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

A selection marker-free method for gene deletion and editing in baculovirus genomes

Here, we develop a simple, efficient, bacmid-based, selection marker-free method for gene deletion and editing in baculovirus genomes. Specifically, based on pFastbac1, a donor plasmid with long left and right homology arms but without a reporter was constructed for disrupting ie1, an essential baculovirus gene. Instead of ligating with a plasmid, the homology arms were introduced to the polyhedrin locus of BmNPV bacmid using the BmNPV bac-to-bac expression system. Two viruses generated from the modified bacmid and unmodified BmNPV bacmid were then used to co-infect BmN cells in order that recombination takes place at the ie1 locus between them. Finally, without multiple rounds of purification, total cellular DNA was isolated, transformed into Cacl₂-treated competent DH10B cells, and then blue colonies were selected for PCR screening. Remarkably, the proportion of blue colonies containing ie1-disrupted bacmid was found to be around 7 %. Moreover, using primers flanking the homology arms further confirmed that all these positive recombinants were double crossovers. These findings indicate that our method is also capable of gene modification if inverse PCR or seamless cloning is used to construct the donor plasmid and sequencing is employed to select positive colonies.

Bracoviruses, ichnoviruses, and virus-like particles from parasitoid wasps retain many features of their virus ancestors

Animal genomes commonly contain genes or sequences that have been acquired from different types of viruses. The vast majority of these endogenous virus elements (EVEs) are inactive or consist of only a small number of components that show no evidence of cooption for new functions or interaction. Unlike most EVEs, bracoviruses (BVs), ichnoviruses (IVs) and virus-like particles (VLPs) in parasitoid wasps have evolved through retention and interaction of many genes from virus ancestors. Here, we discuss current understanding of BV, IV and VLP evolution along with associated implications for what constitutes a virus. We suggest that BVs and IVs are domesticated endogenous viruses (DEVs) that differ in several important ways from other known EVEs.

Systematic Analysis of 42 Autographa Californica Multiple Nucleopolyhedrovirus Genes Identifies An Additional Six Genes Involved in the Production of Infectious Budded Virus

Baculoviruses have been widely used as a vector for expressing foreign genes. Among numerous baculoviruses, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the most frequently used and it encodes 155 open reading frames (ORFs). Here, we systematically investigated the impact of 42 genes of AcMNPV on the production of infectious budded viruses (BVs) by constructing gene-knockout bacmids and subsequently conducting transfection and infection assays. The results showed that among the 39 functionally unverified genes and 3 recently reported genes, 36 are dispensable for infectious BV production, as the one-step growth curves of the gene-knockout viruses were not significantly different from those of the parental virus. Three genes (ac62, ac82 and ac106/107) are essential for infectious BV production, as deletions thereof resulted in complete loss of infectivity while the repaired viruses showed no significant difference in comparison to the parental virus. In addition, three genes (ac13, ac51 and ac120) are important but not essential for infectious BV production, as gene-knockout viruses produced significantly lower BV levels than that of the parental virus or repaired viruses. We then grouped the 155 AcMNPV genes into three categories (Dispensable, Essential, or Important for infectious BV production). Based on our results and previous publications, we constructed a schematic diagram of a potential mini-genome of AcMNPV, which contains only essential and important genes. The results shed light on our understanding of functional genomics of baculoviruses and provide fundamental information for future engineering of baculovirus expression system.

Genome Analysis of a Novel Clade II.b Alphabaculovirus Obtained from Artaxa digramma

Artaxa digramma is a lepidopteran pest distributed throughout southern China, Myanmar, Indonesia, and India. Artaxa digramma nucleopolyhedrovirus (ArdiNPV) is a specific viral pathogen of A. digramma and deemed as a promising biocontrol agent against the pest. In this study, the complete genome sequence of ArdiNPV was determined by deep sequencing. The genome of ArdiNPV contains a double-stranded DNA (dsDNA) of 161,734 bp in length and 39.1% G+C content. Further, 149 hypothetical open reading frames (ORFs) were predicted to encode proteins >50 amino acids in length, covering 83% of the whole genome. Among these ORFs, 38 were baculovirus core genes, 22 were lepidopteran baculovirus conserved genes, and seven were unique to ArdiNPV, respectively. No typical baculoviral homologous regions (hrs) were identified in the genome. ArdiNPV had five multi-copy genes including baculovirus repeated ORFs (bros), calcium/sodium antiporter B (chaB), DNA binding protein (dbp), inhibitor of apoptosis protein (iap), and p26. Interestingly, phylogenetic analyses showed that ArdiNPV belonged to Clade II.b of Group II Alphabaculoviruses, which all contain a second copy of dbp. The genome of ArdiNPV was the closest to Euproctis pseudoconspersa nucleopolyhedrovirus, with 57.4% whole-genome similarity. Therefore, these results suggest that ArdiNPV is a novel baculovirus belonging to a newly identified cluster of Clade II.b Alphabaculoviruses.

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.

Baculovirus infection induces disruption of the nuclear lamina

Baculovirus nucleocapsids egress from the nucleus primarily via budding at the nuclear membrane. The nuclear lamina underlying the nuclear membrane represents a substantial barrier to nuclear egress. Whether the nuclear lamina undergoes disruption during baculovirus infection remains unknown. In this report, we generated a clonal cell line, Sf9-L, that stably expresses GFP-tagged Drosophila lamin B. GFP autofluorescence colocalized with immunofluorescent anti-lamin B at the nuclear rim of Sf9-L cells, indicating GFP-lamin B was incorporated into the nuclear lamina. Meanwhile, virus was able to replicate normally in Sf9-L cells. Next, we investigated alterations to the nuclear lamina during baculovirus infection in Sf9-L cells. A portion of GFP-lamin B localized diffusely at the nuclear rim, and some GFP-lamin B was redistributed within the nucleus during the late phase of infection, suggesting the nuclear lamina was partially disrupted. Immunoelectron microscopy revealed associations between GFP-lamin B and the edges of the electron-dense stromal mattes of the virogenic stroma, intranuclear microvesicles, and ODV envelopes and nucleocapsids within the nucleus, indicating the release of some GFP-lamin B from the nuclear lamina. Additionally, GFP-lamin B phosphorylation increased upon infection. Based on these data, baculovirus infection induced lamin B phosphorylation and disruption of the nuclear lamina.

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.

Autographa californica multiple nucleopolyhedrovirus ORF11 is essential for budded-virus production and occlusion-derived-virus envelopment

ORF11 (ac11) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a highly conserved gene with unknown function. To determine the role of ac11 in the baculovirus life cycle, an ac11 knockout mutant of AcMNPV, Ac11KO, was constructed. Northern blot and 5' rapid amplification of cDNA ends (RACE) analyses revealed that ac11 is an early gene in the life cycle. Microscopy, titration assays, and Western blot analysis revealed that budded viruses (BVs) were not produced in Ac11KO-transfected Sf9 cells. However, quantitative PCR (qPCR) analysis demonstrated that the deletion of ac11 did not affect viral DNA replication. Furthermore, electron microscopy revealed that there was no nucleocapsid in the cytoplasm or plasma membrane of Ac11KO-transfected cells, which demonstrates that the defect in BV production in Ac11KO-transfected cells is due to the inefficient egress of nucleocapsids from the nucleus to the cytoplasm. In addition, electron microscopy observations showed that the nucleocapsids in the nucleus were not enveloped to form occlusion-derived viruses (ODVs) and that their subsequent embedding into occlusion bodies (OBs) was also blocked in Ac11KO-transfected cells, demonstrating that ac11 is required for ODV envelopment. These results therefore demonstrate that ac11 is an early gene that is essential for BV production and ODV envelopment.

IMPORTANCE

Baculoviruses have been extensively used not only as specific, environmentally benign insecticides but also as helper-independent protein expression vectors. Although the function of baculovirus genes in viral replication has been studied by using gene knockout technology, the functions of more than one-third of viral genes, which include some highly conserved genes, are still unknown. In this study, ac11 was proven to play a crucial role in BV production and ODV envelopment. These results will lead to a better understanding of baculovirus infection cycles.

The genome of a baculovirus isolated from Hemileuca sp. encodes a serpin ortholog

The genome sequence of a baculovirus from Hemileuca sp. was determined. The genome is 140,633 kb, has a G+C content of 38.1 %, and encodes 137 putative open-reading frames over 50 amino acids. 126 of these ORFs showed similarity to other baculovirus genes in the database including all 37 core genes. Of the remaining 11 predicted genes, one is related to a lepidopteran serpin gene. This is the first report of a baculovirus encoding a member of this family of serine protease inhibitors, and to our knowledge the first report of a viral serpin outside the Poxviridae. The genome also contained three homologous repeat sequences. Phylogenetic analysis indicated that the virus is a group II Alphabaculovirus and belongs to a lineage that includes Orgyia leucostigma, Ectropis obliqua, Apocheima cinerarium, and Euproctis pseudoconspersa nucleopolyhedroviruses.

Proteotoxic stress induced by Autographa californica nucleopolyhedrovirus infection of Spodoptera frugiperda Sf9 cells

Baculovirus AcMNPV causes proteotoxicity in Sf9 cells as revealed by accumulation of ubiquitinated proteins and aggresomes in the course of infection. Inhibition of proteasomes by lactacystin increased markedly the stock of ubiquitinated proteins indicating a primary role of proteasomes in detoxication. The proteasomes were present in Sf9 cells as 26S and 20S complexes whose protease activity did not change during infection. Proteasome inhibition caused a delay in the initiation of viral DNA replication suggesting an important role of proteasomes at early stages in infection. However, lactacystin did not affect ongoing replication indicating that active proteasomes are not required for genome amplification. At late stages in infection (24-48 hpi), aggresomes containing the ubiquitinated proteins and HSP/HSC70s showed gradual fusion with the vacuole-like structures identified as lysosomes by antibody to cathepsin D. This result suggests that lysosomes may assist in protection against proteotoxicity caused by baculoviruses absorbing the ubiquitinated proteins.

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.

Distribution and phosphorylation of the basic protein P6.9 of Autographa californica nucleopolyhedrovirus

A protamine-like protein named P6.9 is thought to play a role in the condensation of genomes of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) during an infection. Previous studies have shown that P6.9 is phosphorylated immediately upon synthesis and dephosphorylated upon the entry of the P6.9-DNA complex into the capsid. Here, we investigate the dynamic distribution of P6.9 in AcMNPV-infected Spodoptera frugiperda cells using an influenza virus hemagglutinin (HA)-tagged P6.9. Although a portion of P6.9-HA localized to the virogenic stroma, which is the center of viral DNA replication, transcription, and nucleocapsid assembly, the majority of P6.9-HA was distributed near the inner nuclear membrane throughout the course of infection. Antiserum against P6.9 detected specific phosphorylated forms of P6.9 at the edge of, but not within, the electron-dense matte regions of the virogenic stroma. Further analysis using immunoblotting revealed that at least 11 different phosphorylated forms of P6.9, as well as dephosphorylated P6.9, were present in association with occlusion-derived virions, although only dephosphorylated P6.9 was associated with budded virions.

Analysis of genes expression of Spodoptera exigua larvae upon AcMNPV infection

Background

The impact of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) infection on host gene expression in Spodoptera exigua 4th instar larvae was investigated through the use of 454 sequencing-based RNA-seq of cDNA libraries developed from insects challenged with active AcMNPV or heat-inactivated AcMNPV.

Methodology/Principal Findings

By comparing the two cDNA libraries, we show that 201 host genes are significantly up-regulated and 234 genes are significantly down-regulated by active AcMNPV infection. Down-regulated host genes included genes encoding antimicrobial peptides, namely three gloverin isoforms and an attacin, indicating that the viral infection actively repressed the expression of a portion of the host immune gene repertoire. Another interesting group of down-regulated host genes included genes encoding two juvenile hormone binding proteins and a hexamerin, all of which are involved in juvenile hormone regulation. The expression of these genes was enhanced by the topical application of Juvenile Hormone III (JHIII) in the insects challenged with heat-inactivated AcMNPV. However, infection with the active virus strongly suppresses the expression of these three genes, regardless of the absence or presence of JHIII.

Conclusions/Significance

Using RNA-seq, we have identified groups of immune-regulated and juvenile hormone-regulated genes that are suppressed by infection with active AcMNPV. This information and further studies on the regulation of host gene expression by AcMNPV will provide the tools needed to enhance the utility of the virus as an effective protein expression system and as an insecticide.

Phenotypic grouping of 141 BmNPVs lacking viral gene sequences

We constructed a series of gene knockout BmNPVs (KOVs) for each of 141 genes (Gomi et al., 1999; Katsuma et al., 2011) using the BmNPV T3 bacmid system (Ono et al., 2007) and lambda red recombination system (Datsenko and Wanner, 2000). In a subsequent analysis of the properties needed for infection using a marker gene, egfp (enhanced green fluorescent protein gene), inserted into the polyhedrin locus, the knockout viruses (KOVs) were subdivided into four phenotypic types, A to D. Type-A (86 KOVs) showed the ability to expand infections equivalent to the control while type-B (8 KOVs) spread infections more slowly. Type-C (37 KOVs) expressed egfp in transfected-BmN cells but the production of infectious viruses was not observed. Type-D (10 KOVs) showed no ability to express egfp even in the transfection experiments. KOVs lacking genes (pkip (Bm15), gp41 (Bm66), bro-d (Bm131), Bm20, 48, 65, 91, 93, or 101) previously identified as being essential, were placed in the viable type-A and B categories.

Complete sequence, analysis and organization of the Orgyia leucostigma nucleopolyhedrovirus genome

The complete genome of the Orgyia leucostigma nucleopolyhedrovirus (OrleNPV) isolated from the whitemarked tussock moth (Orgyia leucostigma, Lymantridae: Lepidoptera) was sequenced, analyzed, and compared to other baculovirus genomes. The size of the OrleNPV genome was 156,179 base pairs (bp) and had a G+C content of 39%. The genome encoded 135 putative open reading frames (ORFs), which occupied 79% of the entire genome sequence. Three inhibitor of apoptosis (ORFs 16, 43 and 63), and five baculovirus repeated ORFs (bro-a through bro-e) were interspersed in the OrleNPV genome. In addition to six direct repeat (drs), a common feature shared among most baculoviruses, OrleNPV genome contained three homologous regions (hrs) that are located in the latter half of the genome. The presence of an F-protein homologue and the results from phylogenetic analyses placed OrleNPV in the genus Alphabaculovirus, group II. Overall, OrleNPV appears to be most closely related to group II alphabaculoviruses Ectropis obliqua (EcobNPV), Apocheima cinerarium (ApciNPV), Euproctis pseudoconspersa (EupsNPV), and Clanis bilineata (ClbiNPV).

Production of baculovirus defective interfering particles during serial passage is delayed by removing transposon target sites in fp25k

Accumulation of baculovirus defective interfering particle (DIP) and few polyhedra (FP) mutants is a major limitation to continuous large-scale baculovirus production in insect-cell culture. Although overcoming these mutations would result in a cheaper platform for producing baculovirus biopesticides, little is known regarding the mechanism of FP and DIP formation. This issue was addressed by comparing DIP production of wild-type (WT) Autographa californica multiple nucleopolyhedrovirus (AcMNPV) with that of a recombinant AcMNPV (denoted Ac-FPm) containing a modified fp25k gene with altered transposon insertion sites that prevented transposon-mediated production of the FP phenotype. In addition to a reduction in the incidence of the FP phenotype, DIP formation was delayed on passaging of Ac-FPm compared with WT AcMNPV. Specifically, the yield of DIP DNA in Ac-FPm was significantly lower than in WT AcMNPV up to passage 16, thereby demonstrating that modifying the transposon insertion sites increases the genomic stability of AcMNPV. A critical component of this investigation was the optimization of a systematic method based on the use of pulsed-field gel electrophoresis (PFGE) to characterize extracellular virus DNA. Specifically, PFGE was used to detect defective genomes, determine defective genome sizes and quantify the amount of defective genome within a heterogeneous genome population of passaged virus.

Baculovirus infection induces a DNA damage response that is required for efficient viral replication

Several mammalian viruses have been shown to induce a cellular DNA damage response during replication, and in some cases, this response is required for optimal virus replication. However, nothing is known about whether a DNA damage response is stimulated by DNA viruses in invertebrates. Cell cycle arrest and apoptosis are two of the downstream effects of the DNA damage response, and both are stimulated by baculovirus infection, suggesting a possible relationship between baculoviruses and the DNA damage response. In the study described in this report, we found that replication of the baculovirus Autographa californica M nucleopolyhedrovirus (AcMNPV) in the cell line Sf9, derived from the lepidopteran insect Spodoptera frugiperda, stimulated a DNA damage response, as indicated by an increased abundance of the S. frugiperda P53 protein (SfP53) and phosphorylation of the histone variant protein H2AX. Stimulation of the DNA damage response was dependent on viral DNA replication. Inhibition of the DNA damage response prevented both the increase in SfP53 accumulation and H2AX phosphorylation and also caused a 10- to 100-fold reduction in virus production, along with decreased viral DNA replication and late gene expression. However, silencing of Sfp53 expression by RNA interference did not significantly affect AcMNPV replication or induction of apoptosis by a mutant of AcMNPV lacking the antiapoptotic gene p35, indicating that these processes are not dependent on SfP53 in Sf9 cells.

Co-expression of four baculovirus proteins, IE1, LEF3, P143, and PP31, elicits a cellular chromatin-containing reticulate structure in the nuclei of uninfected cells

Baculovirus DNA replication, transcription, and nucleocapsid assembly occur within a subnuclear structure called the virogenic stroma (VS) that consists of two subcompartments. Specific components of the VS sub-compartments have not been identified except for PP31, a DNA-binding protein that localizes specifically to the electron-dense region of VS. Here, we investigate the dynamic structure of VS using a GFP-tagged PP31 molecule (GFP-PP31). GFP-PP31 localizes to the VS throughout the course of infection. At later times post-infection, a PP31 reticulum distributed within VS was also apparent, indicating that VS sub-compartments compose a reticulate structure. Transient expression of PP31 with the viral proteins, IE1, LEF3, and P143, in uninfected cells resulted in the formation of a reticulate structure containing cellular chromatin and the spatial arrangements of the four proteins within the induced reticulum were the same as those within VS reticulum, suggesting that the two reticula are formed by a similar mechanism.

Effect of ac68 knockout and lef3 leading sequence disruption on viral propagation

Orf68 (ac68) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is identified to be an early gene, but its transcription start site remains unknown. The coding sequence of ac68 overlaps 280-bp leading sequence and 159-bp coding sequence of lef3 (ac67). In this study, the transcription start site of ac68 was determined by 5' RACE analysis to be 18 nucleotides upstream from the start codon. In order to investigate the effect of ac68 deletion on virus propagation, we generated a bacmid with an ac68 knockout by deleting 360-bp inside the ac68 gene, which also deleted 220-bp leading sequence of lef3. Production of infectious budded virus and formation of nucleocapsids and occlusion bodies exhibited wild-type patterns of virus propagation in Sf-9 cells infected with the mutant bacmid. The result demonstrated that ac68 was not an essential gene for viral propagation which was confirmed by further deletion of ac68, and disruption of the lef3 leading sequence did not affect viral propagation. Ac68 was the second auxiliary gene discovered besides Ac133 (alk-exo) among the 30 core genes of AcMNPV.

A novel way to purify recombinant baculoviruses by using bacmid

In the present study, we studied the feasibility of deleting essential genes in insect cells by using bacmid and purifying recombinant bacmid in Escherichia coli DH10B cells. To disrupt the orf4 (open reading frame 4) gene of BmNPV [Bm (Bombyx mori) nuclear polyhedrosis virus], a transfer vector was constructed and co-transfected with BmNPV bacmid into Bm cells. Three passages of viruses were carried out in Bm cells, followed by one round of purification. Subsequently, bacmid DNA was extracted and transformed into competent DH10B cells. A colony harbouring only orf4-disrupted bacmid DNA was identified by PCR. A mixture of recombinant (white colonies) and non-recombinant (blue colonies) bacmids were also transformed into DH10B cells. PCR with M13 primers showed that the recombinant and non-recombinant bacmids were separated after transformation. The result confirmed that purification of recombinant viruses could be carried out simply by transformation and indicated that this method could be used to delete essential genes. Orf4-disrupted bacmid DNA was extracted and transfected into Bm cells. Viable viruses were produced, showing that orf4 was not an essential gene.

Characterization of a Bombyx mori nucleopolyhedrovirus with Bmvp80 disruption

A BmNPV Bacmid with the Bmvp80 gene disrupted was constructed using the ET-recombination system in Escherichia coli to investigate the role of Bmvp80 during the baculovirus life cycle. Disruption of Bmvp80 resulted in single cell infection phenotype, whereas a rescue BmBacmid restored budded virus titers to wild type levels; however, the homologous gene Ac104 (Acvp80) from AcMNPV could not complement the BmBacmid lacking a functional Bmvp80 gene. Electron microscopy of cells transfected with BmNPV lacking functional Bmvp80 revealed that the number of nucleocapsids was markedly lower. These results suggest that Bmvp80 is essential for normal budded virus production and nucleocapsid maturation, and is functionally divergent between baculovirus species.

Sf29 gene of Spodoptera frugiperda multiple nucleopolyhedrovirus is a viral factor that determines the number of virions in occlusion bodies

The genome of Spodoptera frugiperda multiple nucleopolyhedrovirus (NPV) was inserted into a bacmid (Sfbac) and used to produce a mutant lacking open reading frame 29 (Sf29null). Sf29null bacmid DNA was able to generate an infection in S. frugiperda. Approximately six times less DNA was present in occlusion bodies (OBs) produced by the Sf29null bacmid in comparison to viruses containing this gene. This reduction in DNA content was consistent with fewer virus particles being packaged within Sf29null bacmid OBs, as determined by fractionation of dissolved polyhedra and comparison of occlusion-derived virus (ODV) infectivity in cell culture. DNA from Sfbac, Sf29null, or Sf29null-repair, in which the gene deletion had been repaired, were equally infectious when used to transfect S. frugiperda. All three viruses produced similar numbers of OBs, although those from Sf29null were 10-fold less infectious than viruses with the gene. Insects infected with Sf29null bacmid died approximately 24 h later than positive controls, consistent with the reduced virus particle content of Sf29null OBs. Transcripts from Sf29 were detected in infected insects 12 h prior to those from the polyhedrin gene. Homologs to Sf29 were present in other group II NPVs, and similar sequences were present in entomopoxviruses. Analysis of the Sf29 predicted protein sequence revealed signal peptide and transmembrane domains, but the presence of 12 potential N-glycosylation sites suggest that it is not an ODV envelope protein. Other motifs, including zinc-binding and threonine-rich regions, suggest degradation and adhesion functions. We conclude that Sf29 is a viral factor that determines the number of ODVs occluded in each OB.

Isolation and characterization of the DNA-binding protein (DBP) of the Autographa californica multiple nucleopolyhedrovirus

DNA-binding protein (DBP) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) was expressed as an N-terminal His(6)-tag fusion using a recombinant baculovirus and purified to near homogeneity. Purified DBP formed oligomers that were crosslinked by redox reagents resulting in predominantly protein dimers and tetramers. In gel retardation assays, DBP showed a high affinity for single-stranded oligonucleotides and was able to compete with another baculovirus SSB protein, LEF-3, for binding sites. DBP binding protected ssDNA against hydrolysis by a baculovirus alkaline nuclease AN/LEF-3 complex. Partial proteolysis by trypsin revealed a domain structure of DBP that is required for interaction with DNA and that can be disrupted by thermal treatment. Binding to ssDNA, but not to dsDNA, changed the pattern of proteolytic fragments of DBP indicating adjustments in protein structure upon interaction with ssDNA. DBP was capable of unwinding short DNA duplexes and also promoted the renaturation of long complementary strands of ssDNA into duplexes. The unwinding and renaturation activities of DBP, as well as the DNA binding activity, were sensitive to sulfhydryl reagents and were inhibited by oxidation of thiol groups with diamide or by alkylation with N-ethylmaleimide. A high affinity of DBP for ssDNA and its unwinding and renaturation activities confirmed identification of DBP as a member of the SSB/recombinase family. These activities and a tight association with subnuclear structures suggests that DBP is a component of the virogenic stroma that is involved in the processing of replicative intermediates.

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.