Characterization of a baculovirus with a deletion of vlf-1

BmHSP19.9 targeting P6.9 and VLF-1 to mediate the formation of defective progeny viruses in the silkworm antiviral variety 871C

The 871C silkworm strain exhibits a high level of resistance to Bombyx mori nucleopolyhedrovirus (BmNPV), making it a valuable variety for the sericulture industry. Understanding the underlying mechanism of its resistance holds great biological significance and economic value in addressing viral disease risks in sericulture. Initially, we infected the resistant strain 871C and its control strain 871 with BmNPV and conducted secondary infection experiments using the progeny occlusion bodies (OBs). As a result, a significant decrease in pathogenicity was observed. Electron microscopy analysis revealed that 871C produces progeny virions with defective DNA packaging, reducing virulence following BmNPV infection. Blood proteomic identification of the silkworm variety 871C and control 871 after BmNPV infection demonstrated the crucial role of the viral proteins P6.9 and VLF-1 in the production of defective viruses by impeding the proper encapsulation of viral DNA. Additionally, we discovered that BmHSP19.9 interacts with P6.9 and VLF-1 and that its expression is significantly upregulated after BmNPV infection. BmHSP19.9 exhibits strong antiviral activity, in part by preventing the entry of the proteins P6.9 and VLF-1 into the nucleus, thereby hindering viral nucleocapsid and viral DNA assembly. Our findings indicate that the antiviral silkworm strain 871C inhibits BmNPV proliferation by upregulating Bmhsp19.9 and impeding the nuclear localization of the viral proteins P6.9 and VLF-1, leading to the production of defective viral particles. This study offers a comprehensive analysis of the antiviral mechanism in silkworms from a viral perspective, providing a crucial theoretical foundation for future antiviral research and the breeding of resistant silkworm strains.

Functional characterization of Microplitis demolitor bracovirus genes that encode nucleocapsid components

IMPORTANCE

Understanding how bracoviruses (BVs) function in wasps is of broad interest in the study of virus evolution. This study characterizes most of the Microplitis demolitor bracovirus (MdBV) genes whose products are nucleocapsid components. Results indicate several genes unknown outside of nudiviruses and BVs are essential for normal capsid assembly. Results also indicate most MdBV tyrosine recombinase family members and the DNA binding protein p6.9-1 are required for DNA processing and packaging into nucleocapsids.

Architecture of the baculovirus nucleocapsid revealed by cryo-EM

Baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) has been widely used as a bioinsecticide and a protein expression vector. Despite their importance, very little is known about the structure of most baculovirus proteins. Here, we show a 3.2 Å resolution structure of helical cylindrical body of the AcMNPV nucleocapsid, composed of VP39, as well as 4.3 Å resolution structures of both the head and the base of the nucleocapsid composed of over 100 protein subunits. AcMNPV VP39 demonstrates some features of the HK97-like fold and utilizes disulfide-bonds and a set of interactions at its C-termini to mediate nucleocapsid assembly and stability. At both ends of the nucleocapsid, the VP39 cylinder is constricted by an outer shell ring composed of proteins AC104, AC142 and AC109. AC101(BV/ODV-C42) and AC144(ODV-EC27) form a C14 symmetric inner layer at both capsid head and base. In the base, these proteins interact with a 7-fold symmetric capsid plug, while a portal-like structure is seen in the central portion of head. Additionally, we propose an application of AlphaFold2 for model building in intermediate resolution density.

A sensitive assay for scrutiny of Autographa californica multiple nucleopolyhedrovirus genes using CRISPR-Cas9

Baculoviruses have very large genomes and previous studies have demonstrated improvements in recombinant protein production and genome stability through the removal of some nonessential sequences. However, recombinant baculovirus expression vectors (rBEVs) in widespread use remain virtually unmodified. Traditional approaches for generating knockout viruses (KOVs) require several experimental steps to remove the target gene prior to the generation of the virus. In order to optimize rBEV genomes by removing nonessential sequences, more efficient techniques for establishing and evaluating KOVs are required. Here, we have developed a sensitive assay utilizing CRISPR-Cas9-mediated gene targeting to examine the phenotypic impact of disruption of endogenous Autographa californica multiple nucleopolyhedrovirus (AcMNPV) genes. For validation, 13 AcMNPV genes were targeted for disruption and evaluated for the production of GFP and progeny virus - traits that are essential for their use as vectors for recombinant protein production. The assay involves transfection of sgRNA into a Cas9-expressing Sf9 cell line followed by infection with a baculovirus vector carrying the gfp gene under the p10 or p6.9 promoters. This assay represents an efficient strategy for scrutinizing AcMNPV gene function through targeted disruption, and represents a valuable tool for developing an optimized rBEV genome. KEY POINTS: [Formula: see text] A method to scrutinize the essentiality of baculovirus genes was developed. [Formula: see text] The method uses Sf9-Cas9 cells, a targeting plasmid carrying a sgRNA, and a rBEV-GFP. [Formula: see text] The method allows scrutiny by only needing to modify the targeting sgRNA plasmid.

Comparison of CRISPR-Cas9 Tools for Transcriptional Repression and Gene Disruption in the BEVS

The generation of knock-out viruses using recombineering of bacmids has greatly accelerated scrutiny of baculovirus genes for a variety of applications. However, the CRISPR-Cas9 system is a powerful tool that simplifies sequence-specific genome editing and effective transcriptional regulation of genes compared to traditional recombineering and RNAi approaches. Here, the effectiveness of the CRISPR-Cas9 system for gene disruption and transcriptional repression in the BEVS was compared. Cell lines constitutively expressing the cas9 or dcas9 gene were developed, and recombinant baculoviruses delivering the sgRNA were evaluated for disruption or repression of a reporter green fluorescent protein gene. Finally, endogenous AcMNPV genes were targeted for disruption or downregulation to affect gene expression and baculovirus replication. This study provides a proof-of-concept that CRISPR-Cas9 technology may be an effective tool for efficient scrutiny of baculovirus genes through targeted gene disruption and transcriptional repression.

3H-31, A Non-structural Protein of Heliothis virescens ascovirus 3h, Inhibits the Host Larval Cathepsin and Chitinase Activities

3h-31 of Heliothis virescens ascovirus 3h (HvAV-3h) is a highly conserved gene of ascoviruses. As an early gene of HvAV-3h, 3h-31 codes for a non-structural protein (3H-31) of HvAV-3h. In the study, 3h-31 was initially transcribed and expressed at 3 h post-infection (hpi) in the infected Spodoptera exigua fat body cells (SeFB). 3h-31 was further inserted into the bacmid of Autographa californica nucleopolyhedrovirus (AcMNPV) to generate an infectious baculovirus (AcMNPV-31). In vivo experiments showed that budded virus production and viral DNA replication decreased with the expression of 3H-31, and lucent tubular structures were found around the virogenic stroma in the AcMNPV-31-infected SeFB cells. In vivo, both LD₅₀ and LD₉₀ values of AcMNPV-31 were significantly higher than those of the wild-type AcMNPV (AcMNPV-wt) in third instar S. exigua larvae. An interesting finding was that the liquefaction of the larvae killed by the infection of AcMNPV-31 was delayed. Chitinase and cathepsin activities of AcMNPV-31-infected larvae were significantly lower than those of AcMNPV-wt-infected larvae. The possible regulatory function of the chitinase and cathepsin for 3H-31 was further confirmed by RNAi, which showed that larval cathepsin activity was significantly upregulated, but chitinase activity was not significantly changed due to the RNAi of 3h-31. Based on the obtained results, we assumed that the function of 3H-31 was associated with the inhibition of host larval chitinase and cathepsin activities, so as to restrain the hosts in their larval stages.

Nucleocapsid Assembly of Baculoviruses

The baculovirus nucleocapsid is formed through a rod-like capsid encapsulating a genomic DNA molecule of 80~180 kbp. The viral capsid is a large oligomer composed of many copies of various protein subunits. The assembly of viral capsids is a complex oligomerization process. The timing of expression of nucleocapsid-related proteins, transport pathways, and their interactions can affect the assembly process of preformed capsids. In addition, the selection of viral DNA and the injection of the viral genome into empty capsids are the critical steps in nucleocapsid assembly. This paper reviews the replication and recombination of baculovirus DNA, expression and transport of capsid proteins, formation of preformed capsids, DNA encapsulation, and nucleocapsid formation. This review will provide a basis for further study of the nucleocapsid assembly mechanism of baculovirus.

Sequential deletion of AcMNPV homologous regions leads to reductions in budded virus production and late protein expression

Homologous regions (hrs) have been predicted to act as origins of baculovirus DNA replication. Hrs have also been shown to function as enhancers of virus transcription. Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) carries eight hrs. In order to assess the role of hrs in virus replication in vivo, we applied a two-step RED recombination system for site-specific mutagenesis to sequentially delete each hr from a bacmid copy of AcMNPV. We then characterized the ability of the bacmids carrying different numbers of hrs or no hr to produce polyhedra and budded virus in transfected cells. We also investigated the ability of virus supernatants from transfected cells to produce budded virus and polyhedra when used to infect cells. We also characterized the expression of specific early and late virus proteins in transfected cells. The results demonstrated that removal of five hrs had little or no effect on virus infection but deleting all eight hrs compromised budded virus production and delayed early and late gene expression but did not completely eliminate assembly of infectious virus. We conclude that multiple hrs ensure an effective virus infection cycle with production of high titers of budded virus and polyhedra.

Autographa californica multiple nucleopolyhedrovirus ac75 is required for egress of nucleocapsids from the nucleus and formation of de novo intranuclear membrane microvesicles

In this study, Autographa californica multiple nucleopolyhedrovirus ac75 was functionally characterized. Ac75 has homologs in all sequenced genomes of alphabaculoviruses, betabaculoviruses, and gammabaculoviruses. It was determined to encode a protein that is associated with the nucleocapsid of budded virus and with both envelope and nucleocapsids of occlusion-derived virus. Sf9 cells transfected by an ac75-knockout bacmid resulted in the infection being restricted to single cells. No budded virus were detected although viral DNA replication and late gene expression were unaffected. Electron microscopy revealed that the virogenic stroma, nucleocapsids and occlusion bodies appeared normal in the cells transfected by an ac75-knockout bacmid. However, the nucleocapsids were unenveloped, the occlusion bodies did not contain any virions or nucleocapsids, and no nucleocapsids were found outside the nucleus or spanning the nuclear membrane. In addition, de novo intranuclear membrane microvesicles that are the precursor of occlusion-derived virus envelopes were absent in the nuclei of transfected cells. Confocal microscopy showed that AC75 protein appeared in the cytoplasm as early as 6 hours post infection. It localized to the ring zone at the periphery of the nucleus from 15 to 24 hours post infection and demonstrated light blocky cloud-like distribution in the center of the nucleus. AC75 was found to co-immunoprecipitate with BV and ODV associated envelope protein ODV-E25. The data from this study suggest that ac75 is essential for induction of the intranuclear membrane microvesicles, it appears to be required for the intranuclear envelopment of nucleocapsids, and is also essential for egress of nucleocapsids from the nuclei, in infected cells.

The Autographa californica Multiple Nucleopolyhedrovirus ac83 Gene Contains a cis-Acting Element That Is Essential for Nucleocapsid Assembly

Baculoviridae is a family of insect-specific viruses that have a circular double-stranded DNA genome packaged within a rod-shaped capsid. The mechanism of baculovirus nucleocapsid assembly remains unclear. Previous studies have shown that deletion of the ac83 gene of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) blocks viral nucleocapsid assembly. Interestingly, the ac83-encoded protein Ac83 is not a component of the nucleocapsid, implying a particular role for ac83 in nucleocapsid assembly that may be independent of its protein product. To examine this possibility, Ac83 synthesis was disrupted by insertion of a chloramphenicol resistance gene into its coding sequence or by deleting its promoter and translation start codon. Both mutants produced progeny viruses normally, indicating that the Ac83 protein is not required for nucleocapsid assembly. Subsequently, complementation assays showed that the production of progeny viruses required the presence of ac83 in the AcMNPV genome instead of its presence in trans Therefore, we reasoned that ac83 is involved in nucleocapsid assembly via an internal cis-acting element, which we named the nucleocapsid assembly-essential element (NAE). The NAE was identified to lie within nucleotides 1651 to 1850 of ac83 and had 8 conserved A/T-rich regions. Sequences homologous to the NAE were found only in alphabaculoviruses and have a conserved positional relationship with another essential cis-acting element that was recently identified. The identification of the NAE may help to connect the data of viral cis-acting elements and related proteins in the baculovirus nucleocapsid assembly, which is important for elucidating DNA-protein interaction events during this process.IMPORTANCE Virus nucleocapsid assembly usually requires specific cis-acting elements in the viral genome for various processes, such as the selection of the viral genome from the cellular nucleic acids, the cleavage of concatemeric viral genome replication intermediates, and the encapsidation of the viral genome into procapsids. In linear DNA viruses, such elements generally locate at the ends of the viral genome; however, most of these elements remain unidentified in circular DNA viruses (including baculovirus) due to their circular genomic conformation. Here, we identified a nucleocapsid assembly-essential element in the AcMNPV (the archetype of baculovirus) genome. This finding provides an important reference for studies of nucleocapsid assembly-related elements in baculoviruses and other circular DNA viruses. Moreover, as most of the previous studies of baculovirus nucleocapsid assembly have been focused on viral proteins, our study provides a novel entry point to investigate this mechanism via cis-acting elements in the viral genome.

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.

Bombyx mori nucleopolyhedrovirus (BmNPV) Bm64 is required for BV production and per os infection

Background

Bombyx mori nucleopolyhedrovirus (BmNPV) orf64 (Bm64, a homologue of ac78) is a core baculovirus gene. Recently, Li et al. reported that Ac78 was not essential for budded viruses (BVs) production and occlusion-derived viruses (ODVs) formation (Virus Res 191:70–82, 2014). Conversely, Tao et al. demonstrated that Ac78 was localized to the BV and ODV envelopes and was required for BV production and ODV formation (J Virol 87:8441–50, 2013). In this study, the function of Bm64 was characterized to determine the role of Bm64 in the BmNPV infection cycle.

Method

The temporal expression of Bm64 was examined using total RNA extracted from BmNPV-infected BmN cells at different time points by reverse-transcription PCR (RT-PCR) and 5’ RACE analysis. To determine the functions of Bm64 in viral replication and the viral phenotype throughout the viral life cycle, a deletion virus (vBm^64KO) was generated via homologous recombination in Escherichia coli. Viral replication and BV production were determined by real-time PCR. Electron microscopy was used to detect virion morphogenesis. The subcellular localization of Bm64 was determined by microscopy, and per os infectivity was used to determine its role in the baculovirus oral infection cycle.

Results

Viral plaque and titer assay results showed that a few infectious BVs were produced by vBm^64KO, suggesting that deletion of Bm64 affected BV production. Viral DNA replication was detected and polyhedra were observed in vBm^64KO-transfected cells. Microscopy analysis revealed that Bm64 was predominantly localized to the ring zone of the nuclei during the infection cycle. Electron microscopy showed that Bm64 was not essential for the formation of ODVs or the subsequent occlusion of ODV into polyhedra. The per os infectivity results showed that the polyhedra of vBm^64KO were unable to infect silkworm larvae.

Conclusion

In conclusion, our results suggest that Bm64 plays an important role in BV production and per os infection, but is not required for viral DNA replication or ODV maturation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-015-0399-9) contains supplementary material, which is available to authorized users.

Proteomic analysis of the occlusion-derived virus of Clostera anachoreta granulovirus

To date, proteomic studies have been performed on occlusion-derived viruses (ODVs) from five members of the family Baculoviridae, genus Alphabaculovirus, but only a single member of the genus Betabaculovirus (Pieris rapae granulovirus). In this study, LC-MS/MS was used to analyse the ODV proteins of Clostera anachoreta granulovirus (ClanGV), another member of the genus Betabaculovirus. The results indicated that 73 proteins, including the products of 27 baculovirus core genes, were present in ClanGV ODVs. This is the largest number of ODV proteins identified in baculoviruses to date. To the best of our knowledge, 24 of these proteins were newly identified as ODV-associated proteins. Twelve of the proteins were shared by all seven of the other baculoviruses that have been analysed by proteomic techniques, including P49, PIF-2, ODV-EC43, P74, P6.9, P33, VP39, ODV-EC27, VP91, GP41, VLF-1 and VP1054. ClanGV shared between 20 and 36 ODV proteins with each of the other six baculoviruses that have been analysed by proteomics. Ten proteins were identified only as ODV components of ClanGV and PrGV: Clan22, Clan27, Clan69, Clan83, Clan84, Clan90, Clan116, Clan94, FGF-3 and ME53, the first seven of which were encoded by betabaculovirus-specific genes. These findings may provide novel insights into baculovirus structure as well as reveal similarities and differences between alphabaculoviruses and betabaculoviruses.

Analysis of BmNPV orf101 disruption: orf101 is essential for mediating budded virus production

In our previous study, Orf101 (Bm101) of Bombyx mori nucleopolyhedrovirus (BmNPV) was identified as a component of the budded virions important for viral late gene expression. In this study we demonstrate that Bm101 is actually a previously unrecognized core gene and that it is essential for mediating budded virus production. To determine the role of Bm101 in the baculovirus life cycle, a Bm101 knockout bacmid containing the BmNPV genome was generated through homologous recombination in Escherichia coli. Furthermore, a Bm101 repair bacmid was constructed by transposing the Bm101 open reading frame with its native promoter region into the polyhedrin locus of the Bm101 knockout bacmid. Bacmid DNA transfection assay revealed that the Bm101 knockout bacmid was unable to produce the infectious budded virus, while the Bm101 repair bacmid rescued this defect, allowing budded-virus titers to reach wild-type levels. Real time PCR analysis indicated that the viral DNA genome in the absence of Bm101 was unaffected in the first 24 h p.t. Thus, studies of a Bm101-null BACmid indicate that Bm101 is required for viral DNA replication during the infection cycle.

Autographa californica multiple nucleopolyhedrovirus orf132 encodes a nucleocapsid-associated protein required for budded-virus and multiply enveloped occlusion-derived virus production

Autographa californica multiple nucleopolyhedrovirus orf132 (named ac132) has homologs in all genome-sequenced group I nucleopolyhedroviruses. Its role in the viral replication cycle is unknown. In this study, ac132 was shown to express a protein of around 28 kDa, which was determined to be associated with the nucleocapsids of both occlusion-derived virus and budded virus. Confocal microscopy showed that AC132 protein appeared in central region of the nucleus as early as 12 h postinfection with the virus. It formed a ring zone at the periphery of the nucleus by 24 h postinfection. To investigate its role in virus replication, ac132 was deleted from the viral genome by using a bacmid system. In the Sf9 cell culture transfected by the ac132 knockout bacmid, infection was restricted to single cells, and the titer of infectious budded virus was reduced to an undetectable level. However, viral DNA replication and the expression of late genes vp39 and odv-e25 and a reporter gene under the control of the very late gene p10 promoter were unaffected. Electron microscopy showed that nucleocapsids, virions, and occlusion bodies were synthesized in the cells transfected by an ac132 knockout bacmid, but the formation of the virogenic stroma and occlusion bodies was delayed, the numbers of enveloped nucleocapsids were reduced, and the occlusion bodies contained mainly singly enveloped nucleocapsids. AC132 was found to interact with envelope protein ODV-E18 and the viral DNA-binding protein P6.9. The data from this study suggest that ac132 possibly plays an important role in the assembly and envelopment of nucleocapsids.

IMPORTANCE

To our knowledge, this is the first report on a functional analysis of ac132. The data presented here demonstrate that ac132 is required for production of the budded virus and multiply enveloped occlusion-derived virus of Autographa californica multiple nucleopolyhedrovirus. This article reveals unique phenotypic changes induced by ac132 deletion on the virus and multiple new findings on ac132.

Identification of a conserved non-protein-coding genomic element that plays an essential role in Alphabaculovirus pathogenesis

Highly homologous sequences 154-157 bp in length grouped under the name of "conserved non-protein-coding element" (CNE) were revealed in all of the sequenced genomes of baculoviruses belonging to the genus Alphabaculovirus. A CNE alignment led to the detection of a set of highly conserved nucleotide clusters that occupy strictly conserved positions in the CNE sequence. The significant length of the CNE and conservation of both its length and cluster architecture were identified as a combination of characteristics that make this CNE different from known viral non-coding functional sequences. The essential role of the CNE in the Alphabaculovirus life cycle was demonstrated through the use of a CNE-knockout Autographa californica multiple nucleopolyhedrovirus (AcMNPV) bacmid. It was shown that the essential function of the CNE was not mediated by the presumed expression activities of the protein- and non-protein-coding genes that overlap the AcMNPV CNE. On the basis of the presented data, the AcMNPV CNE was categorized as a complex-structured, polyfunctional genomic element involved in an essential DNA transaction that is associated with an undefined function of the baculovirus genome.

Effects of Early or Overexpression of the Autographa californica Multiple Nucleopolyhedrovirus orf94 (ODV-e25) on Virus Replication

odv-e25(e25) is one of the core genes of baculoviruses. To investigate how it functions in the replication cycle of a baculovirus, a number of Autographa californica multiple nucleopolyhedrovirus recombinants with e25 under control of the promoter of immediate early gene ie1, or the promoter of the very late hyperexpressed gene p10, were constructed using a bacmid system, and the effects of early expression or overexpression of e25 on replication of the virus were evaluated. Microscopy and titration assays demonstrated that bacmids with e25 under control of ie1 promoter were unable to produce budded viruses; and that the recombinant viruses with e25 under control of p10 promoter generated budded virus normally, but formation of occlusion bodies were dramatically reduced and delayed in the infected cells. Electron microscopy showed that there were no mature virions or intact nucleocapsids present in the cells transfected with a recombinant bacmid with e25 under control of ie1 promoter. Quantitative real-time PCR analysis demonstrated that alteration of the e25 promoter did not affect viral DNA synthesis. The reporter gene expression from the promoter of the major capsid protein gene vp39 was reduced 63% by early expression of e25. Confocal microscopy revealed that E25 was predominantly localized in nuclei by 24 hours post infection with wild-type virus, but it remained in the cytoplasm in the cells transfected with a recombinant bacmid with e25 under control of the ie1 promoter, suggesting that the transport of E25 into nuclei was regulated in a specific and strict time dependent manner.

Autographa californica multiple nucleopolyhedrovirus PK-1 is essential for nucleocapsid assembly

PK-1 (Ac10) is a baculovirus-encoded serine/threonine kinase and its function is unclear. Our results showed that a pk-1 knockout AcMNPV failed to produce infectious progeny, while the pk-1 repair virus could rescue this defect. qPCR analysis demonstrated that pk-1 deletion did not affect viral DNA replication. Analysis of the repaired recombinants with truncated pk-1 mutants demonstrated that the catalytic domain of protein kinases of PK-1 was essential to viral infectivity. Moreover, those PK-1 mutants that could rescue the infectious BV production defect exhibited kinase activity in vitro. Therefore, it is suggested that the kinase activity of PK-1 is essential in regulating viral propagation. Electron microscopy revealed that pk-1 deletion affected the formation of normal nucleocapsids. Masses of electron-lucent tubular structures were present in cell transfected with pk-1 knockout bacmid. Therefore, PK-1 appears to phosphorylate some viral or cellular proteins that are essential for DNA packaging to regulate nucleocapsid assembly.

The Autographa californica multiple nucleopolyhedrovirus ORF78 is essential for budded virus production and general occlusion body formation

ORF78 (ac78) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a baculovirus core gene of unknown function. To determine the role of ac78 in the baculovirus life cycle, an AcMNPV mutant with ac78 deleted, Ac78KO, was constructed. Quantitative PCR analysis revealed that ac78 is a late gene in the viral life cycle. After transfection into Spodoptera frugiperda cells, Ac78KO produced a single-cell infection phenotype, indicating that no infectious budded viruses (BVs) were produced. The defect in BV production was also confirmed by both viral titration and Western blotting. However, viral DNA replication was unaffected, and occlusion bodies were formed. An analysis of BVs and occlusion-derived viruses (ODVs) revealed that AC78 is associated with both forms of the virions and is an envelope structural protein. Electron microscopy revealed that AC78 also plays an important role in the embedding of ODV into the occlusion body. The results of this study demonstrate that AC78 is a late virion-associated protein and is essential for the viral life cycle.

The formation of occlusion-derived virus is affected by the expression level of ODV-E25

Odv-e25 is a core gene of baculoviruses and encodes a 25.5 kDa protein located on both budded virus (BV) and occlusion-derived virus (ODV). Our previous study demonstrated that ODV-E25 was required for the formation of intranuclear microvesicles and ODV, and an odv-e25 deletion mutant could be rescued by re-expression of odv-e25 under its native promoter. To investigate the functions of ODV-E25 expression level on ODV formation, the promoter of ie-1 (pIE1), the odv-e25 native promoter, and the polyhedrin promoter (pPH) were used to direct odv-e25 expression. Our results showed that the production of ODV-E25 under its native promoter was higher than that under pIE1 but lower than that under pPH. Viral DNA replication and budded viruses (BVs) production showed that expression of odv-e25 under pIE1 and pPH could not completely repair the defects caused by the deletion of ODV-E25, while expression under its native promoter did. Electron microscopy showed that intranuclear microvesicles were found in all the constructs transfected cells except the odv-e25-null virus. However, mature ODVs only were detected in cells transfected with virus in which odv-e25 was expressed under its native or polyhedrin promoter. These results indicated that the formation occlusion-derived virus was affected by the expression level of ODV-E25.

Reduced expression of Autographa californica nucleopolyhedrovirus ORF34, an essential gene, enhances heterologous gene expression

Autographa californica multiple nucleopolyhedrovirus ORF34 is part of a transcriptional unit that includes ORF32, encoding a viral fibroblast growth factor (FGF) and ORF33. We identified ORF34 as a candidate for deletion to improve protein expression in the baculovirus expression system based on enhanced reporter gene expression in an RNAi screen of virus genes. However, ORF34 was shown to be an essential gene. To explore ORF34 function, deletion (KO34) and rescue bacmids were constructed and characterized. Infection did not spread from primary KO34 transfected cells and supernatants from KO34 transfected cells could not infect fresh Sf21 cells whereas the supernatant from the rescue bacmids transfection could recover the infection. In addition, budded viruses were not observed in KO34 transfected cells by electron microscopy, nor were viral proteins detected from the transfection supernatants by western blots. These demonstrate that ORF34 is an essential gene with a possible role in infectious virus production.

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.

Characterization of Bombyx mori nucleopolyhedrovirus with a knockout of Bm17

Open reading frame 17 (Bm17) gene of Bombyx mori nucleopolyhedrovirus is a highly conserved gene in lepidopteran nucleopolyhedroviruses, but its function remains unknown. In this report, transient-expression and superinfection assays indicated that BM17 localized in the nucleus and cytoplasm of infected BmN cells. To determine the role of Bm17 in baculovirus life cycle, we constructed a Bm17 knockout virus and characterized its properties in cells. Analysis of the production and infection of budded virions, the level of viral DNA replication revealed showed that there was no significant difference among the mutant, the control, and the Bm17 repaired virus strains. These results suggest that BM17 is not essential for virus replication in cultured cells.

Analysis of the autographa californica multiple nucleopolyhedrovirus overlapping gene pair lef3 and ac68 reveals that AC68 is a per os infectivity factor and that LEF3 is critical, but not essential, for virus replication

Autographa californica multiple nucleopolyhedrovirus ac68 is a core gene that overlaps lef3 which encodes the single-stranded DNA binding protein. A knockout (KO) virus lacking both lef3 and ac68 was generated (lef3-ac68 2×KO) to enable the functional study of ac68. To produce an ac68KO virus that did not impact lef3 expression, the lef3-ac68 2×KO virus was repaired with a DNA fragment containing lef3 and ac68, in which ac68 contained point mutations so that only LEF3 was expressed. Repair of lef3-ac68 2×KO with just ac68 generated an lef3KO virus. Analysis of the ac68KO virus showed that viral DNA replication and budded virus (BV) levels were unaffected compared to levels in the double-repair or wild-type (WT) control virus. Bioassay analyses of Trichoplusia ni larvae injected with BV directly into the hemolymph, bypassing the gut, showed no difference in mortality rates between the ac68KO and the WT viruses. However, in oral bioassays the ac68KO occlusion bodies failed to kill larvae. These results show that the core gene ac68 encodes a per os infectivity factor (pif6). The lef3KO virus was also analyzed, and virus replication was drastically reduced compared to WT virus, but very low levels of lef3KO virus DNA replication and BV production could be detected. In addition, in transfected cells P143 was transported to the nucleus in the absence of LEF3. This study therefore shows for the first time that even though the loss of LEF3 severely impairs virus replication, it is not absolutely essential for P143 nuclear import or viral replication.

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.

Enhanced gene expression in insect cells and silkworm larva by modified polyhedrin promoter using repeated Burst sequence and very late transcriptional factor-1

The Burst of expression from polyhedrin (polh) promoter during very late phase of baculovirus infection requires a sequence located between TAAG and the translation initiation site, typically referred to as burst sequence (BS). The expression of polh promoter is stimulated by specifically binding of very late transcriptional factor 1 (VLF-1) to BS. In order to enhance the production of recombinant proteins the polh promoter was modified via a multiple BS bacmid system in which the number of BSs was increased. Compared to an expression from a normal polh promoter, β-glucuronidase (GUS) activity in High Five insect cells was three times higher with a modified polh promoter containing two BSs. Using a modified polh promoter that contains nine BSs in silkworm expression system, β1-3-N-acetylglucosaminyltransferase 2 (β3GnT2) activity per larva was 6.8-fold higher than control. Furthermore, the co-expression of modified promoters along with VLF-1-enhanced β3GnT activity. Thus, an increased optimal number of BS and its co-expression with VLF-1 leads to the production of higher level of gene expression in insect cells and silkworm larvae. This new modified promoter engineered in the current study is the strongest promoter for overexpressing foreign proteins in an eukaryotic cell and system, thus leading a progress in baculovirus-insect cell and silkworm biotechnology.

Deletion of AcMNPV AC16 and AC17 results in delayed viral gene expression in budded virus infected cells but not transfected cells

This study investigated the combined function of the Autographa californica multiple nucleopolyhedrovirus overlapping genes ac16 (BV/ODV-E26, DA26) and ac17. Ac17 is a late gene and the promoter is within the ac16 open reading frame. A double ac16-ac17 knockout virus was generated to assess the function of each gene independently or together. Loss of ac17 did not affect viral DNA synthesis but budded virus (BV) production was reduced. Deletion of both ac16-ac17 resulted in reduced viral DNA synthesis and a further reduction in BV production. In BV infected Sf9 cells, viral gene expression was delayed up to 12 h in the absence of both AC16 and AC17 but not if either gene was present. Cells infected by transfecting viral DNA, by-passing the BV particle, exhibited no delay in gene expression from the double knockout virus. AC16 and AC17 are therefore required for rapid viral gene expression in cells infected by BV.

Autographa californica multiple nucleopolyhedrovirus LEF-2 is a capsid protein required for amplification but not initiation of viral DNA replication

The late expression factor 2 gene (lef-2) of baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) has been identified as one of the factors essential for origin-dependent DNA replication in transient expression assays and has been shown to be involved in late/very late gene expression. To study the function of lef-2 in the life cycle of AcMNPV, lef-2 knockout and repair bacmids were generated by homologous recombination in Escherichia coli. Growth curve analysis showed that lef-2 was essential for virus production. Interestingly, a DNA replication assay indicated that lef-2 is not required for the initiation of viral DNA replication and that, rather, it is required for the amplification of DNA replication. lef-2 is also required for the expression of late and very late genes, as the expression of these genes was abolished by lef-2 deletion. Temporal and spatial distributions of LEF-2 protein in infected cells were also analyzed, and the data showed that LEF-2 protein was localized to the virogenic stroma in the nuclei of the infected cells. Analysis of purified virus particles revealed that LEF-2 is a viral protein component of both budded and occlusion-derived virions, predominantly in the nucleocapsids of the virus particles. This observation suggests that LEF-2 may be required immediately after virus entry into host cells for efficient viral DNA replication.

Polydnavirus hidden face: the genes producing virus particles of parasitic wasps

Very few obligatory relationships involve viruses to the remarkable exception of polydnaviruses (PDVs) associated with tens of thousands species of parasitic wasps that develop within the body of lepidopteran larvae. PDV particles, injected along with parasite eggs into the host body, act by manipulating host immune defences, development and physiology, thereby enabling wasp larvae to survive in a potentially harmful environment. Particle production does not occur in infected tissues of parasitized caterpillars, but is restricted to specialized cells of the wasp ovaries. Moreover, the genome enclosed in the particles encodes almost no viral structural protein, but mostly factors used to manipulate the physiology of the parasitized host. We recently unravelled the viral nature of PDVs associated with braconid wasps by characterizing a large set of nudivirus genes residing permanently in the wasp chromosome(s). Many of these genes encode structural components of the bracovirus particles and their expression pattern correlates with particle production. They constitute a viral machinery comprising a large number of core genes shared by nudiviruses and baculoviruses. Thus bracoviruses do not appear to be nudiviruses remnants, but instead complex nudiviral devices carrying DNA for the delivery of virulence genes into lepidopteran hosts. This highlights the fact that viruses should no longer be exclusively considered obligatory parasites, and that in certain cases they are obligatory symbionts.

ac109 is required for the nucleocapsid assembly of Autographa californica multiple nucleopolyhedrovirus

ORF109 (Ac109) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a highly conserved gene in all sequenced baculovirus genomes, but its function is not known. This paper describes generation of an ac109 knockout virus (Ac-ac109-KO-GP) and analyses of the influence of ac109 deletion on the virus replication in Sf-9 cells so as to investigate the role of ac109 in the viral life cycle. Results revealed that budded virus (BV) yields and occlusion body synthesis were completely blocked in cells infected with the mutant virus. Electron microscopy demonstrated that ac109 deletion blocked nucleocapsid formation, though infection was initiated and electron-dense bodies associated with the virogenic stroma appeared. The mutant phenotype was rescued by an ac109 rescue virus. On the other hand, real-time PCR analysis indicated that ac109 is not required for viral DNA replication. Thus, these results suggested that ac109 plays an important role in AcMNPV nucleocapsid formation.

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.

Autographa californica multiple nucleopolyhedrovirus 38K is a novel nucleocapsid protein that interacts with VP1054, VP39, VP80, and itself

It has been shown that the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) 38K (ac98) is required for nucleocapsid assembly. However, the exact role of 38K in nucleocapsid assembly remains unknown. In the present study, we investigated the relationship between 38K and the nucleocapsid. Western blotting using polyclonal antibodies raised against 38K revealed that 38K was expressed in the late phase of infection in AcMNPV-infected Spodoptera frugiperda cells and copurified with budded virus (BV) and occlusion-derived virus (ODV). Biochemical fractionation of BV and ODV into the nucleocapsid and envelope components followed by Western blotting showed that 38K was associated with the nucleocapsids. Immunoelectron microscopic analysis revealed that 38K was specifically localized to the nucleocapsids in infected cells and appeared to be distributed over the cylindrical capsid sheath of nucleocapsid. Yeast two-hybrid assays were performed to examine potential interactions between 38K and nine known nucleocapsid shell-associated proteins (PP78/83, PCNA, VP1054, FP25, VLF-1, VP39, BV/ODV-C42, VP80, and P24), three non-nucleocapsid shell-associated proteins (P6.9, PP31, and BV/ODV-E26), and itself. The results revealed that 38K interacted with the nucleocapsid proteins VP1054, VP39, VP80, and 38K itself. These interactions were confirmed by coimmunoprecipitation assays in vivo. These data demonstrate that 38K is a novel nucleocapsid protein and provide a rationale for why 38K is essential for nucleocapsid assembly.

Autographa californica multiple nucleopolyhedrovirus ac53 plays a role in nucleocapsid assembly

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf53 (ac53) is a highly conserved gene existing in all sequenced Lepidoptera and Hymenoptera baculoviruses, but its function remains unknown. To investigate its role in the baculovirus life cycle, an ac53 deletion virus (vAc(ac53KO-PH-GFP)) was generated through homologous recombination in Escherichia coli. Fluorescence and light microscopy and titration analysis revealed that vAc(ac53KO-PH-GFP) could not produce infectious budded virus in infected Sf9 cells. Real-time PCR demonstrated that the ac53 deletion did not affect the levels of viral DNA replication. Electron microscopy showed that many lucent tubular shells devoid of the nucleoprotein core are present in the virogenic stroma and ring zone, indicating that the ac53 knockout affected nucleocapsid assembly. With a recombinant virus expressing an Ac53-GFP fusion protein, we observed that Ac53 was distributed within the cytoplasm and nucleus at 24 h post-infection, but afterwards accumulated predominantly near the nucleus-cytoplasm boundary. These data demonstrate that ac53 is involved in nucleocapsid assembly and is an essential gene for virus production.

A bacmid approach to the genetic manipulation of granuloviruses

A Cydia pomonella granulovirus (CpGV) bacmid has been constructed, which allows rapid and efficient production of recombinant baculoviruses in Escherichia coli. An 8.6kbp bacterial DNA cassette derived from the AcMNPV Bac-to-Bac system was ligated into a unique PacI restriction site within an intergenic region flanking the DNA ligase gene of the CpGV genome. The CpGV bacmids produced in E. coli were transfected into a CpGV-permissive C. pomonella cell line and the transfected cells fed to larvae to amplify the virus. The enhanced green fluorescent protein (EGFP) gene under the constitutive Drosophila heat-shock promoter was transposed into the mini-attTn7 transposition site, using a modified pFASTBAC donor plasmid, to generate a recombinant CpGV bacmid which caused infected larvae to glow under UV light. Targeted homologous recombination was also achieved in a recombinant proficient E. coli strain (BJ5183). A chloramphenicol acetyl transferase (CAT) gene replaced the cathepsin (v-cath) gene in the bacmid to produce a v-cath-deletion mutant. This is the first published report of a granulovirus bacmid, which will allow easy manipulation of the CpGV genome, enabling future studies on granulovirus genes and biology.

Characterization of a nucleopolyhedrovirus with a deletion of the baculovirus core gene Bm67

Open reading frame (ORF) 67 (Bm67) of the Bombyx mori nucleopolyhedrovirus (BmNPV) is a highly conserved gene that is found in all completely sequenced baculoviruses; its function is unknown. In the present study, a Bm67-knockout virus was generated for studying the role of Bm67 in the BmNPV infection cycle. Furthermore, a Bm67-repair bacmid was constructed by transposing the Bm67 native promoter-promoted Bm67 ORF into the polyhedrin locus of the Bm67-knockout bacmid. After these recombinant bacmids were transfected into BmN cells, the Bm67-knockout bacmid caused defects in the production of infectious budded viruses. However, the Bm67-repair bacmid could rescue the defect, and budded virus titres reached wild-type levels. Quantitative real-time PCR analysis indicated that Bm67 is required for normal levels of DNA synthesis or for the stability of nascent viral DNA at the early stage. Electron microscopic analysis revealed that the formation of normal-appearing nucleocapsids is reduced in Bm67-knockout bacmid-transfected cells, and nucleocapsids are rarely found in the cytoplasm. The presence of 'enveloped' nucleocapsids at the nucleoplasm bilayer indicated that they are enveloped abnormally. These results indicated that Bm67 is required for the production of infectious budded viruses and for assembly of envelope and nucleocapsids.

AcMNPV ac143 (odv-e18) is essential for mediating budded virus production and is the 30th baculovirus core gene

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac143 (odv-e18) is a late gene that encodes for a predicted 9.6 kDa structural protein that locates to the occlusion derived viral envelope and viral induced intranuclear microvesicles [Braunagel, S.C., He, H., Ramamurthy, P., and Summers, M.D. (1996). Transcription, translation, and cellular localization of three Autographa californica nuclear polyhedrosis virus structural proteins: ODV-E18, ODV-E35, and ODV-EC27. Virology 222, 100-114.]. In this study we demonstrate that ac143 is actually a previously unrecognized core gene and that it is essential for mediating budded virus production. To examine the role of ac143 in the baculovirus life cycle, we used the AcMNPV bacmid system to generate an ac143 knockout (KO) virus (AcBAC(ac142)(REP-ac143KO)). Fluorescence and light microscopy showed that infection by AcBAC(ac142)(REP-ac143KO) is limited to a single cell and titration assays confirmed that AcBAC(ac142)(REP-ac143KO) was unable to produce budded virus (BV). Progression to very late phases of the viral infection was evidenced by the development of occlusion bodies in the nuclei of transfected cells. This correlated with the fact that viral DNA replication was unaffected in AcBAC(ac142)(REP-ac143KO) transfected cells. The entire ac143 promoter, which includes three late promoter motifs, is contained within the ac142 open reading frame. Different deletion mutants of this region showed that the integrity of the ac142-ac143 core gene cluster was required for the bacmids to display wild-type patterns of viral replication, BV production and RNA transcription.

Autographa californica multiple nucleopolyhedrovirus ac66 is required for the efficient egress of nucleocapsids from the nucleus, general synthesis of preoccluded virions and occlusion body formation

Although orf66 (ac66) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is conserved in all sequenced lepidopteran baculovirus genomes, its function is not known. This paper describes generation of an ac66 knockout AcMNPV bacmid mutant and analyses of the influence of ac66 deletion on the virus replication in Sf-9 cells so as to determine the role of ac66 in the viral life cycle. Results indicated that budded virus (BV) yields were reduced over 99% in ac66-null mutant infected cells in comparison to that in wild-type virus infected cells. Optical microscopy revealed that occlusion body synthesis was significantly reduced in the ac66 knockout bacmid-transfected cells. In addition, ac66 deletion interrupted preoccluded virion synthesis. The mutant phenotype was rescued by an ac66 repair bacmid. On the other hand, real-time PCR analysis indicated that ac66 deletion did not affect the levels of viral DNA replication. Electron microscopy revealed that ac66 is not essential for nucleocapsid assembly, but for the efficient transport of nucleocapsids from the nucleus to the cytoplasm. These results suggested that ac66 plays an important role for the efficient exit of nucleocapsids from the nucleus to the cytoplasm for BV synthesis as well as for preoccluded virion and occlusion synthesis.

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.

In vivo replication kinetics and transcription patterns of the nucleopolyhedrovirus (NeabNPV) of the balsam fir sawfly, Neodiprion abietis

DNA replication and transcription of NeabNPV, the nucleopolyhedrovirus (NPV) of the balsam fir sawfly, Neodiprion abietis (Hymenoptera: Diprionidae), in host larvae were investigated. NPV DNA replication kinetics and gene-expression patterns have been resolved only in lepidopteran cell-culture systems and in limited in vivo experiments with lepidopteran larvae. Furthermore, there are significant differences in pathologies caused by lepidopteran NPVs, which replicate in many tissues, and hymenopteran NPVs, known to replicate in midgut epithelium only. Despite the differences in host specificity and pathology, NeabNPV DNA replication kinetics were similar to those reported for lepidopteran NPVs. Maximal NeabNPV DNA synthesis was observed between 4 and 24 h post-inoculation (p.i.) but, in contrast to lepidopteran NPVs, synthesis continued at a lower rate up to 72 h p.i. Selected NeabNPV genes exhibited a cascade pattern of transcription similar to that of lepidopteran NPVs. RT-PCR products of the NeabNPV lef-1, lef-2 and dnapol transcripts were observed as early as 2 h p.i., whilst lef-8 and lef-9, encoding putative viral RNA polymerase subunits, were detected at 1 and 6 h p.i., respectively. Two structural late transcripts (gp41 and p74) were observed from 6 h p.i. The very late factor 1 (vlf-1) transcript, a transactivator of very late genes, was observed from 12 h p.i., but the very late transcript polh, encoding the major occlusion protein, polyhedrin, was observed from 24 h p.i. This study provides the first insight into DNA replication and gene expression of a non-lepidopteran baculovirus.

Characterization of baculovirus constructs lacking either the Ac 101, Ac 142, or the Ac 144 open reading frame

To investigate the role of the gene products encoded from the open reading frames 101, 142, and 144 of Autographa californica multiple nucleopolyhedrovirus (AcMNPV), a set of bacmid knockout and repair constructs were generated. The repair genes were engineered to contain an HA epitope tag at their C-termini. The results of transfection-infection assays and growth curve analyses showed that the Ac 101, 142, and 144 genes were required for infectious virus production. To better characterize the role of these genes in the baculovirus replication cycle, quantitative DNA replication assays were performed and demonstrated that in cells transfected with the Ac 101, 142, or 144 knockouts, DNA replicated with similar kinetics as a control virus. Western blot analyses of budded virus from cells infected with the repair viruses showed that these proteins are associated with the viral nucleocapsid. Furthermore, immunoelectron microscopy of cells transfected with the knockout bacmids revealed defects in nucleocapsid production for all three constructs. From these results we concluded that the gene products encoded from these open reading frames are essential for virus production and may be involved in DNA processing, packaging, or nucleocapsid morphogenesis.

Characterization of a baculovirus lacking the DBP (DNA-binding protein) gene

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) encodes two proteins that possess properties typical of single-stranded DNA-binding proteins (SSBs), late expression factor-3 (LEF-3), and a protein referred to as DNA-binding protein (DBP). Whereas LEF-3 is a multi-functional protein essential for viral DNA replication, transporting helicase into the nucleus, and forms a stable complex with the baculovirus alkaline nuclease, the role for DBP in baculovirus replication remains unclear. Therefore, to better understand the functional role of DBP in viral replication, a DBP knockout virus was generated from an AcMNPV bacmid and analyzed. The results of a growth curve analysis indicated that the dbp knockout construct was unable to produce budded virus indicating that dbp is essential. The lack of DBP does not cause a general shutdown of the expression of viral genes, as was revealed by accumulation of early (LEF-3), late (VP39), and very late (P10) proteins in cells transfected with the dbp knockout construct. To investigate the role of DBP in DNA replication, a real-time PCR-based assay was employed and showed that, although viral DNA synthesis occurred in cells transfected with the dbp knockout, the levels were less than that of the control virus suggesting that DBP is required for normal levels of DNA synthesis or for stability of nascent viral DNA. In addition, analysis of the viral DNA replicated by the dbp knockout by using field inversion gel electrophoresis failed to detect the presence of genome-length DNA. Furthermore, analysis of DBP from infected cells indicated that similar to LEF-3, DBP was tightly bound to viral chromatin. Assessment of the cellular localization of DBP relative to replicated viral DNA by immunoelectron microscopy indicated that, at 24 h post-infection, DBP co-localized with nascent DNA at distinct electron-dense regions within the nucleus. Finally, immunoelectron microscopic analysis of cells transfected with the dbp knockout revealed that DBP is required for the production of normal-appearing nucleocapsids and for the generation of the virogenic stroma.

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.