Identification of baculovirus gene that promotes Autographa californica nuclear polyhedrosis virus replication in a nonpermissive insect cell line

Leucoma salicis nucleopolyhedrovirus (LesaNPV) genome sequence shed new light on the origin of the Alphabaculovirus orpseudotsugatae species

LesaNPV (Leucoma salicis nucleopolyhedrovirus) is an alphabaculovirus group Ib. Potentially, it can be an eco-friendly agent to control the white satin moth Leucoma salicis population. In this study, we have established the relationship between LesaNPV and other closely related alphabaculoviruses. Environmental samples of late instar of white satin moth collected in Poland infected with baculovirus have been homogenized, polyhedra were purified and subjected to scanning and transmission electron microscopy. Viral DNA was sequenced using the Illumina platform and the whole-genome sequence was established by de novo assembly of paired reads. Genome annotation and phylogenetic analyses were performed with the use of bioinformatics tools. The genome of LesaNPV is 132 549 bp long with 154 ORFs and 54.9% GC content. Whole-genome sequencing revealed deletion of dUTPase as well as ribonucleoside reductases small and large subunits region in LesaNPV genome compared to Dasychira pudibunda nucleopolyhedrovirus (DapuNPV) and Orgyia pseudotsugata multiple nucleopolyhedrovirus (OpMNPV) where this region is complete. Phylogenetic analysis of Baculoviridae family members showed that LesaNPV is less divergent from a common ancestor than closely related species DapuNPV and OpMNPV. This is interesting because their hosts do not occur in the same area. The baculoviruses described in this manuscript are probably isolates of one species and could be assigned to recently denominated species Alphabaculovirus orpseudotsugatae, historically originating from OpMNPV. This finding could have significant implications for the classification and understanding of the phylogeographical spread of baculoviruses.

The complete genome sequence of an alphabaculovirus from the brown tussock moth, Olene mendosa Hübner, expands our knowledge of lymantriine baculovirus diversity and evolution

The complete genome sequence was determined for an apparent alphabaculovirus isolated from larval cadavers of the brown tussock moth, Olene mendosa Hübner, collected during an epizootic in Coimbatore, India. The genome was determined to be a circular 142,291 bp molecule, and 147 ORFs and nine homologous regions were annotated for the sequence. Analysis of the sequence confirmed that this virus, Olene mendosa nucleopolyhedrovirus (OlmeNPV), was a member of genus Alphabaculovirus in family Baculoviridae. Phylogenies inferred from nucleotide and amino acid alignments indicated that OlmeNPV was part of a group of viruses that infect moths of genus Lymantria, suggesting that OlmeNPV may have shifted hosts from a Lymantria species to an ancestral Olene species at some point during its evolutionary history. OlmeNPV was most closely related to Lymantria xylina multiple nucleopolyhedrovirus isolate 5 (LyxyMNPV-5). The genomes of OlmeNPV and LyxyMNPV-5 were distinguished not only by differences in ORF content, but by a 27 kbp region of the genome that is inverted in LyxyMNPV-5 relative to OlmeNPV. Pairwise nucleotide distances between OlmeNPV and other Lymantria spp. alphabaculoviruses indicate that OlmeNPV represents a new baculovirus species.

Carbohydrate metabolism is a determinant for the host specificity of baculovirus infections

Baculoviruses Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Bombyx mori nucleopolyhedrovirus (BmNPV) have highly similar genome sequences but exhibit no overlap in their host range. After baculovirus infects nonpermissive larvae (e.g., AcMNPV infecting B. mori or BmNPV infecting Spodoptera litura), we found that stored carbohydrates, including hemolymph trehalose and fat body glycogen, are rapidly transformed into glucose; enzymes involved in glycolysis and the TCA cycle are upregulated and produce more ATP; adenosine signaling that regulates glycolytic activity is also increased. Subsequently, phagocytosis in cellular immunity and the expression of genes involved in humoral immunity increase significantly. Moreover, inhibiting glycolysis and the expression of gloverins in nonpermissive hosts increased baculovirus infectivity, indicating that the stimulated energy production is designed to support the immune response against infection. Our study highlights that alteration of the host's carbohydrate metabolism is an important factor determining the host specificity of baculoviruses, in addition to viral factors.

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Nonpermissive infections by AcMNPV and BmNPV alter host carbohydrate metabolism

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Increased carbohydrate metabolism produces energy to launch immune responses

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Immune responses including antimicrobial peptide production inhibit virus infection

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Host metabolic alterations affect the determination of virus's host specificity

Antiviral immune responses of Bombyx mori cells during abortive infection with Autographa californica multiple nucleopolyhedrovirus

Lepidopteran cells rely on multiple antiviral responses to defend against baculovirus infections, including apoptosis, global protein synthesis shutdown, and rRNA degradation. Here, we characterized apoptosis and rRNA degradation in Autographa californica multiple nucleopolyhedrovirus (AcMNPV)-infected Bombyx mori cells, a system resulting in abortive infection, in relation to viral DNA replication and viral late gene expression. RNAi-mediated silencing of viral DNA replication-related genes prevented apoptosis, but not rRNA degradation, in B. mori cells infected with p35-deficient AcMNPV. Additionally, AcMNPV, but not B. mori nucleopolyhedrovirus (BmNPV), drastically reduced B. mori cellular iap1 transcript levels and p35-deficient AcMNPV induced more prominent apoptosis than did p35-deficient BmNPV. These results, together with previous results that global protein synthesis shutdown follows viral DNA replication, demonstrate that rRNA degradation is the primary antiviral response that abolishes productive AcMNPV infection of B. mori cells. Our results also demonstrate that B. mori cells induce apoptosis to a different extent depending on NPV species.

HCF-1 encoded by baculovirus AcMNPV is required for productive nucleopolyhedrovirus infection of non-permissive Tn368 cells

Baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) replicates in both Spodoptera frugiperda Sf21 and Trichoplusia ni Tn368 cells, whereas AcMNPV defective in hcf-1 (host cell-factor 1) gene productively infects only Sf21 cells, indicating that HCF-1 is indispensable for the AcMNPV productive infection of Tn368 cells. Here, we demonstrated that HCF-1 protein transiently expressed in Tn368 cells promotes the DNA synthesis of Hyphantria cunea MNPV (HycuMNPV), Orygia pseudotsugata MNPV and Bombyx mori NPV, which are normally unable to replicate in Tn368 cells. We also demonstrated that a recombinant HycuMNPV harboring the hcf-1 gene successfully replicates in Tn368 cells, generating substantial yields of progeny viruses and polyhedra. These results indicate that HCF-1 encoded by AcMNPV is an essential viral factor for productive NPV infection of Tn368 cells. Taken together with the previous findings on HRF-1 (host range factor 1), the present results provide strong evidence that viral genes acquired through horizontal gene transfer play an important role in baculovirus evolution, serving to expand the host range of baculoviruses.

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

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

Generating a host range-expanded recombinant baculovirus

As baculoviruses usually have a narrow insecticidal spectrum, knowing the mechanisms by which they control the host-range is prerequisite for improvement of their applications as pesticides. In this study, from supernatant of culture cells transfected with DNAs of an Autographa californica multiple nucleopolyhedrovirus (AcMNPV) mutant lacking the antiapoptotic gene p35 (vAc(∆P35)) and a cosmid representing a fragment of Spodoptera exigua nucleopolyhedrovirus (SeMNPV), a viral strain was plaque-purified and named vAcRev. vAcRev had a broader host range than either vAc(∆P35) or SeMNPV parental virus, being able to infect not only the permissive hosts of its parental viruses but also a nonpermissive host (Spodoptera litura). Genome sequencing indicated that vAcRev comprises a mixture of two viruses with different circular dsDNA genomes. One virus contains a genome similar to vAc(∆P35), while in the other viral genome, a 24.4 kbp-fragment containing 10 essential genesis replaced with a 4 kbp-fragment containing three SeMNPV genes including a truncated Se-iap3 gene. RNA interference and ectopic expression assays found that Se-iap3 is responsible for the host range expansion of vAcRev, suggesting that Se-iap3 inhibits the progression of apoptosis initiated by viral infection and promotes viral propagation in hosts both permissive and non-permissive for AcMNPV and SeMNPV.

Genome scale transcriptomics of baculovirus-insect interactions

Baculovirus-insect cell technologies are applied in the production of complex proteins, veterinary and human vaccines, gene delivery vectors' and biopesticides. Better understanding of how baculoviruses and insect cells interact would facilitate baculovirus-based production. While complete genomic sequences are available for over 58 baculovirus species, little insect genomic information is known. The release of the Bombyx mori and Plutella xylostella genomes, the accumulation of EST sequences for several Lepidopteran species, and especially the availability of two genome-scale analysis tools, namely oligonucleotide microarrays and next generation sequencing (NGS), have facilitated expression studies to generate a rich picture of insect gene responses to baculovirus infections. This review presents current knowledge on the interaction dynamics of the baculovirus-insect system' which is relatively well studied in relation to nucleocapsid transportation, apoptosis, and heat shock responses, but is still poorly understood regarding responses involved in pro-survival pathways, DNA damage pathways, protein degradation, translation, signaling pathways, RNAi pathways, and importantly metabolic pathways for energy, nucleotide and amino acid production. We discuss how the two genome-scale transcriptomic tools can be applied for studying such pathways and suggest that proteomics and metabolomics can produce complementary findings to transcriptomic studies.

Analysis of the genomic sequence of Philosamia cynthia nucleopolyhedrin virus and comparison with Antheraea pernyi nucleopolyhedrin virus

Background

Two species of wild silkworms, the Chinese oak silkworm (Antheraea pernyi) and the castor silkworm Philosamia cynthia ricini, can acquire a serious disease caused by Nucleopolyhedrin Viruses (NPVs) (known as AnpeNPV and PhcyNPV, respectively). The two viruses have similar polyhedral morphologies and their viral fragments share high sequence similarity. However, the physical maps of the viral genomes and cross-infectivity of the viruses are different. The genome sequences of two AnpeNPV isolates have been published.

Results

We sequenced and analyzed the full-length genome of PhcyNPV to compare the gene contents of the two viruses. The genome of PhcyNPV is 125, 376 bp, with a G + C content of 53.65%, and encodes 138 open reading frames (ORFs) of at least 50 amino acids (aa) (GenBank accession number: JX404026). Between PhcyNPV and AnpeMNPV-L and -Z isolates, 126 ORFs are identical, including 30 baculovirus core genes. Nine ORFs were only found in PhcyNPV. Four genes, cath, v-chi, lef 10 and lef 11, were not found in PhcyNPV. However, most of the six genes required for infectivity via the oral route were found in PhcyNPV and in the two AnpeNPV isolates, with high sequence similarities. The pif-3 gene of PhcyNPV contained 59 aa extra amino acids at the N-terminus compared with AnpeNPV.

Conclusions

Most of the genes in PhcyNPV are similar to the two AnpeNPV isolates, including the direction of expression of the ORFs. Only a few genes were missing from PhcyNPV. These data suggest that PhcyNPV and AnpeNPV might be variants of each other, and that the differences in cross-infection might be caused by gene mutations.

AcMNPV enhances infection by ThorNPV in Sf21 cells and SeMNPV in Hi5 cells

An expression cassette containing the DsRed2 gene, which encodes the red fluorescent protein (RFP), was inserted into the wide-host-range Autographa californica M nucleopolyhedrovirus (AcMNPV) at the polyhedrin locus (vAcDsRed2). An expression cassette containing the enhanced green fluorescent protein (EGFP) gene was inserted at the gp37 locus of the narrow-host-range Thysanoplusia orichalcea MNPV (ThorMNPV) and the p10 locus of Spodoptera exigua MNPV (SeMNPV) to produce vThGFP and vSeGFP, respectively. vThGFP and vSeGFP are poor at infecting Sf21 and Hi5 cells, respectively, whereas vAcDsRed2 is highly infectious to both cell lines. During co-infection, vAcDsRed2 enhanced vThGFP infection in Sf21 cells by approximately 20-fold, and it enhanced vSeGFP infection in Hi5 cells by more than 300-fold, as detected by fluorescence measurements. In contrast, vThGFP reduced vAcDsRed2 infection by 5.4-fold in Sf21 cells, while vSeGFP reduced vAcDsRed2 by 3.2-fold in Hi5 cells. Plaque assay data did not suggest viral recombination, but vThGFP plaques surrounded by vAcDsRed2 plaques were observed. A viral DNA replication assay performed by real-time quantitative PCR suggested that the detected fluorescence correlated with virus replication. Sf21 cells infected with vAcDsRed2 were resistant to superinfection by viruses of the same type expressing EGFP (vAcGFP). These results demonstrated that AcMNPV could enhance replication of ThorMNPV and SeMNPV in non-permissive cells without recombination.

A baculovirus isolated from wild silkworm encompasses the host ranges of Bombyx mori nucleopolyhedrosis virus and Autographa californica multiple nucleopolyhedrovirus in cultured cells

A baculovirus, named BomaNPV S2, was isolated from a diseased larva of the wild silkworm, Bombyx mandarina. Notably, BomaNPV S2 exhibited a distinguishing feature in that its host range covered that of both Bombyx mori nucleopolyhedrosis virus (BmNPV) and Autographa californica multiple nucleopolyhedrovirus (AcMNPV) in cultured cells. It could replicate in cells of B. mori (Bm5 and BmN), Spodoptera frugiperda (Sf9) and Trichoplusia ni (Tn-5B1-4). However, occlusion-derived virions of BomaNPV S2 in B. mori cells contained only a single nucleocapsid, whereas they contained multiple nucleocapsids in Tn-5B1-4 cells. The complete genome sequence of BomaNPV S2, including predicted ORFs, was determined and compared with the genome sequence of its close relatives. The comparison results showed that most of the BomaNPV S2 genome sequence was shared with BmNPV (BmNPV T3) or BomaNPV S1, but several regions seemed more similar to regions of AcMNPV. This observation might explain why BomaNPV S2 covers the host ranges of BmNPV and AcMNPV. Further recombinant virus infection experiments demonstrated that GP64 plays an important role in BomaNPV S2 host-range determination.

Baculovirus Lymantria dispar multiple nucleopolyhedrovirus IAP2 and IAP3 do not suppress apoptosis, but trigger apoptosis of insect cells in a transient expression assay

Ld652Y cells derived from the gypsy moth, Lymantria dispar, are permissive for productive infection with L. dispar multiple nucleopolyhedrovirus (LdMNPV), but undergo apoptosis upon infection with various other NPVs, including those isolated from Bombyx mori, Hyphantria cunea, Spodoptera exigua, Orgyia pseudotsugata, and Spodoptera litura. In this study, we examined whether LdMNPV-encoded inhibitor of apoptosis 2 (Ld-IAP2) and 3 (Ld-IAP3) are involved in apoptosis suppression in LdMNPV-infected Ld652Y cells. We found that neither Ld-IAP2 nor Ld-IAP3 was able to suppress the apoptosis of Ld652Y cells induced by p35-defective Autographa californica MNPV (vAcΔp35). However, both Ld-IAP2 and Ld-IAP3 induced apoptosis in Ld652Y cells in a transient expression assay. The apoptosis induced by Ld-IAP3 was accompanied by the stimulation of caspase-3-like protease activity and cleavage of the B. mori homolog of the initiator caspase Dronc, and was precluded by the LdMNPV-encoded apoptosis suppressor protein Apsup and H. cunea MNPV IAP3. Inconsistent with the results obtained previously in SpIm, Ld652Y and High Five cells infected with NPVs from H. cunea, O. pseudotsugata, and A. californica, respectively, considerable stimulation of caspase-3-like protease activity was not observed in LdMNPV-infected Ld652Y cells, likely due to the strong apoptosis suppression activity of Apsup. These results, together with the previous finding that RNAi-mediated silencing of apsup induces apoptosis of LdMNPV-infected Ld652Y cells, indicate that Apsup, but not Ld-IAP2 or Ld-IAP3, is primarily responsible for the suppression of apoptosis in LdMNPV-infected Ld652Y cells. However, it remains inconclusive whether Ld-IAP2 and Ld-IAP3 function as pro-apoptotic proteins in LdMNPV-infected Ld652Y cells.

Identification of a novel apoptosis suppressor gene from the baculovirus Lymantria dispar multicapsid nucleopolyhedrovirus

Ld652Y cells from Lymantria dispar readily undergo apoptosis upon infection with a variety of nucleopolyhedroviruses (NPVs), while L. dispar multicapsid NPV (LdMNPV) infection of Ld652Y cells results in the production of a high titer of progeny viruses. Here, we identify a novel LdMNPV apoptosis suppressor gene, apsup, which functions to suppress apoptosis induced in Ld652Y cells by infection with vAcΔp35, a p35-defective recombinant Autographa californica MNPV. apsup also suppresses apoptosis of Ld652Y cells induced by actinomycin D and UV exposure. Apsup is expressed in LdMNPV-infected Ld652Y cells late in infection, and RNA interference-mediated apsup ablation induces apoptosis of LdMNPV-infected Ld652Y cells.

Baculovirus infection influences host protein expression in two established insect cell lines

We identified host proteins that changed in response to host cell susceptibility to baculovirus infection. We used three baculovirus-host cell systems utilizing two cell lines derived from pupal ovaries, Hz-AM1 (from Helicoverpa zea) and Hv-AM1 (from Heliothis virescens). Hv-AM1 cells are permissive to Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and semi-permissive to H. zea single nucleopolyhedrovirus (HzSNPV). Hz-AM1 cells are non-permissive to AcMNPV. We challenged each cell line with baculovirus infection and after 24h determined protein identities by MALDI TOF/TOF mass spectrometry. For Hv-AM1 cells, 21 proteins were identified, and for Hz-AM1 cells, 19 proteins were newly identified (with 8 others having been previously identified). In the permissive relationship, 18 of the proteins changed in expression by 70% or more in AcMNPV infected Hv-AM1 cells as compared with non-infected controls; 12 were significantly decreased and 6 cellular proteins were significantly increased. We also identified 3 virus-specific proteins. In the semi-permissive infections, eight proteins decreased by 2-fold or more. Non-permissive interactions did not lead to substantial changes in host cell protein expression. We hypothesize that some of these proteins act in determining host cell specificity for baculoviruses.

Identification of a Hyphantria cunea nucleopolyhedrovirus (NPV) gene that is involved in global protein synthesis shutdown and restricted Bombyx mori NPV multiplication in a B. mori cell line

We previously demonstrated that Bombyx mori nucleopolyhedrovirus (BmNPV) multiplication is restricted in permissive BmN-4 cells upon coinfection with Hyphantria cunea NPV (HycuNPV). Here, we show that HycuNPV-encoded hycu-ep32 gene is responsible for the restricted BmNPV multiplication in HycuNPV-coinfected BmN-4 cells. The only homologue for hycu-ep32 is in Orgyia pseudotsugata NPV. hycu-ep32 could encode a polypeptide of 312 amino acids, and it contains no characteristic domains or motifs to suggest its possible functions. hycu-ep32 is an early gene, and Hycu-EP32 expression reaches a maximum by 6 h postinfection. hycu-ep32-defective HycuNPV, vHycuDeltaep32, was generated, indicating that hycu-ep32 is nonessential in permissive SpIm cells. In BmN-4 cells, HycuNPV infection resulted in a severe global protein synthesis shutdown, while vHycuDeltaep32 did not cause any specific protein synthesis shutdown. These results indicate that the restriction of BmNPV multiplication by HycuNPV is caused by a global protein synthesis shutdown induced by hycu-ep32 upon coinfection with HycuNPV.

Baculovirus interactions in vitro and in vivo

Baculoviruses are promising viral insecticides and are safe for the environment. Interaction of baculoviruses in vitro and in vivo is a basic molecular and ecological question that has practical applications in agriculture. Cellular secretion is also a fundamental property in cell-cell communication. Here, we review recent investigations on how baculoviruses interact with insect cells and insect hosts. We focus particularly on a new interaction mechanism in which a secretion from cells infected with one virus enhances infection by a second virus. We also discuss a hypothesis that the secreted signals may serve as ligands that bind to the receptors on the surface of the cells that harbor the suppressed genomes of Thysanoplusia orichalcea MNPV (ThorMNPV) in Sf21 and Spodoptera exigua MNPV (SeMNPV) in High 5 to initiate signal transduction leading to the activation of genome replication of ThorMNPV in Sf21 and SeMNPV in High 5. We also discuss how the enhanced replication of SeMNPV replication by Autographa californica MNPV (AcMNPV) in nonpermissive insect cells depends on the types of cells. Interaction of baculoviruses in insects focused on mutualism and antagonism, even though the mechanism is not clear on mutualism. The antagonism of a Nucleopolyhedrovirus (NPV) with a Granulovirus (GV) has been extensively studied by a metalloprotein in the capsule of GV that disrupts the peritrophic membrane, a physical barrier to NPV entry to the midgut of larvae, to facilitate NPV infection.

Baculovirus genes affecting host function

Baculoviruses are insect-specific viruses. These large DNA viruses encode many genes in addition to those required to replicate and build new virions. These auxiliary genes provide selective advantages to the virus for invading and infecting host insects. Eight of these genes, which help the virus overcome insect defenses against invasion, are discussed. These include genes whose products help the virus traverse physical or physiological barriers and those that overcome host immune defenses.

Genomic and host range studies of Maruca vitrata nucleopolyhedrovirus

The complete genome of the Maruca vitrata nucleopolyhedrovirus (MaviNPV) isolated from the legume pod borer, Maruca vitrata (Lepidoptera: Pyralidae), was sequenced. It was found to be 111 953 bp in length, with an overall 39 % G+C content, and contained 126 open reading frames (ORFs) encoding predicted proteins of over 50 aa. The gene content and gene order of MaviNPV have the highest similarity to those of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and their shared homologous genes are 100 % collinear. In fact, MaviNPV seems to be a mini-AcMNPV that is native to Taiwan and possesses a smaller genome with fewer auxiliary genes than the AcMNPV type species. Except for one ORF (Mv74), all of the MaviNPV ORFs have homologues in the AcMNPV genome. MaviNPV is the first lepidopteran-specific baculovirus to lack homologues of vfgf and odv-e66. In addition, MaviNPV lacks the baculovirus repeat ORF (bro) gene that corresponds to AcMNPV ORF2. Five homologous regions (hrs) were located within the MaviNPV genome, and these contained a total of 44 imperfect palindromes. Phylogenetic analysis of the whole genome revealed that MaviNPV was separated from the common ancestor of AcMNPV and Bombyx mori nucleopolyhedrovirus before these two viral species diverged from each other. Moreover, replication of MaviNPV in several cell lines and an egfp-MaviNPV infection assay revealed that IPLB-LD-652Y cells are only partially permissive to MaviNPV, which supports our conclusion that MaviNPV is a distinct species of the group I lepidopteran NPVs.

Slow cell infection, inefficient primary infection and inability to replicate in the fat body determine the host range of Thysanoplusia orichalcea nucleopolyhedrovirus

Thysanoplusia orichaceamulticapsid nucleopolyhedrovirus (ThorMNPV) carrying an enhanced green fluorescent protein (EGFP) gene expression cassette (vThGFP) was used to study host-range mechanisms. Infection kinetics showed that vThGFP replication in Sf21 cells was too slow to suppress cell growth. Wide-host-rangeAutographa californicaMNPV (AcMNPV) could speed up vThGFP infection and enhance the vThGFP infection rate in Sf21 cells. The enhancement was not due to recombination, as no recombinant virus was isolated from co-infection by plaque assay. No improvement of vThGFP infection in Sf21 was found by AcMNPV cosmid transactivation assay. However, culture medium from Sf21 cells infected with AcMNPV did enhance vThGFP replication in Sf21. Third-instar larvae ofSpodoptera frugiperda,S. exiguaandHelicoverpa zeawere not killed by feeding with vThGFP polyhedra but were killed by intrahaemocoelic injection using budded viruses (BVs). This suggested that insufficient BVs were generated during the primary infection in the midgut. vThGFP infected haemocytes, tracheae and Malpighian tubules but not fat bodies of larvae ofS. frugiperda,S. exiguaandH. zea. Third-instarS. frugiperdalarvae co-infected by injection with vThGFP and vAcDsRed2, an AcMNPV expressing a red fluorescent protein gene, showed EGFP expression in the fat body. This result suggests that vAcDsRed2 could help vThGFP to replicate in the fat body ortrans-activate EGFP expression in the fat body. All these results suggested that slow cell infection, insufficient primary infection and inability to replicate in the fat body control the host range of ThorMNPV.

Abortive replication of Bombyx mori nucleopolyhedrovirus in Sf9 and High Five cells: defective nuclear transport of the virions

Despite close genetic relationship, Bombyx mori nucleopolyhedrovirus (BmNPV) and Autographa californica multicapsid NPV (AcMNPV) display a distinct host range property. Here, BmNPV replication was examined in Sf9 and High Five cells that were nonproductive for BmNPV infection but supported high titers of AcMNPV replication. Recombinant BmNPV, vBm/gfp/lac, containing bm-ie1 promoter-driven egfp showed that few Sf9 and High Five cells infected with vBm/gfp/lac expressed EGFP, while large proportion of EGFP-expressing cells was observed when transfected with vBm/gfp/lac DNA. Immunocytochemical analysis showed that BmNPV was not imported into the nucleus of these two cell lines, while recombinant BmNPV, vBmDelta64/ac-gp64 possessing AcMNPV gp64 was imported into the nucleus, yielding progeny virions in High Five cells, but not Sf9 cells. These results indicate that the defective nuclear import of infected virions due to insufficient BmNPV GP64 function is involved in the restricted BmNPV replication in Sf9 and High Five cells.

Expression and mutational analysis of Autographa californica nucleopolyhedrovirus HCF-1: functional requirements for cysteine residues

The host cell-specific factor 1 gene (hcf-1) of the baculovirus Autographa californica multiple nucleopolyhedrovirus is required for efficient virus growth in TN368 cells but is dispensable for virus replication in SF21 cells. However, the mechanism of action of hcf-1 is unknown. To begin to understand its function in virus replication we have investigated the expression and localization pattern of HCF-1 in infected cells. Analysis of virus-infected TN368 cells showed that hcf-1 is expressed at an early time in the virus life cycle, between 2 and 12 h postinfection, and localized the protein to punctate nuclear foci. Through coprecipitation experiments we have confirmed that HCF-1 self-associates into dimers or higher-order structures. We also found that overexpression of HCF-1 repressed expression from the hcf-1 promoter in transient reporter assays. Mutagenesis of cysteine residues within a putative RING finger domain in the amino acid sequence of HCF-1 abolished self-association activity and suggests that the RING domain may be involved in this protein-protein interaction. A different but overlapping set of cysteine residues were required for efficient gene repression activity. Functional analysis of HCF-1 mutants showed that the cysteine amino acids required for both self-association and gene repression activities of HCF-1 were also required for efficient late-gene expression and occlusion body formation in TN368 cells. Mutational analysis also identified essential charged and hydrophobic amino acids located between two of the essential cysteine residues. We propose that HCF-1 is a RING finger-containing protein whose activity requires HCF-1 self-association and gene repression activity.

Productive infection of Autographa californica nucleopolyhedrovirus in silkworm Bombyx mori strain Haoyue due to the absence of a host antiviral factor

We have reported that several silkworm strains are permissive to intrahemocoelical infection of Autographa californica nucleopolyhedrovirus (AcNPV), contrary to the general belief that AcNPV cannot infect silkworm. In the present study, we address whether the intrahemocoelical infection of AcNPV to the silkworm was an exceptional phenomenon, and the possible genetic basis underlying it. Wilder range test of 31 strains of silkworm Bombyx mori for intrahemocoelical AcNPV infection led to the identification of 14 permissive strains and 17 nonpermissive strains, indicating that the intrahemocoelical infection of AcNPV to the silkworm was not a rare and isolated phenomenon. Productive infection was shown in permissive silkworms, by EGFP fluorescence in various tissues when expression of reporter gene controlled by a very late viral promoter polh. The viral titer in larval hemolymph of permissive silkworms increased and maintained at a higher level hundredfold more than the initial amount of virus, indicating viral replication. A series of genetic cross experiments suggested the existence of only one dominant host anti-AcNPV gene or a set of genetically linked genes, which prevent AcNPV infection in nonpermissive silkworm strain Qingsong and are absent in permissive silkworm strain Haoyue.

AcMNPV in permissive, semipermissive, and nonpermissive cell lines from Arthropoda

Insect cell lines from Arthropoda represented by Lepidoptera, Coleoptera, Diptera, and Homoptera were evaluated for their ability to support replication of AcMNPV. In addition, some of the cell lines that were refractive to AcMNPV were tested with AcMNPV hsp70 Red, a recombinant carrying the red fluorescent protein (RFP) gene, for their ability to express this protein after inoculation. Of the 10 lepidopteran cell lines tested, only three cell lines from Helicoverpa zea (BCIRL-HZ-AM1), Lymantria dispar (IPLB-LD 65), and Cydia pomonella (CP-169) failed to support detectable viral replication as measured by tissue culture infectious dose 50 (TCID50) assay. Heliothis virescens (BCIRL-HV-AM1) produced the highest viral titer of 2.3 +/- 0.1 x 10(7) TCID50/ml followed by Heliothis subflexa (BCIRL-HS-AM1) at 4.7 +/- 0.1 x 10(6) TCID50/ml and Spodoptera frugiperda (IPLB-SF21) at 4.1 +/- 0.1 x 10(6) TCID50/ml. None of the coleopteran, dipteran, or homopteran cell lines supported AcMNPV replication. However, when studies were performed using AcMNPV hsp70 Red, the dipteran cell lines Aedes aegypti (ATC-10) and Drosophila melanogaster (line 2), both expressed the RFP as well as the refractive lepidopteran cell lines from H. zea and L. dispar. No RFP expression was observed in any of the coleopteran or homopteran cell lines. Cell lines refractive to AcMNPV did not appear to be adversely affected by the virus, as judged by their ability to multiply, nor was there any indication of induced apoptosis, as assessed by deoxyribonucleic acid fragmentation profiles or cell blebbing, or both.

Functional analysis of the baculovirus host range gene, hrf-1

The hrf-1 gene from Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV) prevents translation arrest and promotes Autographa californica multiple nucleopolyhedrovirus (AcMNPV) replication in IPLB-Ld652Y cells (Ld652Y), a non-permissive L. dispar cell line. There are no motifs in the predicted protein sequence to suggest how it might function and the only homolog identified is encoded by another baculovirus, Orgyia pseudotsugata multiple nucleopolyhedrovirus (OpMNPV). In this study, we report a functional analysis of the hrf-1 protein. AcMNPV bearing carboxy- or amino-terminally truncation hrf-1, and hrf-1 mutated by two-amino acid insertions did not replicate Ld652Y cells. Neither OpMNPV hrf-1 nor an OpMNPV/LdMNPV chimeric hrf-1 supported AcMNPV replication. Mutations in a highly acidic domain of hrf-1, in which aspartic acid residues were replaced with alanine, had varied effects on hrf-1 function. They had no effect, abolished hrf-1 function completely, or partially supported protein synthesis in infected Ld652Y cells. A slight increase in protein synthesis was achieved by increasing the expression of hrf-1 acidic domain mutant proteins. Together, these results indicate a critical role for hrf-1 structure and suggest a functional role for the acidic domain.

Expression profiling of baculovirus genes in permissive and nonpermissive cell lines

The baculoviruses Bombyx mori nucleopolyhedrovirus (BmNPV) and Autographa californica multicapsid NPV (AcMNPV) are highly homologous at the genomic level, but they have essentially nonoverlapping host ranges. In order to characterize baculovirus replication in permissive and nonpermissive cell lines, the expression profiles of baculovirus-specific genes (at 2, 6, 12, 24, 36, 48 or 72 h post-infection) were examined in BmN (BmNPV-permissive) or Sf-9 (AcMNPV-permissive) cells that were inoculated with BmNPV or AcMNPV. Surprisingly, nearly all of the 154 genes of AcMNPV appeared to be expressed in both Sf-9 and BmN cells although the peak expression levels of these genes were delayed by roughly 12 h in the nonpermissive BmN cells. In addition, the expression levels of the very late AcMNPV polyhedrin and p10 genes were dramatically reduced in BmN cells, which presumably led to the inability of AcMNPV to form polyhedral inclusion bodies in BmN cells. Nearly all of the 136 genes of BmNPV appeared to be expressed in BmN cells, however, BmNPV gene expression was dramatically reduced in Sf-9 cells inoculated with BmNPV. Experiments in which BmNPV DNAs were transfected to Sf-9 cells suggested that this dramatic reduction in gene expression was not the result of poor attachment, penetration or uncoating of the BmNPV virion into Sf-9 cells. In conclusion, we established a system to monitor global gene expression patterns during baculovirus infection in permissive and nonpermissive cell lines. This system was used to identify global trends in the transcription of baculovirus genes during productive and nonproductive infection.

Host range factor 1 from Lymantria dispar Nucleopolyhedrovirus (NPV) is an essential viral factor required for productive infection of NPVs in IPLB-Ld652Y cells derived from L. dispar

Host range factor 1 (HRF-1) of Lymantria dispar multinucleocapsid nucleopolyhedrovirus promotes Autographa californica MNPV replication in nonpermissive Ld652Y cells derived from L. dispar. Here we demonstrate that restricted Hyphantria cunea NPV replication in Ld652Y cells was not due to apoptosis but was likely due to global protein synthesis arrest that could be restored by HRF-1. Our data also showed that HRF-1 promoted the production of progeny virions for two other baculoviruses, Bombyx mori NPV and Spodoptera exigua MNPV, whose replication in Ld652Y cells is limited to replication of viral DNA without successful production of infectious progeny virions. Thus, HRF-1 is an essential viral factor required for productive infection of NPVs in Ld652Y cells.

Virus entry or the primary infection cycle are not the principal determinants of host specificity of Spodoptera spp. nucleopolyhedroviruses

The multicapsid nucleopolyhedroviruses (NPVs) of Spodoptera exigua (SeMNPV), Spodoptera frugiperda (SfMNPV), and Spodoptera littoralis (SpliNPV) are genetically similar (78 % similarity) but differ in their degree of host specificity. Infection by each of the three NPVs in these three Spodoptera host species was determined by oral inoculation of larvae with occlusion bodies (OBs) or intrahaemocoelic injection with occlusion derived virions (ODVs). RT-PCR analysis of total RNA from inoculated insects, targeted at immediate early (ie-0), early (egt, DNA polymerase), late (chitinase) and very late genes (polyhedrin), indicated that each of the NPVs initiated an infection in all three host species tested. SpliMNPV produced a fatal NPV disease in both heterologous hosts, S. frugiperda and S. exigua, by oral inoculation or injection. SfMNPV was lethal to heterologous hosts, S. exigua and S. littoralis, but infected larvae did not melt and disintegrate, and progeny OBs were not observed. SeMNPV was able to replicate in heterologous hosts and all genes required for replication were present in the genome, as the virus primary infection cycle was observed. However, gene expression was significantly lower in heterologous hosts. SeMNPV pathogenesis in S. frugiperda and S. littoralis was blocked at the haemocoel transmission stage and very nearly cleared. SeMNPV mixtures with SpliMNPV or SfMNPV did not extend the host range of SeMNPV; in all cases, only the homologous virus was observed to proliferate. It is concluded that entry and the primary virus infection cycle are not the only, or the major determinants, for SeMNPV infection of heterologous Spodoptera species.

Baculovirus Late Expression Factors

Autographa californica nuclear polyhedrosis virus, or AcMNPV, is the type member of the baculoviruses, a family of double-stranded DNA viruses with large circular genomes. The successive and concomitant expression of an assortment of early, late and very late genes is instrumental for successful baculovirus infection, and requires a switch from early dependence on a host cell-derived polymerase II to a novel virus-encoded RNA polymerase that is required for transcription later on in infection. A series of repetitive and highly conserved sequences known as homologous regions, or hrs, function both as origins of DNA replication as well as transcriptional enhancers of late gene expression. An array of AcMNPV genes produced early on in infection, known as late expression factors, or LEFs, are essential for both replication and late gene expression. In this review, an overview of baculovirus LEFs and their roles in viral replication and late gene expression is presented. The role of LEFs in determining baculovirus host range is described. Finally, we compare baculovirus replication and transcription machinery with other viral systems.

The role of baculovirus apoptotic suppressors in AcMNPV-mediated translation arrest in Ld652Y cells

Infecting the insect cell line IPLB-Ld652Y with the baculovirus Autographa californica multinucleocapsid nucleopolyhedrovirus (AcMNPV) results in global translation arrest, which correlates with the presence of the AcMNPV apoptotic suppressor, p35. In this study, we investigated the role of apoptotic suppression on AcMNPV-induced translation arrest. Infecting cells with AcMNPV bearing nonfunctional mutant p35 did not result in global translation arrest. In contrast, global translation arrest was observed in cells infected with AcMNPV in which p35 was replaced with Opiap, Cpiap, or p49, baculovirus apoptotic suppressors that block apoptosis by different mechanisms than p35. These results indicated that suppressing apoptosis triggered translation arrest in AcMNPV-infected Ld652Y cells. Experiments using the DNA synthesis inhibitor aphidicolin and temperature shift experiments, using the AcMNPV replication mutants ts8 and ts8deltap35, indicated that translation arrest initiated during the early phase of infection, but events during the late phase were required for global translation arrest. Peptide caspase inhibitors could not substitute for baculovirus apoptotic suppressors to induce translation arrest in Ld652Y cells infected with a p35-null virus. However, if the p35-null-AcMNPV also carried hrf-1, a novel baculovirus host range gene, progeny virus was produced and treatment with peptide caspase inhibitors enhanced translation of a late viral gene transcript. Together, these results indicate that translation arrest in AcMNPV-infected Ld652Y cells is due to the anti-apoptotic function of p35, but suggests that rather than simply preventing caspase activation, its activity enhances signaling to a separate translation arrest pathway, possibly by stimulating the late stages of the baculovirus infection cycle.

Insect defenses against virus infection: the role of apoptosis

Insects, with their lack of an adaptive immune response, provide a unique animal model to examine the effects of apoptosis on viral infection. Several members of the baculovirus family of insect viruses have been shown to induce apoptosis during infection of cultured insect cells, and depending on the virus-host combination this apoptotic response can severely limit viral replication. In response to this evolutionary pressure, all baculoviruses studied to date carry antiapoptotic genes, including members of the p35 and IAP (inhibitor of apoptosis) gene families. Recent work has characterized the apoptotic response during infection of the host insect, and the results directly demonstrate the power of apoptosis as an antiviral response.

Analysis of host specificity of two closely related baculoviruses in permissive and nonpermissive cell lines

The baculoviruses Bombyx mori nucleopolyhedrovirus (BmNPV) and Autographa californica multinucleocapsid nucleopolyhedrovirus (AcMNPV) share about 90% identity at the genomic level but they have non-overlapping host range and show a high degree of host specificity. We have demonstrated here that AcMNPV undergoes DNA replication and early gene expression in Bombyx-derived BmN cells but fails to show very late gene expression or produce budded virion (BV) particles. Coinfection with BmNPV supported BV production from AcMNPV in BmN cells at low levels but not very late gene expression or polyhedral inclusion body formation. BV production and very late gene expression from BmNPV, on the contrary, were adversely affected in coinfections. In Spodoptera frugiperda-derived Sf21 cell lines, BmNPV DNA replication, BV production, and very late gene expression took place only when coinfected with AcMNPV. BmNPV exerted a less profound effect on AcMNPV multiplication and very late gene expression in permissive host cell lines. AcMNPV shuts down cellular and viral protein synthesis completely when infected alone or coinfected with BmNPV in BmN cells, whereas BmNPV infection did not affect cellular and viral protein synthesis in Sf21 cells. Overall, AcMNPV showed a more dominant effect by complementing the multiplication of BmNPV in nonpermissive host cells while inhibiting it in BmN cells.

Induction of apoptosis in an insect cell line, IPLB-Ld652Y, infected with nucleopolyhedroviruses

Ld652Y cells derived from the gypsy moth, Lymantria dispar, were infected with seven different nucleopolyhedroviruses (NPVs) including those from Autographa californica, Bombyx mori (BmNPV), Hyphantria cunea (HycuNPV), Spodoptera exigua (SeMNPV), L. dispar, Orgyia pseudotsugata (OpMNPV) and Spodoptera litura (SpltMNPV). The results showed that Ld652Y cells infected with BmNPV, HycuNPV, SeMNPV, OpMNPV and SpltMNPV underwent apoptosis, displaying apoptotic bodies, characteristic DNA fragmentation and increased caspase-3-like protease activity; HycuNPV induced the most severe apoptosis. In HycuNPV-infected Ld652Y cells, a considerable amount of viral DNA was synthesized although there was no detectable yield of budded virions and polyhedrin. Northern blot and immunoblot analyses revealed that HycuNPV inhibitor of apoptosis 3 (IAP3), which has been shown to function in Sf9 cells, was expressed in HycuNPV-infected Ld652Y cells at a level higher than or comparable with that in HycuNPV-infected SpIm cells, which produced a high titre of progeny virions without any apoptotic response. These results imply that the relative ease of apoptosis induction in NPV-infected Ld652Y cells is largely dependent on inherent cellular properties rather than functions of the respective NPVs, and indicate that the defect in progeny virion production is not merely due to the virus-induced apoptosis in HycuNPV-infected Ld652Y cells.

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

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

Identification and molecular characterization of the baculovirus CfMNPV early genes: ie-1, ie-2 and pe38

Three early virus genes, ie-1, ie-2 and pe38, were identified and localized in the XbaI G region (91.2-98.6 m.u.) of the genome of Choristoneura fumiferana nucleopolyhedrovirus (CfMNPV), a baculovirus pathogenic to spruce budworm. Nucleotide sequence analysis indicated that these genes share varied sequence similarity with their homologues in other baculoviruses where they are involved in regulating virus gene expression and DNA replication. Sequence motifs characteristic of DNA binding and transactivation found in other baculovirus regulatory genes were conserved in the CfMNPV genes. Northern analysis demonstrated that all three CfMNPV genes were transcriptionally active in virus infected cells and followed the temporal expression pattern of immediate early baculovirus genes. Primer extension experiments revealed that typical baculovirus early transcription start sites (CAGT) were used for ie-1 and pe38 transcription initiation. Two regions of highly repetitive DNA were found in the odv-e56 to ie-2 and ie-2 and pe38 intergenic regions. These sequences are predicted to function as transcriptional enhancers and viral origins of DNA replication.

The genome sequence and evolution of baculoviruses

Comparative analysis of the complete genome sequences of 13 baculoviruses revealed a core set of 30 genes, 20 of which have known functions. Phylogenetic analyses of these 30 genes yielded a tree with 4 major groups: the genus Granulovirus (GVs), the group I and II lepidopteran nucleopolyhedroviruses (NPVs), and the dipteran NPV, CuniNPV. These major divisions within the family Baculoviridae were also supported by phylogenies based on gene content and gene order. Gene content mapping has revealed the patterns of gene acquisitions and losses that have taken place during baculovirus evolution, and it has highlighted the fluid nature of baculovirus genomes. The identification of shared protein phylogenetic profiles provided evidence for two putative DNA repair systems and for viral proteins specific for infection of lymantrid hosts. Examination of gene order conservation revealed a core gene cluster of four genes, helicase, lef-5, ac96, and 38K(ac98), whose relative positions are conserved in all baculovirus genomes.

Global protein synthesis shutdown in Autographa californica nucleopolyhedrovirus-infected Ld652Y cells is rescued by tRNA from uninfected cells

Global protein synthesis arrest occurs in Autographa californica nucleopolyhedrovirus (AcNPV)-infected Ld652Y cells at late times postinfection (p.i.). A Lymantria dispar nucleopolyhedrovirus gene, hrf-1, precludes this protein synthesis arrest. We used in vitro translation assays to characterize the translation defect. Cell-free lysates prepared from uninfected Ld652Y cells, AcNPV-infected cells harvested at early times p.i., and cells infected with vAchrf-1, a recombinant AcNPV bearing hrf-1, all supported translation. Lysates prepared from AcNPV-infected Ld652Y cells at late times p.i. did not support translation, but activity was restored by adding small RNA species from mock-, vAchrf-1- (24 or 48 h p.i.), and AcNPV- (6 h p.i. ) infected cells. Small RNA species (24 and 48 h p.i.) from AcNPV-infected cells did not rescue translation. Assays of RNA species further fractionated by ion exchange chromatography demonstrated that tRNA rescued translation. Although specific defective tRNA species were not revealed by comparative two-dimensional gel analysis, analysis of (32)P-labeled tRNAs showed a reduction in de novo synthesis of small RNA isolated from AcNPV-infected cells compared with mock- and vAchrf-1-infected cells. This study suggests a mechanism of translation arrest involving defective or depleted tRNA species in AcNPV-infected Ld652Y cells.

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

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

Isolation of an apoptosis suppressor gene of the Spodoptera littoralis nucleopolyhedrovirus

Spodoptera frugiperda SF9 cells infected with mutants of the Autographa californica nucleopolyhedrovirus (AcMNPV) which lack a functional p35 gene undergo apoptosis, aborting the viral infection. The Spodoptera littoralis nucleopolyhedrovirus (SlNPV) was able to suppress apoptosis triggered by vDeltaP35K/pol+, an AcMNPV p35 null mutant. To identify the putative apoptotic suppressor gene of SlNPV, overlapping cosmid clones representing the entire SlNPV genome were individually cotransfected along with genomic DNA of vDeltaP35K/pol+. Using this complementation assay, we isolated a SlNPV DNA fragment that was able to rescue the vDeltaP35K/pol+ infection in SF9 cells. By further subcloning and rescue, we identified a novel SlNPV gene, Slp49. The Slp49 sequence predicted a 49-kDa polypeptide with about 48.8% identity to the AcMNPV apoptotic suppressor P35. SLP49 displays a potential recognition site, TVTDG, for cleavage by death caspases. Recombinant AcMNPVs deficient in p35 bearing the Slp49 gene did not induce apoptosis and showed successful productive infections in SF9 cells, indicating that Slp49 is a functional homologue of p35. A 1.5-kbp Slp49-specific transcript was identified in SF9 cells infected with SlNPV or with vAc496, a vDeltaP35K/pol+-recombinant bearing Slp49. The discovery of Slp49 contributes to the identification of important functional motifs conserved in p35-like apoptotic suppressors and to the future isolation of p35-like genes from other baculoviruses.

Four genotypic variants of a Spodoptera exigua Nucleopolyhedrovirus (Se-SP2) are distinguishable by a hypervariable genomic region

Four genotypes named SP2A, SP2B, SP2C and SP2D were obtained in vivo by infecting S. exigua larvae with limiting dilutions of the Spanish field isolate Spodoptera exigua Nucleopolyhedrovirus (Se-SP2) of SeMNPV. The cloning of variants SP2A, SP2B and SP2C took 1, 6, and 3 passages, respectively, before the DNA profiles showed all bands in equimolar concentrations, and they remained constant for at least six further passages indicating the stability of their genotypes. The SP2D variant isolation took over ten passages and it was genetically less stable. Physical maps of their genomes were constructed for the restriction enzymes BamHI, BglII, PstI, and XbaI. The region between 8-10 m.u. was highly variable and characteristic of each cloned genotype and, hence, can be used as RFLP markers for all four genotypic variants. This region, included in the PstI-MB fragment, was cloned and sequenced showing that all the Se-SP2 variants contained a homologous region (hr) with a variable number of 98 bp sequences tandemly repeated, which were used to distinguish genotypic variants from each other. The biological activity of the genotypic variants SP2A, SP2B, and SP2C when compared in terms of LD50 and LT50, were not significantly different. However, the SP2D genotypic variant was found to be significantly less infective (higher LD50). The emergence of new genotypes in the Se-SP2 field populations is discussed.