ac18 is not essential for the propagation of Autographa californica multiple nucleopolyhedrovirus

Genome-wide nonessential gene identification of Autographa californica multiple nucleopolyhedrovirus

The Baculovirus Expression Vector System (BEVS) is an insect cell-based heterologous protein expression system that possesses powerful potential in the development of protein drugs and vaccines. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the most widely-used vector in BEVS with 151 open reading frames (ORFs) containing essential and nonessential genes. Deletion of nonessential genes has many advantages including increased foreign gene insertion. In this study, the λ red recombination system was used to knock out genes in a modified AcMNPV that carried an enhanced yellow fluorescent protein (eYFP) at the Ac126-Ac127 locus. Eighty genes were almost completely deleted respectively and 69 gene knockout AcMNPVs (KOVs) were obtained to evaluate their infection efficiency. After infecting Spodoptera frugiperda 9 (Sf9) cells, 51 KOVs including 62 genes showed similar infectivity as wide type (WT) and hence were defined as nonessential genes. However, 18 KOVs produced fewer infectious virions, indicating that these genes were influential in the production of progeny viruses. Combining our research with previous studies, a desired minimal AcMNPV genome containing 86 ORFs and all of the homologous regions (hrs) was brought up, facilitating genetic modification of baculovirus vectors and improvement of recombinant protein expression in the future.

Improvement of protein production in baculovirus expression vector system by removing a total of 10 kb of nonessential fragments from Autographa californica multiple nucleopolyhedrovirus genome

Baculovirus expression vector system (BEVS) is a powerful and versatile platform for recombinant protein production in insect cells. As the most frequently used baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) encodes 155 open reading frames (ORFs), including a considerable number of non-essential genes for the virus replication in cell culture. Studies have shown that protein production in BEVS can be improved by removing some viral dispensable genes, and these AcMNPV vectors also offer the possibility of accommodating larger exogenous gene fragments. In this study, we, respectively, deleted 14 DNA fragments from AcMNPV genome, each of them containing at least two contiguous genes that were known nonessential for viral replication in cell culture or functionally unknown. The effects of these fragment-deletions on virus replication and exogenous protein production were examined. The results showed that 11 of the 14 fragments, containing 43 genes, were dispensable for the virus replication in cultured cells. By detecting the expression of intracellularly expressed and secreted reporter proteins, we demonstrated that nine of the fragment-deletions benefited protein production in Sf9 cells and/or in High Five cells. After combining the deletion of some dispensable fragments, we obtained two AcMNPV vectors shortened by more than 10 kb but displayed an improved capacity for recombinant protein production. The deletion strategies used in this study has the potential to further improve the BEVS.

Identification of Loci Associated with Enhanced Virulence in Spodoptera litura Nucleopolyhedrovirus Isolates Using Deep Sequencing

Spodoptera litura is an emerging pest insect in cotton and arable crops in Central Asia. To explore the possibility of using baculoviruses as biological control agents instead of chemical pesticides, in a previous study we characterized a number of S. litura nucleopolyhedrovirus (SpltNPV) isolates from Pakistan. We found significant differences in speed of kill, an important property of a biological control agent. Here we set out to understand the genetic basis of these differences in speed of kill, by comparing the genome of the fast-killing SpltNPV-Pak-TAX1 isolate with that of the slow-killing SpltNPV-Pak-BNG isolate. These two isolates and the SpltNPV-G2 reference strain from China were deep sequenced with Illumina. As expected, the two Pakistani isolates were closely related with >99% sequence identity, whereas the Chinese isolate was more distantly related. We identified two loci that may be associated with the fast action of the SpltNPV-Pak-TAX1 isolate. First, an analysis of rates of synonymous and non-synonymous mutations identified neutral to positive selection on open reading frame (ORF) 122, encoding a viral fibroblast growth factor (vFGF) that is known to affect virulence in other baculoviruses. Second, the homologous repeat region hr17, a putative enhancer of transcription and origin of replication, is absent in SpltNPV-Pak-TAX1 suggesting it may also affect virulence. Additionally, we found there is little genetic variation within both Pakistani isolates, and we identified four genes under positive selection in both isolates that may have played a role in adaptation of SpltNPV to conditions in Central Asia. Our results contribute to the understanding of the enhanced activity of SpltNPV-Pak-TAX1, and may help to select better SpltNPV isolates for the control of S. litura in Pakistan and elsewhere.

Bombyx mori nucleopolyhedrovirus protein Bm11 is involved in occlusion body production and occlusion-derived virus embedding

Bombyx mori nucleopolyhedrovirus (BmNPV) orf11 (bm11) is a highly conserved gene with unknown function. It is homologous to AcMNPV orf19. In this study, a bm11 knockout virus was constructed and its role was investigated. Expression analysis indicated that bm11 is a late gene and confocal microscopy analysis demonstrated that Bm11 localizes predominantly in the nuclear ring zone at the late phase of infection. The bm11 deletion did not affect budded virus (BV) production or viral genome replication, but markedly reduced the production of occlusion bodies (OBs) and the embedding of occlusion-derived viruses (ODVs). Bio-assays showed that Bm11 was involved in BmNPV infectivity in vivo by direct injection. In conclusion, our results demonstrated that although Bm11 is not essential for BV production or mature ODV formation, it affects OB production and ODV occlusion.

Disruption of Autographa Californica Multiple Nucleopolyhedrovirus ac111 Results in Reduced per os Infectivity in a Host-Dependent Manner

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac111 gene is highly conserved in lepidopteran-specific baculoviruses, and its function in the AcMNPV life cycle is still unknown. To investigate the function of ac111, an ac111-knockout AcMNPV (vAc111KO) was constructed through homologous recombination in Escherichia coli. Viral growth curve analysis and plaque assays showed that the deletion of ac111 had no effect on infectious budded virion production. Quantitative real-time polymerase chain reaction analysis confirmed that viral DNA replication was unaffected in the absence of ac111. Electron microscopy revealed that the ac111 deletion did not affect nucleocapsid assembly, occlusion-derived virion formation, or the embedding of occlusion-derived virions into the occlusion bodies. However, in vivo bioassays showed that although the deletion of ac111 did not affect the per os infectivity of AcMNPV in Spodoptera exigua larvae, it led to an approximately five-fold reduction in infectivity of AcMNPV in Trichoplusia ni larvae, and vAc111KO took approximately 21 h longer to kill Trichoplusia ni larvae than the wild-type viruses. Taken together, our results demonstrated that although ac111 is not essential for virus replication in vitro, it plays an important role in the per os infectivity of AcMNPV in a host-dependent manner.

Protein composition analysis of polyhedra matrix of Bombyx mori nucleopolyhedrovirus (BmNPV) showed powerful capacity of polyhedra to encapsulate foreign proteins

Polyhedra can encapsulate other proteins and have potential applications as protein stabilizers. The extremely stable polyhedra matrix may provide a platform for future engineered micro-crystal devices. However, the protein composition of the polyhedra matrix remains largely unknown. In this study, the occlusion-derived virus (ODV)-removed BmNPV polyhedra matrix fraction was subjected to SDS-PAGE and then an LC-ESI-MS/MS analysis using a Thermo Scientific Q Exactive mass spectrometer. In total, 28 host and 91 viral proteins were identified. The host components were grouped into one of six categories, i.e., chaperones, ubiquitin and related proteins, host helicases, cytoskeleton-related proteins, RNA-binding proteins and others, according to their predicted Pfam domain(s). Most viral proteins may not be essential for polyhedra assembly, as evidenced by studies in the literature showing that polyhedra formation occurs in the nucleus upon the disruption of individual genes. The structural role of these proteins in baculovirus replication will be of significant interest in future studies. The immobilization of enhanced green fluorescent protein (eGFP) into the polyhedra by fusing with the C-terminus of BM134 that is encoded by open reading frame (ORF) 134 suggested that the polyhedra had a powerful capacity to trap foreign proteins, and BM134 was a potential carrier for incorporating proteins of interest into the polyhedra.

De novo transcriptome analysis of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) genes in latently infected Se301 cells

Cells of the P8-Se301-C1 strain are Spodoptera exigua cell clones that each harbor a partial version of the S. exigua multiple nucleopolyhedrovirus (SeMNPV) genome and which are resistant to homologous SeMNPV infections. The cells produce no viral progeny, suggesting that the infection is a latent-like viral infection. To investigate the SeMNPV genes harbored in the P8-Se301-C1 cells, the de novo transcriptomes of P8-Se301-C1 cells and S. exigua Se301 cells were analyzed and compared. A total of 54,569,296 reads were obtained from the P8-Se301-C1 cells that yielded 112,565 final unigenes with a mean length of 1,093 nt. A total of 56,865,504 reads were obtained from the Se301 cells that yielded 102,996 final unigenes with a mean length of 1,082 nt. Ten SeMNPV gene transcripts (se5, se7, se8, se12, se43, se45, se89, se90, se124, and se126) were detected in the P8-Se301-C1 cells by RNA-Seq but not in the Se301 cells, which was verified by RTPCR. 5'/3' RACE analyses showed that the 3'- or 5'-end sequences of the viral transcripts are aligned to the host gene sequences in P8-Se301-C1 cells, suggesting that the SeMNPV genes may integrate into and be transcribed with the host genes in the P8-Se301-C1 cells. Furthermore, six additional viral gene transcripts, se11, se42, se44, se88, se91, and se127 (incorporated into chimeric fusion transcripts in the P8-Se301-C1 cells), were detected in the RACE analyses. Taken together, sixteen SeMNPV transcripts were identified in the P8-Se301-C1 cell strain. This study provides information to develop the understanding of baculovirus latent infections and superinfection exclusion.

Functional Regulation of an Autographa californica Nucleopolyhedrovirus-Encoded MicroRNA, AcMNPV-miR-1, in Baculovirus Replication

An Autographa californica nucleopolyhedrovirus-encoded microRNA (miRNA), AcMNPV-miR-1, downregulates the ac94 gene, reducing the production of infectious budded virions and accelerating the formation of occlusion-derived virions. In the current study, four viruses that constitutively overexpress AcMNPV-miR-1 were constructed to further explore the function of the miRNA. In addition to the ac94 gene, two new viral gene targets (ac18 and ac95) of AcMNPV-miR-1 were identified, and the possible interacting proteins were verified and tested. In the context of AcMNPV-miR-1 overexpression, ac18 was slightly upregulated, and ac95 was downregulated. Several interacting proteins were identified, and a functional pathway for AcMNPV-miR-1 was deduced. AcMNPV-miR-1 overexpression decreased budded virus infectivity, reduced viral DNA replication, accelerated polyhedron formation, and promoted viral infection efficiency in Trichoplusia ni larvae, suggesting that AcMNPV-miR-1 restrains virus infection of cells but facilitates virus infection of larvae.

IMPORTANCE

Recently, microRNAs (miRNAs) have been widely reported as moderators or regulators of mammalian cellular processes, especially disease-related pathways in humans. However, the roles played by miRNAs encoded by baculoviruses, which infect numerous beneficial insects and agricultural pests, have rarely been described. To explore the actions of virus-encoded miRNAs, we investigated an miRNA encoded by Autographa californica nucleopolyhedrovirus (AcMNPV-miR-1). We previously identified this miRNA through the exogenous addition of AcMNPV-miR-1 mimics. In the current study, we constitutively overexpressed AcMNPV-miR-1 and analyzed the resultant effects to more comprehensively assess what is indeed the function of this miRNA during viral infection. In addition, we widely explored the target genes for the miRNA in the viral and host genomes and proposed a possible functional network for AcMNPV-miR-1, which provides a better general understanding of virus-encoded miRNAs. In brief, our study implied that AcMNPV-miR-1 constrains viral replication and cellular infection but enhances larval infection.

Disruption of the baculovirus core gene ac78 results in decreased production of multiple nucleocapsid-enveloped occlusion-derived virions and the failure of primary infection in vivo

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac78 gene is one of the baculovirus core genes. Recent studies showed that ac78 is essential for budded virion (BV) production and the embedding of occlusion-derived virion (ODV) into occlusion body during the AcMNPV life cycle. Here, we report that an ac78-knockout AcMNPV (vAc78KO) constructed in this study had different phenotypes than those described in the previous studies. A few infectious BVs were detected using titer assays, immunoblot analyses and plaque assays, indicating that ac78 is not essential for BV formation. Electron microscopy confirmed that the ac78 deletion did not affect nucleocapsid assembly and ODV formation. However, the numbers of multiple nucleocapsid-enveloped ODVs and ODV-embedded occlusion bodies were significantly decreased. Subsequently, the highly conserved amino acid residues 2-25 and 64-88 of Ac78, which are homologous to an oxidoreductase and cytochrome c oxidase, respectively, were demonstrated to play a crucial role in the morphogenesis of multiple nucleocapsid-enveloped ODV. Immunoblot analysis found that Ac78 was an ODV envelope-associated protein. Consistently, amino acid residues 56-93 of Ac78 were identified as an inner nuclear membrane sorting motif, which may direct the localization of Ac78 to the ODV envelope. In vivo infectivity assays showed that the occlusion bodies of vAc78KO were unable to establish primary infection in the midgut of Trichoplusia ni larvae. Taken together, our results suggest that ac78 plays an important role in BV production and proper multiple nucleocapsid-enveloped ODV formation, as well as AcMNPV primary infection in vivo.

Genomic diversity in European Spodoptera exigua multiple nucleopolyhedrovirus isolates

Key virus traits such as virulence and transmission strategies rely on genetic variation that results in functional changes in the interactions between hosts and viruses. Here, comparative genomic analyses of seven isolates of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) with differing phenotypes were employed to pinpoint candidate genes that may be involved in host-virus interactions. These isolates obtained after vertical or horizontal transmission of infection in insects differed in virulence. Apart from one genome containing a piggyBac transposon, all European SeMNPV isolates had a similar genome size and content. Complete genome analyses of single nucleotide polymorphisms and insertions/deletions identified mutations in 48 ORFs that could result in functional changes. Among these, 13 ORFs could be correlated with particular phenotypic characteristics of SeMNPV isolates. Mutations were found in all gene functional classes and most of the changes we highlighted could potentially be associated with differences in transmission. The regulation of DNA replication (helicase, lef-7) and transcription (lef-9, p47) might be important for the establishment of sublethal infection prior to and following vertical transmission. Virus-host cell interactions also appear instrumental in the modulation of viral transmission as significant mutations were detected in virion proteins involved in primary (AC150) or secondary infections (ME35) and in apoptosis inhibition (IAP2, AC134). Baculovirus populations naturally harbour high genomic variation located in genes involved at different levels of the complex interactions between virus and host during the course of an infection. The comparative analyses performed here suggest that the differences in baculovirus virulence and transmission phenotypes involve multiple molecular pathways.

Proteomic analyses of baculovirus Anticarsia gemmatalis multiple nucleopolyhedrovirus budded and occluded virus

Baculoviruses infect insects, producing two distinct phenotypes during the viral life cycle: the budded virus (BV) and the occlusion-derived virus (ODV) for intra- and inter-host spread, respectively. Since the 1980s, several countries have been using Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV) as a biological control agent against the velvet bean caterpillar, A. gemmatalis. The genome of AgMNPV isolate 2D (AgMNPV-2D) carries at least 152 potential genes, with 24 that possibly code for structural proteins. Proteomic studies have been carried out on a few baculoviruses, with six ODV and two BV proteomes completed so far. Moreover, there are limited data on virion proteins carried by AgMNPV-2D. Therefore, structural proteins of AgMNPV-2D were analysed by MALDI- quadrupole-TOF and liquid chromatography MS/MS. A total of 44 proteins were associated with the ODV and 33 with the BV of AgMNPV-2D. Although 38 structural proteins were already known, we found six new proteins in the ODV and seven new proteins carried by the AgMNPV-2D BV. Eleven cellular proteins that were found on several other enveloped viruses were also identified, which are possibly carried with the virion. These findings may provide novel insights into baculovirus biology and their host interaction. Moreover, our data may be helpful in subsequent applied studies aiming to improve AgMNPV use as a biopesticide and a biotechnology tool for gene expression or delivery.

Bm91 is an envelope component of ODV but is dispensable for the propagation of Bombyx mori nucleopolyhedrovirus

Orf91 (Bm91) of Bombyx mori nucleopolyhedrovirus (BmNPV) is a highly conserved gene that encodes a predicted 105-amino-acid protein, but its function remains unknown. In the current study, 5'-RACE revealed that the transcription initiation site of Bm91 was - 12 nucleotides upstream of the start codon ATG, transcription of Bm91 was detected from 12 to 96 h postinfection (p.i.) and Bm91 protein was detected from 24 to 96 h p.i. in BmNPV-infected BmN cells. Furthermore, Western blot analysis revealed that Bm91 was in occlusion-derived virus (ODV) but not in budded virus (BV). To investigate the role of Bm91 in baculovirus life cycle, a Bm91-knockout virus was constructed by bacmid recombination in E. coli. Fluorescence and light microscopy showed that the production of BV and occlusion bodies (OBs) in Bm91-deficient-virus-infected BmN cells were similar to those in wild-type-virus-infected ones. Bioassay results showed that genetic deletion of Bm91 did not significantly affect BmNPV infectivity, but extended the median lethal time (LT50). Taken together, these results indicate that Bm91 is not essential for viral propagation in vitro, but absence of the gene may affect the virulence of ODVs in silkworm larvae.

Genome of Epinotia aporema granulovirus (EpapGV), a polyorganotropic fast killing betabaculovirus with a novel thymidylate kinase gene

BACKGROUND

Epinotia aporema (Lepidoptera: Tortricidae) is an important pest of legume crops in South America. Epinotia aporema granulovirus (EpapGV) is a baculovirus that causes a polyorganotropic infection in the host larva. Its high pathogenicity and host specificity make EpapGV an excellent candidate to be used as a biological control agent.

RESULTS

The genome of Epinotia aporema granulovirus (EpapGV) was sequenced and analyzed. Its circular double-stranded DNA genome is 119,082 bp in length and codes for 133 putative genes. It contains the 31 baculovirus core genes and a set of 19 genes that are GV exclusive. Seventeen ORFs were unique to EpapGV in comparison with other baculoviruses. Of these, 16 found no homologues in GenBank, and one encoded a thymidylate kinase. Analysis of nucleotide sequence repeats revealed the presence of 16 homologous regions (hrs) interspersed throughout the genome. Each hr was characterized by the presence of 1 to 3 clustered imperfect palindromes which are similar to previously described palindromes of tortricid-specific GVs. Also, one of the hrs (hr4) has flanking sequences suggestive of a putative non-hr ori. Interestingly, two more complex hrs were found in opposite loci, dividing the circular dsDNA genome in two halves. Gene synteny maps showed the great colinearity of sequenced GVs, being EpapGV the most dissimilar as it has a 20 kb-long gene block inversion. Phylogenetic study performed with 31 core genes of 58 baculoviral genomes suggests that EpapGV is the baculovirus isolate closest to the putative common ancestor of tortricid specific betabaculoviruses.

CONCLUSIONS

This study, along with previous characterization of EpapGV infection, is useful for the better understanding of the pathology caused by this virus and its potential utilization as a bioinsecticide.

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.

Helicoverpa armaigera nucleopolyhedrovirus ORF50 is an early gene not essential for virus propagation in vitro and in vivo

Homologs of Helicoverpa armigera nucleopolyhedrovirus ORF50 (HA50) are found in most alphabaculoviruses, but their functions remain unknown. Here, we characterized whether Ha50 is indispensable for virus progration. Ha50 transcript was first detected at 3 h post-infection from HearNPV-infected HzAM1 cells. 3'RACE analysis showed that Ha50 transcript was polyadenlylated. 5'RACE analysis revealed two transcription initiation sites, one of which was mapped to the canonical baculovirus early transcription initiator motif CAGT. HA50 protein could be detected from infected cells harvested at 12 h post-infection. Transient expression assays showed that GFP-fused HA50 localized in the cytoplasm and nucleus of HzAM1 cells with or without superinfection. To further examine the role of Ha50 in the virus life cycle, a Ha50 knockout bacmid and a repair bacmid carrying Ha50 under the control of its native promoter elements were constructed using bacmid technology. One-step growth curve analysis showed that the kinetics of infectious budded virus production of Ha50 knockout virus was similar to that of the parental virus or the repair virus. Analysis of the expression of viral early protein IE-1, late protein VP39 and very late protein suggested that viral protein expression was not affected by Ha50 inactivation. Electron microscopy revealed that HaBacΔ50-PH-G occluded viruses (ODVs) and occlusion bodies were indistinguishable from those of the wild-type virus. Similarly, bioassays showed no significant difference in the LC(50) values between Ha50 deletion virus and wild-type virus. Our results together demonstrate that Ha50 is an early gene dispensable for virus propagation in vitro and in vivo.

Protein composition of the occlusion derived virus of Chrysodeixis chalcites nucleopolyhedrovirus

Chrysodeixis chalcites nucleopolyhedrovirus (ChchNPV) is a group II NPV and its genome has 151 predicted open reading frames. In this study, the protein composition of ChchNPV occlusion derived virus (ODV) was determined by LC-MS/MS. Fifty-three proteins were identified in ChchNPV ODV particles. One ODV-protein is encoded by a gene so far unique to ChchNPV (Chch105). The two DNA photolyases PHR1 and PHR2, which are characteristic for ChchNPV and thought to be involved in repairing UV damage in viral DNA, were not detected in the ODVs. Comparison of the ODV proteins identified in ChchNPV and in three other baculoviruses enabled the identification of ten conserved ODV proteins (ODV-E18, ODV-E56, ODV-EC27, ODV-EC43, P6.9, P33, P49, P74, GP41, and VP39). In addition, the baculovirus per os infectivity factors PIF1, PIF2 and PIF3 were all detected in ChchNPV and these should be considered as conserved ODV proteins as well as they are absolutely required for oral infection. With the LC-MS/MS method used 22 viral proteins were detected, which were not identified as ODV proteins in previous studies.

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.

Genetic modification of a baculovirus vector for increased expression in insect cells

Generating large amounts of recombinant protein in transgenic animals is often challenging and has a number of drawbacks compared to cell culture systems. The baculovirus expression vector system (BEVS) uses virus-infected insect cells to produce recombinant proteins to high levels, and these are usually processed in a similar way to the native protein. Interestingly, since the development of the BEVS, the virus most often used (Autographa californica multi-nucleopolyhedovirus; AcMNPV) has been little altered genetically from its wild-type parental virus. In this study, we modified the AcMNPV genome in an attempt to improve recombinant protein yield, by deleting genes that are non-essential in cell culture. We deleted the p26, p10 and p74 genes from the virus genome, replacing them with an antibiotic selection cassette, allowing us to isolate recombinants. We screened and identified recombinant viruses by restriction enzyme analysis, PCR and Western blot. Cell viability analysis showed that the deletions did not improve the viability of infected cells, compared to non-deletion viruses. However, expression studies showed that recombinant protein levels for the deletion viruses were significantly higher than the expression levels of non-deletion viruses. These results confirm that there is still great potential for improving the BEVS, further increasing recombinant protein expression yields and stability in insect cells.

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 highly conserved baculovirus gene p48 (ac103) is essential for BV production and ODV envelopment

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) p48 (ac103) is a highly conserved baculovirus gene of unknown function. In the present study, we generated a knockout of the p48 gene in an AcMNPV bacmid and investigated the role of P48 in baculovirus life cycle. The p48-null Bacmid vAc(P48-KO-PH-GFP) was unable to propagate in cell culture, while a 'repair' Bacmid vAc(P48-REP-PH-GFP) was able to replicate in a manner similar to a wild-type Bacmid vAc(PH-GFP). Titration assays and Western blotting confirmed that vAc(P48-KO-PH-GFP) was unable to produce budded viruses (BVs). qPCR analysis showed that p48 deletion did not affect viral DNA replication. Electron microscopy indicated that P48 was required for nucleocapsid envelopment to form occlusion-derived viruses (ODVs) and their subsequent occlusion. Confocal analysis showed that P48 prominently condensed in the centre of the nucleus. Our results demonstrate that P48 plays an essential role in BV production and ODV envelopment in the AcMNPV life cycle.