Transcription analysis of the EcoRI D region of the baculovirus Autographa californica nuclear polyhedrosis virus identifies an early 4-kilobase RNA encoding the essential p143 gene

The 38K-Mediated Specific Dephosphorylation of the Viral Core Protein P6.9 Plays an Important Role in the Nucleocapsid Assembly of Autographa californica Multiple Nucleopolyhedrovirus

Encapsidation of the viral genomes, leading to the assembly of the nucleocapsids to form infectious progeny virions, is a key step in many virus life cycles. Baculovirus nucleocapsid assembly is a complex process that involves many proteins. Our previous studies showed that the deletion of the core gene 38K (ac98) interrupted the nucleocapsid assembly by producing capsid sheaths devoid of viral genomes by an unknown mechanism. All homologs of 38K contain conserved motifs of the haloacid dehalogenase superfamily, which are involved in phosphoryl transfer. The requirements of these motifs for nucleocapsid assembly, confirmed in the present study, suggest that 38K may be a functioning haloacid dehalogenase. P6.9 is also encoded by a core gene (ac100) and is required for viral genome encapsidation. It has been reported that multiple phosphorylated species of P6.9 are present in virus-infected cells, while only an unphosphorylated species is detected in the budded virus. Therefore, whether 38K mediates the dephosphorylation of P6.9 was investigated. An additional phosphorylated species of P6.9 in 38K-deleted or -mutated virus-transfected cells was detected, and the dephosphorylated sites mediated by 38K were determined by mass spectrometry. To assess the effects of dephosphorylation of P6.9 mediated by 38K on virus replication, these sites were mutated to glutamic acids (phosphorylation-mimic mutant) or to alanines (phosphorylation-deficient mutant). Studies showed that the nucleocapsid assembly was interrupted in phosphorylation-mimic mutant virus-transfected cells. Taken together, our findings demonstrate that 38K mediates the dephosphorylation of specific sites at the C terminus of P6.9, which is essential for viral genome encapsidation.IMPORTANCE Genome packaging is a fundamental process in the virus life cycle, and viruses have different strategies to perform this step. For several double-stranded DNA (dsDNA) viruses, the procapsid is formed before genome encapsidation, which may require basic proteins that help to neutralize the nucleic acid charge repulsion to facilitate the compaction of the genome within the confined capsid space. Baculovirus encodes a small basic protein, P6.9, which is required for a variety of processes in the virus infection cycle. The phosphorylation of P6.9 is thought to result in nucleocapsid uncoating, while the dephosphorylation of P6.9 is involved in viral DNA encapsidation during nucleocapsid assembly. Here, we demonstrate that a haloacid dehalogenase homolog encoded by baculovirus core gene 38K is involved in nucleocapsid assembly by mediating the dephosphorylation of 5 specific sites at the C terminus of P6.9. This finding contributes to the understanding of the mechanisms of virus nucleocapsid assembly.

The transcriptome of the baculovirus Autographa californica multiple nucleopolyhedrovirus in Trichoplusia ni cells

Baculoviruses are important insect pathogens that have been developed as protein expression vectors in insect cells and as transduction vectors for mammalian cells. They have large double-stranded DNA genomes containing approximately 156 tightly spaced genes, and they present significant challenges for transcriptome analysis. In this study, we report the first comprehensive analysis of AcMNPV transcription over the course of infection in Trichoplusia ni cells, by a combination of strand-specific RNA sequencing (RNA-Seq) and deep sequencing of 5' capped transcription start sites and 3' polyadenylation sites. We identified four clusters of genes associated with distinctive patterns of mRNA accumulation through the AcMNPV infection cycle. A total of 218 transcription start sites (TSS) and 120 polyadenylation sites (PAS) were mapped. Only 29 TSS were associated with a canonical TATA box, and 14 initiated within or near the previously identified CAGT initiator motif. The majority of viral transcripts (126) initiated within the baculovirus late promoter motif (TAAG), and late transcripts initiated precisely at the second position of the motif. Analysis of 3' ends showed that 92 (77%) of the 3' PAS were located within 30 nucleotides (nt) downstream of a consensus termination signal (AAUAAA or AUUAAA). A conserved U-rich region was found approximately 2 to 10 nt downstream of the PAS for 58 transcripts. Twelve splicing events and an unexpectedly large number of antisense RNAs were identified, revealing new details of possible regulatory mechanisms controlling AcMNPV gene expression. Combined, these data provide an emerging global picture of the organization and regulation of AcMNPV transcription through the infection cycle.

Autographa californica multiple nucleopolyhedrovirus ac76 is involved in intranuclear microvesicle formation

In this study, we characterized Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf76 (ac76), which is a highly conserved gene of unknown function in lepidopteran baculoviruses. Transcriptional analysis of ac76 revealed that transcription of multiple overlapping multicistronic transcripts initiates from a canonical TAAG late-transcription start motif but terminates at different 3' ends at 24 h postinfection in AcMNPV-infected Sf9 cells. To investigate the role of ac76 in the baculovirus life cycle, an ac76-knockout virus was constructed using an AcMNPV bacmid system. Microscopy, titration assays, and Western blot analysis demonstrated that the resulting ac76-knockout virus was unable to produce budded viruses. Quantitative real-time PCR analysis demonstrated that ac76 deletion did not affect viral DNA synthesis. Electron microscopy showed that virus-induced intranuclear microvesicles as well as occlusion-derived virions were never observed in cells transfected with the ac76-knockout virus. Confocal microscopy analysis revealed that Ac76 was predominantly localized to the ring zone of nuclei during the late phase of infection. This suggests that ac76 plays a role in intranuclear microvesicle formation. To the best of our knowledge, this is the first baculovirus gene identified to be involved in intranuclear microvesicle formation.

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

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

The AcMNPV pp31 gene is not essential for productive AcMNPV replication or late gene transcription but appears to increase levels of most viral transcripts

The pp31 gene of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) encodes a phosphorylated DNA binding protein that associates with virogenic stroma in the nuclei of infected cells. Prior studies of pp31 by transient late expression assays suggested that pp31 may play an important role in transcription of AcMNPV late genes [Todd, J. W., Passarelli, A. L., and Miller, L. K. (1995). Eighteen baculovirus genes, including lef-11, p35, 39K, and p47, support late gene expression. J. Virol. 69, 968-974] although genetic studies of the closely related BmNPV pp31 gene suggested that pp31 may be dispensable [Gomi, S., Zhou, C. E., Yih, W., Majima, K., and Maeda, S. (1997). Deletion analysis of four of eighteen late gene expression factor gene homologues of the baculovirus, BmNPV. Virology 230 (1), 35-47]. In the current study, we examined the role of the pp31 gene in the context of the AcMNPV genome during infection. We used a BACmid-based system to generate a pp31 knockout in the AcMNPV genome. The pp31 knockout was subsequently rescued by reinserting the pp31 gene into the polyhedrin locus of the same virus genome. We found that pp31 was not essential for viral replication although the absence of pp31 resulted in a lower viral titer. Analysis of viral DNA replication in the absence of pp31 showed that the kinetics of viral DNA replication were unaffected. An AcMNPV oligonucleotide microarray was used to compare gene expression from all AcMNPV genes in the presence or absence of pp31. In the absence of pp31, a modest reduction in transcripts was detected for many viral genes (99 genes) while no substantial increase or decrease was observed for 43 genes. Transcripts from 6 genes (p6.9, ORF 97, ORF 60, ORF 98, ORF 102 and chitinase) were reduced by 66% or more compared to the levels detected from the control virus. Microarray results were further examined by qPCR analysis of selected genes. In combination, these data show that deletion of the pp31 gene was not lethal and did not appear to affect viral DNA replication but resulted in an apparent modest down-regulation of a subset of AcMNPV genes that included both early and late genes.

Autographa californica multiple nucleopolyhedrovirus nucleocapsid assembly is interrupted upon deletion of the 38K gene

38K (ac98) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a highly conserved baculovirus gene whose function is unknown. To determine the role of 38K in the baculovirus life cycle, a 38K knockout bacmid containing the AcMNPV genome was generated through homologous recombination in Escherichia coli. Furthermore, a 38K repair bacmid was constructed by transposing the 38K open reading frame with its native promoter region into the polyhedrin locus of the 38K knockout bacmid. After transfection of these viruses into Spodoptera frugiperda cells, the 38K knockout bacmid led to a defect in production of infectious budded virus, while the 38K repair bacmid rescued this defect, allowing budded-virus titers to reach wild-type levels. Slot blot analysis indicated that 38K deletion did not affect the levels of viral DNA replication. Subsequent immunoelectron-microscopic analysis revealed that masses of electron-lucent tubular structures containing the capsid protein VP39 were present in cells transfected with 38K knockout bacmids, suggesting that nucleocapsid assembly was interrupted. In contrast, the production of normal nucleocapsids was restored when the 38K knockout bacmid was rescued with a copy of 38K. Recombinant virus that expresses 38K fused to green fluorescent protein as a visual marker was constructed to monitor protein transport and localization within the nucleus during infection. Fluorescence was first detected along the cytoplasmic periphery of the nucleus and subsequently localized to the center of the nucleus. These results demonstrate that 38K plays a role in nucleocapsid assembly and is essential for viral replication in the AcMNPV life cycle.

Identification, expression and phylogenetic analysis of the Anticarsia gemmatalis multicapsid nucleopolyhedrovirus (AgMNPV) Helicase gene

The helicase gene from Anticarsia gemmatalis multicapsid nucleopolyhedrovirus (AgMNPV) was identified and localized in the 58.85-65.90 m.u. of the viral genomic map. This gene encodes a putative polypeptide of 1221 amino acids containing motifs homologous to those found in the helicase superfamily. Expression of the AgMNPV helicase was observed as early as 4h post-infection (p.i.) up until 10 h p.i. A unique early transcription initiation site was observed upstream a putative TATA box. Phylogenetic analysis of the helicase genes of 23 baculoviruses indicated that the AgMNPV helicase is closely related to the helicase genes from Epiphyas postvitanna multicapsid nucleopolyhedrovirus and Choristoneura fumiferana defective nucleopolyhedrovirus.

Characterization of the interaction between P143 and LEF-3 from two different baculovirus species: Choristoneura fumiferana nucleopolyhedrovirus LEF-3 can complement Autographa californica nucleopolyhedrovirus LEF-3 in supporting DNA replication

The baculovirus protein P143 is essential for viral DNA replication in vivo, likely as a DNA helicase. We have demonstrated that another viral protein, LEF-3, first described as a single-stranded DNA binding protein, is required for transporting P143 into the nuclei of insect cells. Both of these proteins, along with several other early viral proteins, are also essential for DNA replication in transient assays. We now describe the identification, nucleotide sequences, and transcription patterns of the Choristoneura fumiferana nucleopolyhedrovirus (CfMNPV) homologues of p143 and lef-3 and demonstrate that CfMNPV LEF-3 is also responsible for P143 localization to the nucleus. We predicted that the interaction between P143 and LEF-3 might be critical for cross-species complementation of DNA replication. Support for this hypothesis was generated by substitution of heterologous P143 and LEF-3 between two different baculovirus species, Autographa californica nucleopolyhedrovirus and CfMNPV, in transient DNA replication assays. The results suggest that the P143-LEF-3 complex is an important baculovirus replication factor.

Expression and localization of LEF-11 in Autographa californica nucleopolyhedrovirus-infected Sf9 cells

The Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) lef-11 gene was found previously to be necessary to support optimal levels of transient expression from an AcMNPV late promoter. The lef-11 gene is unusual in that it overlaps both upstream (orf38) and downstream (pp31) genes. In this study, the expression and cellular localization of LEF-11 were examined. The lef-11 transcripts were detected from 4 to 36 h post-infection (p.i.). The 1.5 kb lef-11 mRNA initiates 196 nt upstream of the lef-11 translation initiation codon, within the upstream orf38 gene. This relatively long 5' upstream region encodes a potential small upstream open reading frame (ORF) of 58 amino acids that overlaps the lef-11 ORF. The 3' end of the lef-11 mRNA was mapped as co-terminal with mRNAs from the downstream pp31 gene. Using affinity purified anti-LEF-11 antibodies, levels of LEF-11 expression were found to be maximal between approximately 8 and 24 h p.i., although LEF-11 could be detected as late as 72 h p.i. Using immunofluorescence microscopy, it was determined that LEF-11 localized to dense regions of infected cell nuclei, consistent with its role as a possible late transcription factor.

A baculovirus single-stranded DNA binding protein, LEF-3, mediates the nuclear localization of the putative helicase P143

The intracellular localization of the baculovirus Autographa californica nuclear polyhedrosis virus p143 gene product (P143) was investigated by immunofluoresence staining of infected and transfected cells. As expected for a protein essential for viral DNA replication, under these conditions, P143 was localized to the nucleus. However, when a plasmid directing the synthesis of P143 from its own promoter was co-transfected with a plasmid expressing IE-1, P143 was found in the cytoplasm. Cotransfection of cells with a series of plasmids expressing viral genes sufficient for stimulation of plasmid replication resulted in nuclear localization of P143 suggesting that one of the gene products required for viral DNA replication may also assist nuclear localization of P143. Sequential deletion of various genes from this assay revealed that when the plasmid expressing LEF-3 was deleted from the mixture, P143 remained in the cytoplasm. Plasmids were constructed where the synthesis of LEF-3 and P143 were directed by the ie-1 promoter. When these plasmids were cotransfected, both LEF-3 and P143 colocalized to the nucleus suggesting that an important function of LEF-3 is intracellular trafficking of P143 and directing it to the nucleus.

A highly conserved genomic region in baculoviruses: sequence analysis of an 11.3 kbp DNA fragment (46.5-55.1 m.u.) of the Spodoptera exigua multicapsid nucleopolyhedrovirus

A DNA fragment of 11.3 kilobase pairs (kbp) in size of the baculovirus Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) genome (46.5 to 55.1 m.u.) was completely sequenced. Analysis of the sequence revealed eleven potential open reading frames (ORF). Ten of these ORFs showed significant amino acid identity to Autographa californica MNPV (AcMNPV) and Orgyia pseudotsugata MNPV (OpMNPV) genes p6.9, lef5, 38K, p19, p143, p25, p18, vp33, lef4, and vp39. One ORF (XC12) has no homolog in other baculoviruses and may be unique to SeMNPV. All but three of these putative genes are preceded by the consensus baculovirus late promoter element (5'-ATAAG-3'). The genetic organization and the putative map of transcripts of this fragment suggested that this region is highly similar to a region in AcMNPV fragment EcoRI-D. Comparison of the genetic organization of this 11.3 kbp fragment with the genomes of AcMNPV, OpMNPV, Bombyx mori NPV (BmNPV) and SeMNPV revealed that this region is highly conserved among baculovirus genomes. This is in contrast to the genetic organization of the polyhedrin-p10 region, which is much more diverged, but has been taken as point of reference to orient baculovirus physical maps. Through its diversity the latter region, however, would be an excellent candidate to determine baculovirus relatedness and phylogeny. The presence of conserved and diverged regions in baculovirus genomes with respect to gene order is reminiscent to the situation in other large DNA viruses, such as herpes- and poxviruses, where conserved central and diverged terminal parts are common characteristics. The role of this feature in the genomic organization of large DNA viruses is discussed with particular emphasis on virus replication and evolution.

Actin binding and nucleation by Autographa california M nucleopolyhedrovirus

The budded form of Autographa californica M nucleopolyhedrovirus enters permissive cells via adsorptive endocytosis. Shortly after nucleocapsid penetration into the cytoplasm, thick actin cables form, which frequently project toward the nucleus. These actin cables are transient structures, formed in association with viral nucleocapsids prior to viral gene expression and concomitant with nucleocapsid transport to the nucleus. In this paper we report that nucleocapsids are capable of nucleating actin polymerization in vitro in a concentration-dependent manner. Two viral-encoded capsid proteins, p39 and p78/83, were found to bind actin directly and therefore could be involved in the observed acceleration of actin polymerization. When nucleocapsids were added to actin in the presence of cytochalasin D, actin polymerization was reduced to levels below those obtained with actin and cytochalasin D alone, suggesting that the nucleocapsids bound to the pointed ends of actin filaments. Finally, treatment of infected cells with the myosin inhibitor 2,3-butanedione monoxime delayed nucleocapsid transport to the nucleus. We postulate that upon entering the cytoplasm, AcMNPV nucleocapsids induce the polymerization of actin cables, which, in conjunction with a myosin-like motor, facilitate their transport to and/or into the nucleus.

Baculovirus gp64 gene expression: negative regulation by a minicistron

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

Splicing is required for transactivation by the immediate early gene 1 of the Lymantria dispar multinucleocapsid nuclear polyhedrosis virus

A region of the Lymantria disper multinucleocapsid nuclear polyhedrosis virus (LdMNPV) genome containing the homolog of the baculovirus ie-1 gene was identified using a series of overlapping cosmids and individual plasmids in a transient transcriptional expression assay. Sequence analysis of the active region identified two ORFs, one of which is 32% identical to AcMNPV ORF141 (ie-0) and contains a putative splice donor site and the other of which is 29% identical to AcMNPV ie-1 and contains a highly conserved splice acceptor consensus sequences. Plasmids containing the LdMNPV ORF141 and ie-1 regions were able to stimulate expression of a GUS reporter gene, while plasmids containing the ie-1 region alone were inactive, suggesting that only the spliced, IE-0 form of the gene product is an active transactivator. Primer extension analysis confirmed the presence of spliced ie-0 mRNA transcripts starting at 6 hr and continuing throughout the time course of viral infection of the L dispar cell line Ld652Y. Using a plasmid containing the ie-0 spliced form of the gene as a transactivator, hr4, one of the eight homologous regions of LdMNPV, was shown to act as a transcriptional enhancer. In contrast, a reporter plasmid containing the AcMNPV hr5 enhancer did not show increased activity when cotransfected with LdMNPV ie-0, suggesting that these enhancer sequences are viral specific. In a transient replication assay system. LdMNPV ie-0 acted as an essential replication gene, but LdMNPV ie-1 was inactive. These results indicate that splicing is required to obtain an active gene product in LdMNPV in the Ld652Y cell line.

Sequencing of the putative DNA helicase-encoding gene of the Bombyx mori nuclear polyhedrosis virus and fine-mapping of a region involved in host range expansion

The complete sequence of a 3666-nucleotide (nt) open reading frame (ORF) and its flanking regions (58.1-62.1 map units (m.u.) from Bombyx mori nuclear polyhedrosis virus (BmNPV)) was determined. This ORF, BmNPV dnahel, encoded a predicted protein of 143623 Da which possessed seven consensus motifs found in proteins which unwind duplex DNAs, indicating that it is a DNA helicase. A 572-bp SacI-HindIII fragment, BmScH, that was previously shown to expand the host range of Autographa californica NPV (AcNPV) following homologous recombination [Maeda et al. (1993) J. Virol. 67, 6234-6238], was localized within BmNPV dnahel. By cotransfection experiments, two adjacent nt (A and T) that appeared to be the minimal essential sequence necessary to expand the host range of AcNPV, were mapped within BmScH. These adjacent nt encoded a single amino acid difference between BmNPV (Asp) and AcNPV (Ser).

Autographa californica nuclear polyhedrosis virus p143 gene product is a DNA-binding protein

We have identified the protein product of the Autographa californica nuclear polyhedrosis virus (AcMNPV) p143 gene by constructing a recombinant baculovirus overexpressing the gene product P143. The overexpressed protein exhibited a relative mobility of approximately 140 kDa and was stable for at least 12 hr after synthesis. Immunoblotting using a monoclonal antibody developed against the overexpressed protein identified a similar polypeptide in AcMNPV-infected cells which was detectable by 4 hr postinfection. P143 was present within infected cell nuclei at relatively constant amounts until at least 72 hr after infection, suggesting that P143 may perform other functions at late times after infection. P143, purified from infected cell nuclei by chromatography over hydroxylapatite and DNA cellulose, bound in a sequence-independent fashion to double-stranded but not to single-stranded DNA to form a ladder of retarded protein-DNA complexes. Together, these data are consistent with the essential role of P143 for viral DNA replication and suggest that P143 may function by direct binding to DNA.

Abortive infection of the baculovirus Autographa californica nuclear polyhedrosis virus in Sf-9 cells after mutation of the putative DNA helicase gene

Homologous recombination between the Autographa californica nuclear polyhedrosis virus (AcNPV) genome and a 0.6-kbp-long DNA fragment derived from the putative DNA helicase gene of Bombyx mori nuclear polyhedrosis virus generates eh2-AcNPV, an expanded-host-range AcNPV mutant (S. Maeda, S.G. Kamita, and A. Kondo, J. Virol. 67:6234-6238, 1993). After inoculation at a high multiplicity of infection (MOI), eh2-AcNPV replicates efficiently in both the Sf-9 (AcNPV-permissive) and BmN (non-AcNPV-permissive) cell lines. In this study, we found that after the inoculation of Sf-9 cells at a low MOI (i.e., 1 and 0.1 PFU per cell), the release of eh2-AcNPV virions was dramatically reduced (approximately 900- and 10,000-fold, respectively, at 72 h postinoculation) compared with that of wild-type AcNPV. In addition, the titer of eh2-AcNPV determined by plaque assay on Sf-9 cells was approximately 200-fold lower than that determined by plaque assay on BmN cells. Analyses of gene expression and viral DNA replication after low-MOI eh2-AcNPV inoculation of Sf-9 cells indicated that viral early genes were expressed normally. However, DNA replication and late-gene expression were significantly reduced. These findings suggested that abortive infection occurred at the stage of viral DNA replication in nearly all low-MOI eh2-AcNPV-infected Sf-9 cells. In the larvae of Spodoptera frugiperda, the organism from which Sf-9 cells are derived, the infectivity of eh2-AcNPV was lower than that of AcNPV; however, abortive infection was not found.

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

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

Nucleotide sequence and transcriptional analysis of the DNA polymerase gene of Bombyx mori nuclear polyhedrosis virus

A gene encoding a putative DNA polymerase (pol) of Bombyx mori nuclear polyhedrosis virus (BmSNPV) was cloned and sequenced. The gene included an open reading frame (ORF) encoding a polypeptide of 988 amino acids with a predicted molecular mass of 114.65 kDa. The deduced amino acid sequence of the BmSNPV pol ORF showed an overall identity of 96 and 45% to those of the Autographa californica NPV (AcMNPV) pol ORF and the Lymantria dispar NPV pol ORF, respectively, and contained sequences conserved in a variety of eukaryotic and viral replicative DNA polymerases. The BmSNPV pol lacked a canonical TATAA element but contained a G+C-rich sequence in the transcriptional initiation region. Analyses by Northern blot hybridization, RNase protection assay, primer extension, and 3' and 5' RACE (rapid amplification of cDNA ends) showed that at least seven different transcripts of approximately 3.1 kb that shared a common 3' end were expressed from the BmSNPV pol. The expression of these transcripts from BmSNPV pol was regulated differentially during virus infection. Transcription of five of the seven species initiated in the close vicinity of and within the motif 5'-GCGTGCT-3'. One transcript placed its initiation site within the motif 5'-AGAGCGT-3' and the remaining one within the motif 5'-GGCGGTGG-3'. The motifs 5'-GCGTGCT-3' and 5'-AGAGCGT-3' have been identified in pol and other genes of AcMNPV as conserved sequences containing transcriptional initiation sites, whereas the motif 5'-GGCGGTGG-3', which is arranged as a direct repeat in BmSNPV pol but not in AcMNPV pol, has not been defined as the sequence responsible for transcriptional initiation sites. The BmSNPV pol transcripts were detectable at 2 hr postinfection (p.i.), peaked at 10 hr p.i., and declined to a low level by 18 hr p.i. The expression of BmSNPV pol was not inhibited but delayed dramatically by the protein synthesis inhibitor cycloheximide upon treatment of infected cells, whereas aphidicolin, an inhibitor of DNA polymerase, inhibited BmSNPV pol transcription. These results suggest a complicated and unique mechanism for the regulation of BmSNPV pol expression.

Temporal regulation of a complex and unconventional promoter by viral products

The DNA polymerase (dnapol) gene of Autographa californica nuclear polyhedrosis virus presents a complex promoter organization. It lacks the usual TATA box and start site, and its RNA accumulation initially increases and then decreases dramatically during infection. We investigated dnapol temporal regulation. Transiently expressed dnapol gene was transcribed at a low level from minor start sites. Coexpression with ie0 and/or ie1 immediate-early genes dramatically enhanced dnapol transcription, specifically from a new start site. Moreover, the ie1 transactivation required little or no information in front of this nonconventional proximal promoter. We showed that IE0 and IE1 proteins were stably expressed during infection and that the dnapol mRNA level decrease was not a consequence of the disappearance of these proteins. The dnapol promoter region contains a putative overlapping open reading frame (ORF) in the opposite direction. We showed that ORF-2 was indeed highly expressed late, when the dnapol mRNA level decreased, and that during that time, dnapol mRNA stability was not significantly altered, excluding a destabilizing antisense effect. Additionally, we showed that the dnapol promoter was inhibited late but not early during the infection of cells transiently expressing constructs carrying either the intact or the altered ORF-2 promoter. Therefore, ORF-2 initiation of transcription and dnapol promoter inhibition are two coincidental nonrelated phenomena. Finally, we showed that both IE1 transactivation and late inhibition occurred in the same limited region around the dnapol promoter.

Extension of Autographa californica nuclear polyhedrosis virus host range by interspecific replacement of a short DNA sequence in the p143 helicase gene

Recombinant baculoviruses obtained by coinfection of insect cells with Autographa californica and Bombyx mori nuclear polyhedrosis viruses (AcNPV and BmNPV, respectively) possess a wider in vitro host range than either parent virus. To localize the DNA sequences responsible for this species specificity, we used a two-step method of production and selection of recombinant viruses with altered specificity. Sf9 cells, which are permissive for AcNPV, were first cotransfected with genomic AcNPV DNA and a complete or incomplete set of BmNPV restriction fragments. AcNPV-BmNPV recombinants from the Sf9 supernatant were then selected on the basis of ability to replicate in B. mori Bm5 cells, which are not permissive for AcNPV. Cotransfection of AcNPV DNA with the 7.6-kbp BmNPV Sma I-C fragment was sufficient to produce recombinants able to infect both Sf9 and Bm5 cells. A series of cotransfections with subclones of this fragment defined a 79-nt sequence within the p143 helicase gene capable of extending AcNPV host range in vitro. In this 79-nt region, BmNPV and AcNPV differ at six positions, corresponding to four amino acid substitutions. The involvement of the 79-nt region in species specificity control was confirmed by cotransfecting AcNPV DNA and gel-purified polymerase chain reaction products derived from the BmNPV p143 gene. Replacement in the AcNPV genome of three AcNPV-specific amino acids by the three corresponding BmNPV-specific amino acids at positions 556, 564, and 577 of the p143 protein extends AcNPV host range to B. mori larvae.

Inhibition of Bombyx mori nuclear polyhedrosis virus (NPV) replication by the putative DNA helicase gene of Autographa californica NPV

Coinfection of Bombyx mori nuclear polyhedrosis virus (BmNPV) with Autographa californica NPV (AcNPV) in the BmNPV-permissive BmN cell line resulted in the complete inhibition of BmNPV replication. Coinfected BmN cells exhibited an atypical cytopathic effect (CPE) and synthesis of viral and host proteins was dramatically attenuated by 5 h postinfection (p.i.) and nearly completely blocked by 24 h p.i. Viral transcription, however, appeared to occur normally during both early (5-h-p.i.) and late (24-h-p.i.) stages of infection. Superinfection of BmN cells with AcNPV at 5 and 12 h post-BmNPV infection resulted in limited inhibition of BmNPV replication. BmN cells singly infected with AcNPV also showed similar CPE, premature inhibition of viral and host protein synthesis, and apparently normal viral transcription. BmNPV replication occurred normally following coinfection of BmNPV and eh2-AcNPV, an AcNPV mutant identical to AcNPV except for a 572-bp region in its putative DNA helicase gene originating from BmNPV (S. Maeda, S. G. Kamita, and A. Kondo, J. Virol. 67:6234-6238, 1993). Furthermore, atypical CPE and premature attenuation of host and viral protein synthesis were not observed. These results indicated that the inhibition of BmNPV replication was caused either directly or indirectly at the translational level by the putative AcNPV DNA helicase gene.

Sequence and temporal appearance of the early transcribed baculovirus gene HE65

We have identified the early transcribed HE65 gene by screening a cDNA library from polyadenylated RNA which was isolated at 1 h after infection of Spodoptera frugiperda cells with Autographa californica nuclear polyhedrosis virus (AcNPV). Nucleotide sequencing analysis of the HE65-specific cDNA clone reveals one open reading frame of 1,662 nucleotides from which a protein of 65 kDa in size can be predicted. The HE65 gene is located downstream of the late transcribed p80 gene and upstream of the homologous region hr4left, which overlaps the 5' sequences of the HE65 gene. An HE65-specific transcript of about 1,800 nucleotides is detectable 2 h postinfection and remains stable during the late phases of infection. RNase protection and primer extension analyses demonstrate that transcripts from the early start site of HE65 continue to accumulate from 2 to 48 h postinfection, even in the presence of aphidicolin. Furthermore, transcriptional analysis of the HE65 gene indicates a lower intensity of early transcription in comparison with the very early transcribed genes IEN, PE38, and ME53.

Alternative transcriptional initiation as a novel mechanism for regulating expression of a baculovirus trans activator

In this report, we show that the Orgyia pseudotsugata nuclear polyhedrosis virus p34 gene, which is homologous to the Autographa californica nuclear polyhedrosis virus PE-38 gene, is a trans activator. The predicted p34 protein contains a number of motifs that are similar to those found in other eukaryotic transcriptional trans activators, including a putative zinc finger DNA-binding domain, a glutamine-rich domain, and a leucine zipper. Northern (RNA) blot analysis showed that the p34 gene is expressed as a 1.1-kb mRNA from 1 to 48 h postinfection and as a 0.7-kb mRNA from 18 to 120 h postinfection. Mapping of these transcripts showed that they were 3' coterminal but initiated at different 5' start sites. The 1.1-kb transcript initiates at a baculovirus early gene motif (CACAGT) and encodes the entire p34 open reading frame (ORF). The 0.7-kb transcript initiates at a baculovirus late gene start site (GTAAG) internal to the p34 ORF. Western blot (immunoblot) analysis using p34 antisera showed that the 0.7-kb transcript is translated as an amino-terminally truncated 20-kDa form of the full length 34-kDa protein. Functional analysis indicated that the 34-kDa protein transcriptionally trans activates the IE-2 promoter whereas the 20-kDa protein does not. Therefore, p34 produces two functionally different proteins from the same ORF, using the novel mechanism of alternative transcriptional initiation.

Sequence, transcriptional mapping, and overexpression of p47, a baculovirus gene regulating late gene expression

A 2.8-kb region of the Autographa californica nuclear polyhedrosis virus genome was sequenced and found to contain an open reading frame (p47) which was capable of rescuing a previously characterized temperature-sensitive mutant, ts317 (S. Partington, H. Yu, A. Lu, and E. B. Carstens, Virology 157:91-102, 1990). Transcriptional mapping demonstrated that an early 4.2-kb RNA encoded the p47 open reading frame and probably overlapped the 39K delayed-early gene. The p47 open reading frame was cloned behind the polyhedrin promoter in a baculovirus transfer plasmid, which was then used to prepare a recombinant baculovirus overexpressing the p47 polypeptide. The overexpressed polypeptide was used to prepare p47-specific monoclonal antibodies. These antibodies detected a polypeptide of 47 kDa in A. californica nuclear polyhedrosis virus-infected cells, demonstrating that p47 is expressed as an authentic viral product. The p47 gene product was localized to the nucleus of infected cells, supporting the hypothesis that it is involved in regulating viral transcription at late times postinfection.

The structural and functional organization of the Autographa californica nuclear polyhedrosis virus genome

Baculoviruses are used as biological control agents of insect pests in agriculture and forestry. The multiple-nucleocapsid nuclear polyhedrosis virus of Autographa californica (AcMNPV) is the prototype baculovirus. Recently, this virus has become widely used as vector for the high-level expression of foreign genes in insect cells. An updated physical map of restriction sites as well as the location of open reading frames (ORFs) and transcripts are presented. Most characteristic is the dispersal of "early", "late", and "very late" genes over the genome and the presence of nested sets of 3' and 5' coterminal transcripts.

Promoter influence on baculovirus-mediated gene expression in permissive and nonpermissive insect cell lines

The activities of viral and insect promoters were examined in a range of insect cell lines permissive and nonpermissive for the replication of the baculovirus Autographa californica nuclear polyhedrosis virus. Recombinant baculoviruses were constructed to place the bacterial chloramphenicol acetyltransferase gene under the control of promoters strongly active in the early, late, or very late stages of virus replication. In fully permissive cells, expression from a very late promoter was 2- to 3-fold higher than expression from a late promoter and 10- to 20-fold higher than expression from an early promoter or from a virus-borne insect promoter. In cell lines that do not support the efficient production of viral progeny, late-promoter-driven expression was similar to or surpassed very late promoter-driven expression. In nonpermissive insect cell lines, expression driven by an insect promoter derived from Drosophila melanogaster was higher than expression from the three viral promoters and was especially high in the Drosophila cell line tested. Surprisingly, late-promoter-driven expression, which is dependent on DNA replication, was higher than early-promoter-driven expression in three of four nonpermissive lines. In contrast, very late promoter-driven expression was quite limited in nonpermissive cell lines. The results indicate that the promoter used to drive foreign-gene expression strongly influences the range of insect cells which can efficiently support the production of the foreign protein during infection with recombinant baculoviruses.

Nucleotide sequence and transcriptional analysis of the p80 gene of Autographa californica nuclear polyhedrosis virus: a homologue of the Orgyia pseudotsugata nuclear polyhedrosis virus capsid-associated gene

The 67.2- to 68.5-m.u. region of Autographa californica nuclear polyhedrosis virus was sequenced. A large open reading frame (ORF) was identified in the clockwise direction on the circular genome map which could potentially encode an 80-kDa polypeptide. Analysis of the predicted amino acid sequence of this ORF indicated that it was a homologue of the p87 capsid-associated gene of Orgyia pseudotsugata MNPV with an overall amino acid similarity of 34.3%. A late transcript of 2.1 kb was mapped to this open reading frame. An antisense 3.1-kb transcript partially overlapped the 5' end of the 2.1-kb RNA. Anti-extracellular virus sera reacted with a fusion protein consisting of a portion of the AcMNPV p80 gene fused to the bacterial trpE gene product, suggesting that the AcMNPV p80 gene product was also a component of the virus capsid.