus CAGT sequence in TATA-independent transcription was also examined. Single-nucleotide substitution mutations in the CAGT sequence indicated that certain nucleotides within the CAGT start site were essential. In addition to the start site sequence and two upstream elements, a fourth essential element was identified in the 5' untranslated leader region (5'UTR). While the 5'UTR was not necessary for TATA-dependent transcription, deletion of a 10-bp 5'UTR sequence resulted in the loss of TATA-independent transcriptional activity. Although necessary, neither the GATA, CACGTG, start site region, nor 5'UTR element was alone sufficient for accurately initiated TATA-independent transcription from the consensus CAGT start site. Thus, the gp64 efp early promoter contains overlapping TATA-dependent and TATA-independent transcriptional activities. A number of common sequence elements (GATA, CACGTG, and start site CAGT) are involved in both of these activities, while one element (in the 5'UTR) is required only in the TATA-independent context.

Identification and nucleotide sequence of an ecdysteroid UDP-glucosyltransferase gene of Spodoptera littoralis multicapsid nuclear polyhedrosis virus

The Spodoptera littoralis multicapsid nuclear polyhedrosis virus (SlMNPV) is a member of the Baculoviridae that shows a distant genetic relationship to the prototype Autographa californica MNPV (AcMNPV). Using an AcMNPV gene-specific probe, we identified and mapped an ecdysteroid UDP-glucosyltransferase (egt) gene in the genome of SlMNPV. Sequence determination of a part from the hybridizing DNA fragment revealed an open reading frame of 1548 nucleotides that exhibits 38% and 44% identity to the egt amino acid sequences of AcMNPV and Lymantria dispar MNPV (LdMNPV), respectively. Sequences flanking the SlMNPV egt gene, including the promoter region, were found to be unique to the virus. The presence of this nonstructural gene in SlMNPV and several other baculoviruses points to the importance of egt for the viral infection process.

Early transcription of the ie-1 transregulator gene of Autographa californica nuclear polyhedrosis virus is regulated by DNA sequences within its 5' noncoding leader region

The ie-1 gene of Autographa californica nuclear polyhedrosis virus (AcMNPV) encodes a transregulatory protein (IE1) which accelerates the expression of early and late virus genes. Transcription of ie-1 occurs immediately upon infection from a conserved CAGT motif and continues into the late phases. To examine the mechanisms by which ie-1 expression is regulated, cis-acting control elements within the ie-1 promoter were identified by constructing hybrid early promoters and by using site-directed mutagenesis. The ie-1 upstream activating region, extending from nucleotide -546 to the TATA element at -34, stimulated ie-1 basal promoter activity more than 1,000-fold when transfected into uninfected Spodoptera frugiperda SF21 cells. However, when introduced into the genome of AcMNPV recombinants, the ie-1 upstream activating region had only a minimal twofold effect early in infection. Instead, maximum steady-state levels of early ie-1 RNAs required sequences within the 5' noncoding leader region extending from +11 to +24 relative to the RNA start site (+1). The +11 to +24 noncoding region did not influence the stability of ie-1 transcripts. When assayed by in vitro transcription, deletion of the +11 to +24 region reduced the levels of ie-1 runoff RNAs. Thus, this downstream activating sequence controlled the rate of early ie-1 transcription. A larger overlapping region from +11 to +36 affected steady-state levels of ie-1 RNAs late (24 h) in infection. Deletion of sequences that included the conserved CAGT start site abolished early ie-1 transcription. Thus, ie-1 is the first example of an early baculovirus gene in which essential cis-acting regulatory elements reside within the 5' noncoding region and include sequences comprising the RNA start site.