A mutant baculovirus with a temperature-sensitive IE-1 transregulatory protein

Entomopathogenic Viruses in the Neotropics: Current Status and Recently Discovered Species

The market for biological control of insect pests in the world and in Brazil has grown in recent years due to the unwanted ecological and human health impacts of chemical insecticides. Therefore, research on biological control agents for pest management has also increased. For instance, insect viruses have been used to protect crops and forests around the world for decades. Among insect viruses, the baculoviruses are the most studied and used viral biocontrol agent. More than 700 species of insects have been found to be naturally infected by baculoviruses, with 90% isolated from lepidopteran insects. In this review, some basic aspects of baculovirus infection in vivo and in vitro infection, gene content, viral replication will be discussed. Furthermore, we provide examples of the use of insect viruses for biological pest control and recently characterized baculoviruses in Brazil.

Characterization of cDNAs encoding p53 of Bombyx mori and Spodoptera frugiperda

Complementary DNAs encoding homologs of the tumor suppressor gene, p53, were characterized from two lepidopteran insects, Bombyx mori (Bm) and Spodoptera frugiperda (Sf). They encoded predicted proteins of 368 (41.2 kDa) (Bm) and 374 (42.5 kDa) (Sf) amino acids. The sequences shared 44% amino acid and 60% nucleotide sequence identity with each other, but exhibited less than 20% amino acid and 46% nucleotide sequence identity to Drosophila melanogaster p53. Despite the sequence diversity, conserved amino acids involved in DNA and zinc binding were present in the lepidopteran sequences. Expression of Sfp53-induced apoptosis in S. frugiperda cells, and antiserum made against recombinant Sfp53 recognized a protein whose abundance increased after treatment with DNA damaging agents.

High-level expression and improved folding of proteins by using the vp39 late promoter enhanced with homologous DNA regions

Some recombinant proteins expressed by baculovirus expression vector systems (BEVS) aggregate because the BEVS can produce large amounts of protein late during infection, when post-translational modification and protein quality control mechanisms are inactive. For expression during earlier stages than that driven by the polyhedrin (polh) very late promoter, transfer vectors were generated in which this promoter was replaced with a green fluorescent protein (GFP) gene controlled by a vp39 late promoter modified to contain HR3, one of the homologous DNA regions (HRs) of Bombyx mori nuclear polyhedrosis virus (BmNPV). The rise times of the fluorescence of GFP expressed by using recombinant viruses carrying the modified vp39 promoter were earlier than those associated with either the polh promoter or the native vp39 promoter lacking HR3. In transient expression assays, the vp39 late promoter in transfer vectors behaved like a delayed-early promoter, and was enhanced by HR3, and required IE-1 protein and various viral gene products encoded on both sides of BmNPV polh. When the vp39 promoter with HR3 was used, the aggregation of several foreign proteins expressed by the BEVS was markedly decreased. This study provides a new option for the expression of sufficiently quality-controlled proteins by using the vp39 promoter and HR3 in BEVS early in baculovirus infection, when the infection has caused little damage in the host cells.

Baculovirus DNA replication-specific expression factors trigger apoptosis and shutoff of host protein synthesis during infection

Apoptosis is an important antivirus defense. To define the poorly understood pathways by which invertebrates respond to viruses by inducing apoptosis, we have identified replication events that trigger apoptosis in baculovirus-infected cells. We used RNA silencing to ablate factors required for multiplication of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). Transfection with double-stranded RNA (dsRNA) complementary to the AcMNPV late expression factors (lefs) that are designated as replicative lefs (lef-1, lef-2, lef-3, lef-11, p143, dnapol, and ie-1/ie-0) blocked virus DNA synthesis and late gene expression in permissive Spodoptera frugiperda cells. dsRNAs specific to designated nonreplicative lefs (lef-8, lef-9, p47, and pp31) blocked late gene expression without affecting virus DNA replication. Thus, both classes of lefs functioned during infection as defined. Silencing the replicative lefs prevented AcMNPV-induced apoptosis of Spodoptera cells, whereas silencing the nonreplicative lefs did not. Thus, the activity of replicative lefs or virus DNA replication is sufficient to trigger apoptosis. Confirming this conclusion, AcMNPV-induced apoptosis was suppressed by silencing the replicative lefs in cells from a divergent species, Drosophila melanogaster. Silencing replicative but not nonreplicative lefs also abrogated AcMNPV-induced shutdown of host protein synthesis, suggesting that virus DNA replication triggers inhibition of host biosynthetic processes and that apoptosis and translational arrest are linked. Our findings suggest that baculovirus DNA replication triggers a host cell response similar to the DNA damage response in vertebrates, which causes translational arrest and apoptosis. Pathways for detecting virus invasion and triggering apoptosis may therefore be conserved between insects and mammals.

Transactivator IE1 is required for baculovirus early replication events that trigger apoptosis in permissive and nonpermissive cells

Immediate early viral protein IE1 is a potent transcriptional activator encoded by baculoviruses. Although the requirement of IE1 for multiplication of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) is well established, the functional roles of IE1 during infection are unclear. Here, we used RNA interference to ablate IE1, plus its splice variant IE0, and thereby define in vivo activities of these early proteins, including gene-specific regulation and induction of host cell apoptosis. Confirming an essential replicative role, simultaneous ablation of IE1 and IE0 by gene-specific double-stranded RNAs inhibited AcMNPV late gene expression, reduced yields of budded virus by more than 1,000-fold, and blocked production of occluded virus particles. Depletion of IE1 and IE0 had no effect on early expression of the envelope fusion protein gene gp64 but abolished early expression of the caspase inhibitor gene p35, which is required for prevention of virus-induced apoptosis. Thus, IE1 is a positive, gene-specific transactivator. Whereas an AcMNPV p35 deletion mutant caused widespread apoptosis in permissive Spodoptera frugiperda cells, ablation of IE1 and IE0 prevented this apoptosis. Silencing of ie-1 also prevented AcMNPV-induced apoptosis in nonpermissive Drosophila melanogaster cells. Thus, de novo synthesis of IE1 is required for virus-induced apoptosis. We concluded that IE1 causes apoptosis directly or contributes indirectly by promoting virus replication events that subsequently trigger cell death. This study reveals that IE1 is a gene-selective transcriptional activator which is required not only for expedition of virus multiplication but also for blocking of its own proapoptotic activity by upregulation of baculovirus apoptotic suppressors.

Functional and structural characterisation of AgMNPV ie1

We have located and cloned the Anticarsia gemmatalis multicapsid nucleopolyhedrovirus isolate 2D (AgMNPV-2D) genomic DNA fragment containing the immediate early 1 ORF and its flanking regions. Computer assisted analysis of the complete ie1 locus nucleotide sequence information was used to locate regulatory signals in the upstream region and conserved nucleotide and amino acid sequences. Comparative studies led to the identification of several characteristic protein motifs and to the conclusion that AgMNPV-2D is more closely related to Choristoneura fumiferana defective NPV than to other Group I nucleopolyhedrovirus. We have also shown that the AgMNPV IE1 protein was able to transactivate an early Autographa californica MNPV promoter and its own promoter in transient expression assays. In order to investigate the biological functionality of the ie1 promoter, the ie1 upstream activating region (UAR) was molecularly dissected and cloned upstream of the E. coli lacZ ORF. The results obtained, after transfection of UFL-AG-286 insect cells, leading us to find that the -492 and -357 versions contains sequence motifs important for the level of the lacZ reporter gene expression.

Focal distribution of baculovirus IE1 triggered by its binding to the hr DNA elements

In BmN cells infected with the baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV), IE1, a principal transcriptional activator, localizes to sites of viral DNA replication. IE1 initially displays focal distribution in BmNPV-infected cells prior to DNA synthesis, whereas the protein expressed by transfection with the ie1 gene is distributed throughout the nucleoplasm instead of localized to discrete subnuclear structures. To identify the inducer of focus formation for IE1, we conducted transfection experiments with an IE1-GFP construct and found that cotransfection with genomic DNA fragments bearing the homologous region (hr) sequences caused the formation of IE1-green fluorescent protein (GFP) foci. The transfection of insect cells with a single plasmid containing exclusively the hr3 sequence and the IE1-GFP gene was sufficient to form IE1-GFP foci. These results suggest that hr elements are a primary determinant of the focal distribution of IE1. An analysis of a series of hr3 deletion mutants showed that a single copy of the direct repeat could induce the formation of IE1 foci. Targeted mutagenesis within the hr-binding domain of IE1-GFP caused impairment of the hr-dependent IE1 localization, suggesting that binding of IE1 to the hr elements is essential for the onset of IE1 focus formation. The observation of BmNPV IE1 foci in non-BmNPV-susceptible cells suggests that no species-specific factors are required for hr-dependent IE1 focus formation.

Analysis of baculovirus IE1 in living cells: dynamics and spatial relationships to viral structural proteins

IE1, a principal transcriptional activator of the baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV), is an essential factor for viral DNA replication. During viral infection, IE1 accumulates in discrete subnuclear structures where viral DNA replication occurs. To analyse the dynamic properties of IE1, we monitored green fluorescent protein-tagged IE1 (IE1-GFP) in BmNPV-infected B. mori cells by live-cell microscopy. Time-lapse imaging showed that IE1-associated structures gradually expanded and occasionally fused with one another, while photobleaching experiments revealed that IE1-GFP was relatively immobile inside the IE1-associated structures. To investigate the spatial relationships between IE1 and viral structural proteins in infected cells, three GFP-tagged viral components were expressed together with DsRed-tagged IE1. Two structural proteins that constitute the occlusion-derived virus (ODV), P91-GFP and GFP-ODV-E25, localized to the periphery of the IE1-associated structures. While local accumulations of these proteins were often in contact with the IE1-associated structures, they did not extend beyond the boundaries of the structures. In contrast, the major capsid protein VP39-GFP predominantly accumulated within the IE1-associated structures. These data indicated, in conjunction with the finding of a high DNA content in the structures, that IE1 localizes to the virogenic stroma and therefore support the prediction previously proposed that the virogenic stroma is a site for viral DNA replication as well as for the assembly of nucleocapsids.

Genetic requirements for homologous recombination in Autographa californica nucleopolyhedrovirus

It is known that baculovirus infection promotes high-frequency recombination between its genomes and plasmid DNA during the construction of recombinant viruses for foreign gene expression. However, little is known about the viral genes necessary to promote homologous recombination (HR). We developed an assay to identify viral genes that are necessary to stimulate HR. In this assay, we used two plasmids containing extensive sequence homology that yielded a visible and quantifiable phenotype if HR occurred. The plasmids contained the green fluorescent protein gene (gfp) that was mutated at either the N or the C terminus and a viral origin of DNA replication. When the plasmids containing these mutant gfp genes were transfected into insect cells alone or together, few green fluorescent protein (GFP)-positive cells were observed, confirming that the host cell machinery alone was not able to promote high levels of HR. However, if viral DNA or viral genes involved in DNA replication were cotransfected into cells along with the mutant gfp-containing plasmids, a dramatic increase in GFP-positive cells was observed. The viral genes ie-1, ie-2, lef-7, and p35 were found to be important for efficient HR in the presence of all other DNA replication genes. However, ie-1 and ie-2 were sufficient to promote HR in the absence of other viral genes. Recombination substrates lacking a viral origin of replication had similar genetic requirements for recombination but were less dependent on ie-1. Interestingly, even though HR was stimulated by the presence of a viral origin of DNA replication, virally stimulated HR could proceed in the presence of the DNA synthesis inhibitor aphidicolin.

Oligomerization mediated by a helix-loop-helix-like domain of baculovirus IE1 is required for early promoter transactivation

IE1 is a principal transcriptional regulator of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). Transactivation by IE1 is stimulated when early viral promoters are cis linked to homologous-region (hr) enhancer sequences of AcMNPV. This transcriptional enhancement is correlated with the binding of IE1 as a dimer to the 28-bp palindromic repeats comprising the hr enhancer. To define the role of homophilic interactions in IE1 transactivation, we have mapped the IE1 domains required for oligomerization. We report here that IE1 oligomerizes by a mechanism independent of enhancer binding, as demonstrated by in vitro pull-down assays using fusions of IE1 (582 residues) to the C terminus of glutathione S-transferase. In vivo oligomerization of IE1 was verified by immunoprecipitation of IE1 complexes from extracts of plasmid-transfected SF21 cells. Analyses of a series of site-directed IE1 insertion mutations indicated that a helix-loop-helix (HLH)-like domain extending from residue 543 to residue 568 is the primary determinant of oligomerization. Replacement of residues within the hydrophobic face of the putative dimerization domain disrupted IE1 homophilic interactions and caused loss of IE1 transactivation of hr-dependent promoters in plasmid transfection assays. Thus, oligomerization is required for IE1 transcriptional stimulation. HLH mutations also reduced IE1 stability and abrogated transactivation of non-hr-dependent promoters. These data support a model wherein IE1 oligomerizes prior to DNA binding to facilitate proper interaction with the symmetrical recognition sites within the hr enhancer and thereby promote the transcription of early viral genes.

Viruses and apoptosis

Successful viral replication requires not only the efficient production and spread of progeny, but also evasion of host defense mechanisms that limit replication by killing infected cells. In addition to inducing immune and inflammatory responses, infection by most viruses triggers apoptosis or programmed cell death of the infected cell. This cell response often results as a compulsory or unavoidable by-product of the action of critical viral replicative functions. In addition, some viruses seem to use apoptosis as a mechanism of cell killing and virus spread. In both cases, successful replication relies on the ability of certain viral products to block or delay apoptosis until sufficient progeny have been produced. Such proteins target a variety of strategic points in the apoptotic pathway. In this review we summarize the great amount of recent information on viruses and apoptosis and offer insights into how this knowledge may be used for future research and novel therapies.

In vivo and in vitro analysis of baculovirus ie-2 mutants

Upon transient expression in cell culture, the ie-2 gene of Autographa californica nuclear polyhedrosis virus (AcMNPV) displays three functions: trans activation of viral promoters, direct or indirect stimulation of virus origin-specific DNA replication, and arrest of the cell cycle. The ability of IE2 to trans stimulate DNA replication and coupled late gene expression is observed in a cell line derived from Spodoptera frugiperda but not in a cell line derived from Trichoplusia ni. This finding suggested that IE-2 may exert cell line-specific or host-specific effects. To examine the role of ie-2 in the context of infection and its possible influence on the host range, we constructed recombinants of AcMNPV containing deletions of different functional regions within ie-2 and characterized them in cell lines and larvae of S. frugiperda and T. ni. The ie-2 mutant viruses exhibited delays in viral DNA synthesis, late gene expression, budded virus production, and occlusion body formation in SF-21 cells but not in TN-5B1-4 cells. In TN-5B1-4 cells, the ie-2 mutants produced more budded virus and fewer occlusion bodies but the infection proceeded without delay. Examination of the effects of ie-2 and the respective mutants on immediate-early viral promoters in transient expression assays revealed striking differences in the relative levels of expression and differences in responses to ie-2 and its mutant forms in different cell lines. In T. ni and S. frugiperda larvae, the infectivities of the occluded form of ie-2 mutant viruses by the normal oral route of infection was 100- and 1,000-fold lower, respectively, than that of wild-type AcMNPV. The reduction in oral infectivity was traced to the absence of virions within the occlusion bodies. The infectivity of the budded form of ie-2 mutants by hemocoelic injection was similar to that of wild-type virus in both species. Thus, ie-2 mutants are viable but exhibit cell line-specific effects on temporal regulation of the infection process. Due to its effect on virion occlusion, mutants of IE-2 were essentially noninfectious by the normal route of infection in both species tested. However, since budded viruses exhibited normal infectivity upon hemocoelic injection, we conclude that ie-2 does not affect host range per se. The possibility that IE-2 exerts tissue-specific effects has not been ruled out.

Characterization of the acidic domain of the IE1 regulatory protein from Orgyia pseudotsugata multicapsid nucleopolyhedrovirus

This study presents a detailed analysis of the acidic N-terminal region of the Orgyia pseudotsugata multicapsid nucleopolyhedrovirus (OpMNPV) transactivator IE1. The N-terminal region of IE1 is rich in acidic amino acids and has been hypothesized to be an acidic activation domain. Removal of the N-terminal 126 amino acids containing the acidic domain of IE1 resulted in complete loss of transactivation activity, indicating that this region is essential for transactivation. The OpMNPV acidic domain was replaced with the archetype acidic activation domain from VP16 and the acid-rich region of Autographa californica multicapsid NPV (AcMNPV) IE1. These chimeric constructs were fully capable of transactivation in transient assays. The chimeric OpMNPV IE1s containing the herpes simplex virus VP16 and AcMNPV IE1 acidic activation domains consistently transactivated a reporter gene to higher levels than the OpMNPV IE1 acidic activation domain. Transactivation by the chimeric constructs is enhanced synergistically when cotransfected with IE2 into Lymantria dispar and Spodoptera frugiperda cells. Both N- to C-terminal and C- to N-terminal deletions of the OpMNPV acidic activation domain were constructed to define functional domains within the OpMNPV IE1 acidic activation domain. At least two potential activation domains were identified. Within each of these domains, two core regions at amino acids 28-43 and amino acids 113-124 were identified that were similar to core regions of VP16 and GAL4, which contain predominately acidic and bulky hydrophobic amino acids.

The early baculovirus he65 promoter: On the mechanism of transcriptional activation by IE1

We have initiated studies on the mechanism of early transcriptional activation of the early he65 promoter during infection with Autographa californica multicapsid nuclear polyhedrosis virus. This analysis is based on a comparison of the sequences required for he65 promoter activation with those sequences that support specific protein binding. The he65 promoter is located immediately downstream of the homologous region (hr) 4a. The sequences of hr4a are characterized by two imperfect palindromes of 24 bp. The results of transient expression assays indicate promoter activation in the presence of both the proximal palindrome and the known viral trans-regulator IE1. The results of mobility shift assays and DNaseI footprinting analyses reveal differences in specific protein binding at and close to the proximal palindrome depending on whether the nuclear protein extracts are prepared from uninfected or infected cells. The analysis of the protein binding complex at the proximal inverted repeat with extracts from infected cells suggests the involvement of both IE1 and IE0 as oligomers. The minimal protein binding sequences include the left half-site of the 24 bp repeat with 9 additional bp of the flanking sequences. The right half-site of the repeat also directs binding although with lower affinity as confirmed by phenanthroline-copper footprinting assays. Both half-sites of the repeat are thus essential for he65 promoter activation, suggesting that IE1 acts via cooperative binding. We conclude that the proximal inverted repeat is able to interact with both IE1 and IE0 although IE1 is sufficient for activation at least in transient expression assays.

Identification of two independent transcriptional activation domains in the Autographa californica multicapsid nuclear polyhedrosis virus IE1 protein

The Autographa californica multicapsid nuclear polyhedrosis virus immediate-early protein, IE1, is a 582-amino-acid phosphoprotein that regulates the transcription of early viral genes. Deletion of N-terminal regions of IE1 in previous studies (G. R. Kovacs, J. Choi, L. A. Guarino, and M. D. Summers, J. Virol. 66:7429-7437, 1992) resulted in the loss of transcriptional activation, suggesting that this region may contain an acidic activation domain. To identify independently functional transcriptional activation domains, we developed a heterologous system in which potential regulatory domains were fused with a modified Escherichia coli Lac repressor protein that contains a nuclear localization signal (NLacR). Transcriptional activation by the resulting NLacR-IE1 chimeras was measured with a basal baculovirus early promoter containing optimized Lac repressor binding sites (lac operators). Chimeras containing IE1 peptides dramatically activated transcription of the basal promoter only when lac operator sequences were present. In addition, transcriptional activation by NLacR-IE1 chimeras was allosterically regulated by the lactose analog, isopropyl-beta-D-thiogalactopyranoside (IPTG). For a more detailed analysis of IE1 regulatory domains, the M1 to T266 N-terminal portion of IE1 was subdivided (on the basis of average amino acid charge) into five smaller regions which were fused in various combinations to NLacR. Regions M1 to N125 and A168 to G222 were identified as independent transcriptional activation domains. Some NLacR-IE1 chimeras exhibited retarded migration in sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels. As with wild-type IE1, this aberrant gel mobility was associated with phosphorylation. Mapping studies with the NLacR-IE1 chimeras indicate that the M1 to A168 region of IE1 is necessary for this phosphorylation-associated effect.

DNA-dependent transregulation by IE1 of Autographa californica nuclear polyhedrosis virus: IE1 domains required for transactivation and DNA binding

IE1 is the principal early transregulator of Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV). The 582-residue protein stimulates viral transcription and binds as a dimer to 28-bp palindromic repeats (28-mers) comprising the AcMNPV homologous region (hr) transcription enhancers. To define IE1 domains responsible for hr-dependent transactivation, we first constructed a series of IE1 fusions to the DNA binding domain of the yeast GAL4 transactivator. In transfection assays, GAL4-IE1 fusions stimulated transcription from a TATA-containing AcMNPV promoter only upon cis linkage to GAL4 DNA binding sites. IE1 N-terminal residues 8 to 118 were sufficient for GAL4-binding-site-dependent transactivation. To identify IE1 residues required for hr interaction, we tested a series of IE1 mutations for 28-mer binding by using electrophoretic mobility shift assays. Deletion of IE1 residues other than the N-terminal transactivation domain eliminated 28-mer binding. Of 14 insertion mutations, only IE1(I425) and IE1(I553) failed to bind the 28-mer either as homodimers or as heterodimers with functional IE1. In contrast to insertion IE1(I425), IE1(I553) also failed to compete with wild-type IE1 for DNA binding and suggested a defect in oligomerization. Consistent with loss of oligomerization, substitutions within a hydrophobic repeat (residues 543 to 568) at the IE1 C terminus abolished 28-mer binding and demonstrated that this helix-loop-helix-like domain is required for DNA interaction. These data confirm that IE1 contains separable domains for transactivation and oligomerization-dependent DNA binding. Furthermore, they support a model wherein hr-mediated transactivation by IE1 involves sequence-specific DNA binding that contributes to transcriptional stimulation by interaction with components of the basal transcription complex.

DNA replication promotes high-frequency homologous recombination during Autographa californica multiple nuclear polyhedrosis virus infection

The relative ease with which foreign genes can be incorporated into the genome of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) indicates that a highly efficient recombinational process exists within infected cells. However, it is unclear whether this is due to marker transfer mediated by host cell enzymes or recombination events promoted by AcMNPV itself. To address the latter possibility, a pair of inverted repeat IS50 elements derived from the bacterial transposon Tn5 was inserted into the polyhedrin gene locus of the AcMNPV genome. Inversion of Tn5 sequences arising from recombination between its IS50 repeats could be readily detected in this virus, indicating that AcMNPV DNA undergoes high-frequency recombination during infection. To further characterize this process, a transient recombination assay was developed and used to identify the cis- and trans-acting requirements for Tn5 inversion in AcMNPV. A transfected Tn5-containing plasmid was found to undergo the same sequence inversion events seen in the viral genome, but only if it also contained a putative AcMNPV origin of replication (homologous region 2) in cis and was replicated by AcMNPV gene products supplied in trans. Taken together, these results indicated that recombination events which occur in infected cells were strictly dependent upon AcMNPV-mediated DNA replication. Direct support for this hypothesis was provided by the observation that the minimal set of AcMNPV genes that was essential for plasmid DNA replication also promoted recombination events leading to Tn5 inversion in the absence of any other viral function. Finally, using a panel of deletion mutants of the IS50 elements in Tn5, sequence inversion was shown to be the result of homologous rather than site-specific recombination, since it occurred independently of a discrete sequence within the transposon. These results demonstrate that the AcMNPV DNA replication machinery exhibits a strong propensity to promote homologous recombination events during infection and is likely to play a role in the high frequency of marker transfer observed in this virus.

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.

Induction of apoptosis by baculovirus transactivator IE1

Apoptosis is induced upon infection of SF-21 cells by mutants of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) lacking a functional p35 gene which encodes a stoichiometric inhibitor of members of the interleukin-1beta converting enzyme family of cysteine proteases (N.J. Bump et al, Science 269:1885-1888, 1995; R.J. Clem, M. Fechheimer, and L.K. Miller, Science 254:1388-1390, 1991). We found that transfection of SF-21 cells with the AcMNPV ie-1 gene was sufficient to induce apoptosis, which was characterized by fragmentation of cellular DNA into oligonucleosomal fragments and apoptotic body formation. No signs of apoptosis were observed in Trichoplusia ni TN-368 cells transfected with ie-1, a result which is consistent with the observation that p35 mutants of AcMNPV do not induce apoptosis in this cell line. Cotransfection of SF-21 cells with p35 blocked ie-1-induced apoptosis, indicating that expression of ie-1 activates apoptosis through a p35-inhibitable cysteine protease pathway. Cotransfection with Cp-iap, an active member of another family of antiapoptotic inhibitors of apoptosis (iaps), also inhibited IE1-induced apoptosis. Thus, ie-1 may participate in inducing apoptosis in AcMNPV-infected cells, although the dependence of induction on DNA replication suggests that ie-1 is not the direct apoptotic signal during infection. The ie-1 gene product, IE1, is known to be a potent transactivator of baculovirus gene expression that interacts with specific palindromic sequences which can act as transcriptional enhancers and as origins of DNA replication in transient assays.

Factors regulating baculovirus late and very late gene expression in transient-expression assays

Eighteen genes of Autographa californica nuclear polyhedrosis virus are necessary and sufficient to transactivate expression from the late vp39 promoter in transient-expression assays in SF-21 cells. These 18 genes, known as late expression factor genes (lefs), are also required to transactivate the very late promoter of the polyhedrin gene, polh, but expression from this promoter is relatively weak compared with expression from the vp39 promoter. To further define the factors required for late and very late promoter expression, we first determined that the eighteen lefs were also required for expression from two other major baculovirus promoters: the late basic 6.9-kDa protein gene, p6.9, and the very late 10-kDa protein gene, p10. We next examined the effect of the very late expression factor 1 gene (vlf-1), a gene previously identified by analysis of a temperature-sensitive mutant, in the transient expression assay and found that vlf-1 specifically transactivated the two very late promoters but not the two late promoters. We then surveyed the Autographa californica nuclear polyhedrosis virus genome for additional genes which might specifically regulate very late gene expression; no additional vlf genes were detected, suggesting that VLF-1 is the primary regulator of very late gene expression. Finally, we found that the relative contribution of the antiapoptosis gene p35, which behaves as a lef in these transient-expression assays, depended on the nature of the other viral genes provided in the cotransfection mixtures, suggesting that other viral genes also contribute to the ability of the virus to block apoptosis.

Differential requirements for baculovirus late expression factor genes in two cell lines

A plasmid library of 18 late expression factor (LEF) genes (LEF library) from the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) supports transient expression from a late viral promoter in the SF-21 cell line, derived from Spodoptera frugiperda. We found, however, that this LEF library was unable to support expression from the same promoter in the TN-368 cell line, derived from Trichoplusia ni, which is also permissive for AcMNPV replication. To identify the additional factor(s) required for expression in TN-368 cells, we cotransfected the LEF library with clones representing portions of the AcMNPV genome not represented in the LEF library. A single additional gene was identified; this gene corresponded to ORF70 of the complete AcMNPV sequence and potentially encodes a 34-kDa cysteine-rich polypeptide. Because of its differential effect on late gene expression in the two cell lines, we renamed ORF70 hcf-1 (for host cell-specific factor 1). hcf-1 was involved in expression from reporter plasmids under late and very late but not early promoter control, indicating that it was also a LEF gene. Plasmid DNA replication assays indicated that HCF-1 was involved in virus origin-specific DNA replication in TN-368 cells. Three LEF genes, ie-2, lef-7, and p35, required for optimal virus origin-specific plasmid DNA replication or stability in SF-21 cells had little or no influence in TN-368 cells. Thus, as determined by transient-expression assays, cell line-specific and potentially host-specific factors are required for origin-specific DNA replication or stability.

Transcriptional enhancer activity of hr5 requires dual-palindrome half sites that mediate binding of a dimeric form of the baculovirus transregulator IE1

The hr5 enhancer element stimulates early viral transcription and may function as an origin of DNA replication for Autographa californica nuclear polyhedrosis virus (AcMNPV). The smallest functional unit of hr5 is a 28-bp repeat consisting of an imperfect palindrome (28-mer). To identify essential sequences and examine the molecular basis of hr5 activity, the effects of site-directed mutations on transcriptional enhancement by the 28-mer and binding of the AcMNPV transregulator IE1 were investigated. In transfection assays and infections with AcMNPV recombinants, activation of a basal viral promoter required sequences within both halves of the 28-mer. Basal promoter activation also required a critical spacing between these half sites. Mobility shift assays indicated that hr5 probes containing a single 28-mer were bound by in vitro-synthesized IE1. Competition assays using DNA fragments that contained mutated 28-mers demonstrated that both half sites were required for optimal binding of IE1. Similar assays using mutated 28-mer DNAs and nuclear extracts indicated that the relative affinity with which AcMNPV infection-specific proteins bound to the 28-mer was similar to that of in vitro-synthesized IE1. By using a combination of DNA binding and antibody supershift assays, it was demonstrated that IE1 binds to the 28-mer as a dimer. Collectively, these findings support a model in which symmetrical IE1 binding and simultaneous interaction with each half site are required for IE1-mediated transcriptional enhancement by hr5. Thus, sequence-specific binding may be one of the mechanisms by which IE1 directly or indirectly transregulates baculovirus gene expression.

The baculovirus transactivator IE1 binds to viral enhancer elements in the absence of insect cell factors

The transregulatory IE1 protein of Autographa californica nuclear polyhedrosis virus binds to the viral enhancer element hr5. To test whether IE1 binds independently of host cell factors, IE1 was translated in rabbit reticulocyte extracts and tested for DNA binding activity by an electrophoretic mobility shift assay. Complexes with the hr5 probe were detected with translation reaction mixtures primed with ie1 RNA but not with control translation reaction mixtures. However, the DNA-protein complexes formed with IE1 translated in vitro migrated more slowly than complexes formed with IE1 that was transiently expressed in insect cells. Phosphatase treatment of the translation reactions resulted in an increase in the mobility of the DNA-protein complexes, suggesting that hyperphosphorylation was responsible for the altered migration. To further verify that IE1 was capable of binding DNA in the absence of host cell factors, an N-terminal truncation of IE1 was synthesized in vitro, and shown to interact with hr5. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of IE1 translated in vitro revealed that the mobility of the protein was heterogeneous. This pattern was altered by translation in the presence of an oligonucleotide corresponding to the IE1 specific binding site but was not affected by translation in the presence of a nonspecific DNA. These results suggest that binding of IE1 to DNA causes a conformational change in the protein that alters the accessibility of IE1 to protein kinases.

The CAGT motif functions as an initiator element during early transcription of the baculovirus transregulator ie-1

The highly conserved tetranucleotide CAGT is located at the RNA start site of the transregulator gene ie-1 of Autographa californica nuclear polyhedrosis virus (AcMNPV). The presence of this motif within numerous baculovirus early promoters and its similarity to transcriptional initiators suggested a fundamental role in viral transcription regulation. To determine the function of the CAGT motif, site-specific mutations were introduced within the ie-1 promoter fused to a reporter gene within AcMNPV recombinants. In previous studies, deletion of the CAGT motif (nucleotides -1 to +3) and the adjacent downstream activating region (nucleotides +11 to +24) abolished ie-1 transcription. Here, we show that nucleotide replacements within the CAGT motif reduced steady-state levels of ie-1 RNAs from the proper start site (+1), both early and late in infection. These CAGT mutations caused comparable reductions in the yield of ie-1 runoff RNAs from in vitro transcription reactions using nuclear extracts from AcMNPV-infected cells; the CA dinucleotide was most sensitive to substitution. Thus, the CAGT motif affects the rate of ie-1 transcription. Deletions upstream and downstream from the ie-1 RNA start site demonstrated that nucleotides -6 to +11 encompassing the CAGT motif were sufficient for proper transcription in a TATA-independent manner. Nonetheless, additional regulatory elements, which included the ie-1 TATA element, the ie-1 downstream activating region, and a heterologous upstream activating region, stimulated transcription from the motif. Thus, by all criteria examined, the ie-1 CAGT motif functions as a transcriptional initiator by its capacity to determine the position of the RNA start site and to regulate the rate of transcription. These findings suggest that by stimulating early transcription through the recruitment of host factors, the CAGT initiator accelerates expression of viral genes, such as ie-1, that are critical to establishing a productive infection.

The roles of eighteen baculovirus late expression factor genes in transcription and DNA replication

A set of 18 plasmid subclones of the Autographa californica nuclear polyhedrosis virus genome supports expression from a late viral promoter in transient expression assays (J. W. Todd, A. L. Passarelli, and L. K. Miller, J. Virol. 69:968-974, 1995). Using this set of plasmids, we have assigned a role for each of the 18 genes required for optimal late gene expression with respect to its involvement at the levels of transcription, translation, and/or DNA replication. RNase protection analyses demonstrated that all of the known late expression factor genes (lefs) affected the steady-state level of reporter gene RNA. Thus, none of the lefs appeared to be specifically involved in translation. A subset of the lefs supported plasmid replication; ie-1, lef-1, lef-2, lef-3, p143, and p35 were essential for plasmid replication, while ie-n, lef-7, and dnapol had stimulatory effects. The predicted sequence of lef-7 suggests that it is a homolog of herpesvirus single-stranded DNA-binding protein (UL29). The role of p35 in plasmid replication appears to be suppression of apoptosis, because p35 could be functionally replaced in the replication assay by either Cp-iap or Op-iap, two heterologous baculovirus genes which suppress apoptosis by a mechanism which appears to differ from that of p35. Thus, one or more of the replication-related lefs or the process of plasmid replication appears to induce cellular apoptosis. Our results indicate that the remaining lefs, lefs 4 through 11, p47, and 39K (pp31), function either at the level of transcription or at that of mRNA stabilization.

Early transcription of the ie-1 transregulator gene of Autographa californica nuclear polyhedrosis virus is regulated by DNA sequences within its 5' noncoding leader region

The ie-1 gene of Autographa californica nuclear polyhedrosis virus (AcMNPV) encodes a transregulatory protein (IE1) which accelerates the expression of early and late virus genes. Transcription of ie-1 occurs immediately upon infection from a conserved CAGT motif and continues into the late phases. To examine the mechanisms by which ie-1 expression is regulated, cis-acting control elements within the ie-1 promoter were identified by constructing hybrid early promoters and by using site-directed mutagenesis. The ie-1 upstream activating region, extending from nucleotide -546 to the TATA element at -34, stimulated ie-1 basal promoter activity more than 1,000-fold when transfected into uninfected Spodoptera frugiperda SF21 cells. However, when introduced into the genome of AcMNPV recombinants, the ie-1 upstream activating region had only a minimal twofold effect early in infection. Instead, maximum steady-state levels of early ie-1 RNAs required sequences within the 5' noncoding leader region extending from +11 to +24 relative to the RNA start site (+1). The +11 to +24 noncoding region did not influence the stability of ie-1 transcripts. When assayed by in vitro transcription, deletion of the +11 to +24 region reduced the levels of ie-1 runoff RNAs. Thus, this downstream activating sequence controlled the rate of early ie-1 transcription. A larger overlapping region from +11 to +36 affected steady-state levels of ie-1 RNAs late (24 h) in infection. Deletion of sequences that included the conserved CAGT start site abolished early ie-1 transcription. Thus, ie-1 is the first example of an early baculovirus gene in which essential cis-acting regulatory elements reside within the 5' noncoding region and include sequences comprising the RNA start site.

Identification and characterization of vlf-1, a baculovirus gene involved in very late gene expression

We have identified a gene required for strong expression of the polyhedrin gene by characterizing a mutant, tsB837, of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) which is temperature sensitive (ts) for occluded virus production at the nonpermissive temperature. Marker rescue experiments utilizing an overlapping set of AcMNPV genomic clones revealed that the gene responsible for the ts mutant phenotype mapped to a region between 46 and 48 map units. Fragments (2.2 kb) from both wild-type AcMNPV and tsB837 genomes spanning the mutated region were sequenced, and a single nucleotide difference was observed. This mutation is predicted to substitute a single amino acid within a 44.4-kDa polypeptide. Analysis of protein synthesis in wild-type- and mutant-infected cells at the nonpermissive temperature indicated that polyhedrin synthesis was dramatically reduced in the mutant. Northern (RNA) blot analysis revealed that the mutant had markedly reduced levels of polyhedrin transcripts. Transcripts of another very late gene, p10, were also reduced but to a lesser degree. The transcription of two late genes (603 ORF and vp39) was neither reduced nor temporally delayed. Thus, the gene encoding this very late expression factor, designated vlf-1, regulates the levels of very late gene transcripts, and the tsB837 mutation affects the levels of polyhedrin gene transcripts more strongly than those of p10 transcripts. The product of the newly identified gene has a surprising but significant relationship to a family of integrases and resolvases.

Identification of three late expression factor genes within the 33.8- to 43.4-map-unit region of Autographa californica nuclear polyhedrosis virus

A transient transactivation assay system was used in combination with an overlapping Autographa californica nuclear polyhedrosis virus clone library to identify genes involved in late and very late baculovirus gene expression. We have identified three genes within the 33.8- to 43.4-map-unit region of the A. californica nuclear polyhedrosis virus genome which contribute to expression from promoters of the vp39 major capsid protein and polyhedrin genes. One of these three genes corresponds to the previously identified DNA polymerase gene, while the other two genes encode previously unidentified polypeptides of 59,418 and 8,706 Da. None of these genes were required for expression from the early etl promoter.

Identification and transcriptional regulation of the baculovirus lef-6 gene

We identified an Autographa californica nuclear polyhedrosis virus (AcMNPV) gene, the late expression factor 6 gene (lef-6), which is involved in expression from late and very late AcMNPV gene promoters but not from an early AcMNPV gene promoter in transient expression assays. This gene was located within the PstI I fragment of the AcMNPV genome (14.7 to 17.9 map units), immediately downstream of Ac-iap, the AcMNPV homolog of a baculovirus gene family involved in blocking apoptotic programmed cell death. The nature and temporal regulation of both Ac-iap and lef-6 transcripts was examined. Ac-iap and lef-6 were cotranscribed as bicistronic messages at both early and late times postinfection. In addition, lef-6 was transcribed as a monocistronic mRNA by initiation from an early promoter within Ac-iap.