In vitro transcription of pe38/polyhedrin hybrid promoters reveals sequences essential for recognition by the baculovirus-induced RNA polymerase and for the strength of very late viral promoters

Actin Contributes to the Hyperexpression of Baculovirus Polyhedrin (polh) and p10 as a Component of Transcription Initiation Complex (TIC)

Hyperexpression of polh and p10, two very late genes, is one of the remarkable characteristics in the baculovirus life cycle. However, the mechanisms underlying the hyperexpression of these two genes are still incompletely understood. In this study, actin was identified as a highly potential binding partner of polh and p10 promoters by conducting DNA pull-down and LC–MS/MS analyses. Inhibiting actin dynamics delayed and decreased the transcription of polh and p10. Actin interacted with viral RNA polymerase and transcription regulators, and the nuclear import of viral polymerase was inhibited with the disruption of actin dynamics. Simultaneously, the high enrichment of actin in polh and p10 promoters discovered via a chromatin immunoprecipitation (ChIP) assay indicated that actin was a component of the viral polymerase TIC. Moreover, overexpression of actin surprisingly upregulated the expression of luciferase (Luc) under the control of polh and p10 promoters. Taken together, actin participated in the hyperexpression of polh and p10 as a component of TIC. These results facilitate the promotion of the expression efficiency of foreign genes in the baculovirus expression vector system (BEVS).

Molecular mechanism responsible for the hyperexpression of baculovirus polyhedrin

One of the amazing phenomena in the baculovirus life cycle is the hyperexpression of the very late gene, polyhedrin (polh), causing the production of the occlusion bodies where progeny virions are embedded. However, to date, the molecular mechanism underlying its hyperexpression is not completely elucidated. Considering that, in this review, the mechanism responsible for its hyperexpression from the previous studies up to now was comprehensively summarized from three aspects, namely, the structure characteristics of the polh promoter and transcription regulation, the structure and translation regulation of the polh mRNA, and especially the regulators that influence the expression of polh gene. Moreover, this review will help us obtain a better understanding about the hyperexpression of polh, and also provide guidance for improving the expression efficiency of the foreign proteins by adopting the baculovirus expression vector system.

A review of alternative promoters for optimal recombinant protein expression in baculovirus-infected insect cells

Generating recombinant proteins in insect cells has been made possible via the use of the Baculovirus Expression Vector System (BEVS). Despite the success of many proteins via this platform, some targets remain a challenge due to issues such as cytopathic effects, the unpredictable nature of co-infection and co-expressions, and baculovirus genome instability. Many promoters have been assayed for the purpose of expressing diverse proteins in insect cells, and yet there remains a lack of implementation of those results when reviewing the landscape of commercially available baculovirus vectors. In advancing the platform to produce a greater variety of proteins and complexes, the development of such constructs cannot be avoided. A better understanding of viral gene regulation and promoter options including viral, synthetic, and insect-derived promoters will be beneficial to researchers looking to utilize BEVS by recruiting these intricate mechanisms of gene regulation for heterologous gene expression. Here we summarize some of the developments that could be utilized to improve the expression of recombinant proteins and multi-protein complexes in insect cells.

Introduction to Baculovirus Molecular Biology

The development of baculovirus expression vector systems has accompanied a rapid expansion of our knowledge about the genes, their function and regulation in insect cells. Classification of these viruses has also been refined as we learn more about differences in gene content between isolates, how this affects virus structure and their replication in insect larvae. Baculovirus gene expression occurs in an ordered cascade, regulated by early, late and very late gene promoters. There is now a detailed knowledge of these promoter elements and how they interact first with host cell-encoded RNA polymerases and later with virus-encoded enzymes. The composition of this virus RNA polymerase is known. The virus replication process culminates in the very high level expression of both polyhedrin and p10 gene products in the latter stages of infection. It has also been realized that the insect host cell has innate defenses against baculoviruses in the form of an apoptotic response to virus invasion. Baculoviruses counter this by encoding apoptotic-suppressors, which also appear to have a role in determining the host range of the virus. Also of importance to our understanding of baculovirus expression systems is how the virus can accumulate mutations within genes that affect recombinant protein yield in cell culture. The summary in this chapter is not exhaustive, but should provide a good preparation to those wishing to use this highly successful gene expression system.

Widespread genome reorganization of an obligate virus mutualist

The family Polydnaviridae is of interest because it provides the best example of viruses that have evolved a mutualistic association with their animal hosts. Polydnaviruses in the genus Bracovirus are strictly associated with parasitoid wasps in the family Braconidae, and evolved ∼100 million years ago from a nudivirus. Each wasp species relies on its associated bracovirus to parasitize hosts, while each bracovirus relies on its wasp for vertical transmission. Prior studies establish that bracovirus genomes consist of proviral segments and nudivirus-like replication genes, but how these components are organized in the genomes of wasps is unknown. Here, we sequenced the genome of the wasp Microplitis demolitor to characterize the proviral genome of M. demolitor bracovirus (MdBV). Unlike nudiviruses, bracoviruses produce virions that package multiple circular, double-stranded DNAs. DNA segments packaged into MdBV virions resided in eight dispersed loci in the M. demolitor genome. Each proviral segment was bounded by homologous motifs that guide processing to form mature viral DNAs. Rapid evolution of proviral segments obscured homology between other bracovirus-carrying wasps and MdBV. However, some domains flanking MdBV proviral loci were shared with other species. All MdBV genes previously identified to encode proteins required for replication were identified. Some of these genes resided in a multigene cluster but others, including subunits of the RNA polymerase that transcribes structural genes and integrases that process proviral segments, were widely dispersed in the M. demolitor genome. Overall, our results indicate that genome dispersal is a key feature in the evolution of bracoviruses into mutualists.

Characterization of N-glycan structures and biofunction of anti-colorectal cancer monoclonal antibody CO17-1A produced in baculovirus-insect cell expression system

Advantages of the baculovirus insect cell expression system for production of recombinant proteins include high capacity, flexibility, and glycosylation capability. In this study, this expression system was exploited to produce anti-cancer monoclonal antibody (mAb) CO17-1A, which recognizes the antigen GA733. The heavy chain (HC) and light chain (LC) genes of mAb CO17-1A were cloned under the control of P(10) and Polyhedrin promoters in the pFastBac dual vector, respectively. Gene expression cassettes carrying the HC and LC genes were transposed into a bacmid in Escherichia coli (DH10Bac). The transposed bacmid was transfected to Sf9 insect cells to generate baculovirus expressing mAb CO17-1A. Confocal immunofluorescence and Western blot analyses confirmed expression of mAb CO17-1A in baculovirus-infected insect cells. The optimum conditions for mAb expression were evaluated at 24, 48, and 72 h after the virus infection at an optimum virus multiplicity of infection of 1. Expression of mAb CO17-1A in insect cells significantly increased at 72 h after infection. HPLC analysis of glycosylation status revealed that the insect-derived mAb (mAb(I)) CO17-1A had insect specific glycan structures. ELISA showed that the purified mAb(I) from cell culture supernatant specifically bound to SW948 human colorectal cancer cells. Fluorescence-activated cell sorting analysis showed that, although mAb(I) had insect specific glycan structures that differed from their mammalian counterparts, mAb(I) similarly interacted with CD64 (FcgammaRI) and Fc of IgG, compared to the interactions of mammalian-derived mAb. These results suggest that the baculovirus insect cell expression system is able to express, assemble, and secrete biofunctional full size mAb.

Serine/threonine kinase (pk-1) is a component of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) very late gene transcription complex and it phosphorylates a 102 kDa polypeptide of the complex

The baculovirus gene, protein kinase-I (pk-1) encodes a serine/threonine kinase that is essential for very late gene expression. Late and very late genes of the baculoviruses are transcribed by an alpha-amanitin resistant RNA polymerase. The very late gene promoter transcription initiation complex was isolated from nuclei of Autographa californica multiple nucleopolyhedrovirus (AcMNPV)-infected Sf9 cells by DNA affinity purification and found to contain 4 major polypeptides of sizes approximately 102, 38, 32, and 18 kDa. The 32 kDa polypeptide was immunoreactive to AcMNPV anti-pk-1 antibody and phosphorylated the 102 kDa polypeptide, earlier reported as late gene expression factor LEF-8. Electrophoretic mobility shift assays with anti-pk-1 antibody indicated the binding of promoter DNA with recombinant AcMNPV-pk-1 and transcription initiation complex proteins. All these results suggested AcMNPV-pk-1 to be a component of the viral very late gene transcription initiation complex.

Introduction to baculovirus molecular biology

The development of baculovirus expression vector systems has accompanied a rapid expansion of our knowledge about the genes, their function, and regulation in insect cells. Classification of these viruses has also been refined as we learn more about differences in gene content between isolates, how this affects virus structure, and their replication in insect larvae. Baculovirus gene expression occurs in an ordered cascade, regulated by early, late, and very late gene promoters. There is now a detailed knowledge of these promoter elements and how they interact first with host cell-encoded RNA polymerases and later with virus-encoded enzymes. The composition of this virus RNA polymerase is known. The virus replication process culminates in the very high level expression of both polyhedrin and p10 gene products in the latter stages of infection. It has also been realized that the insect host cell has innate defenses against baculoviruses in the form of an apoptotic response to virus invasion. Baculoviruses counter this by encoding apoptotic-suppressors, which also appear to have a role in determining the host range of the virus. Also of importance to our understanding of baculovirus expression systems is how the virus can accumulate mutations within genes that affect recombinant protein yield in cell culture. The summary in this chapter is not exhaustive, but should provide a good preparation to those wishing to use this highly successful gene expression system.

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.

Temporal transcription program of recombinant Autographa californica multiple nucleopolyhedrosis virus

Baculoviruses, a family of large, rod-shaped viruses that mainly infect lepidopteran insects, have been widely used to transduce various cells for exogenous gene expression. Nonetheless, how a virus controls its transcription program in cells is poorly understood. With a custom-made baculovirus DNA microarray, we investigated the recombinant Autographa californica multiple nucleopolyhedrosis virus (AcMNPV) gene expression program in lepidopteran Sf21 cells over the time course of infection. Our analysis of transcription kinetics in the cells uncovered sequential viral gene expression patterns possibly regulated by different mechanisms during different phases of infection. To gain further insight into the regulatory network, we investigated the transcription program of a mutant virus deficient in an early transactivator (pe38) and uncovered several pe38-dependent and pe38-independent genes. This study of baculovirus dynamic transcription programs in different virus genetic backgrounds provides new molecular insights into how gene expression in viruses is regulated.

Sequence analysis and organization of the Neodiprion abietis nucleopolyhedrovirus genome

Of 30 baculovirus genomes that have been sequenced to date, the only nonlepidopteran baculoviruses include the dipteran Culex nigripalpus nucleopolyhedrovirus and two hymenopteran nucleopolyhedroviruses that infect the sawflies Neodiprion lecontei (NeleNPV) and Neodiprion sertifer (NeseNPV). This study provides a complete sequence and genome analysis of the nucleopolyhedrovirus that infects the balsam fir sawfly Neodiprion abietis (Hymenoptera, Symphyta, Diprionidae). The N. abietis nucleopolyhedrovirus (NeabNPV) is 84,264 bp in size, with a G+C content of 33.5%, and contains 93 predicted open reading frames (ORFs). Eleven predicted ORFs are unique to this baculovirus, 10 ORFs have a putative sequence homologue in the NeleNPV genome but not the NeseNPV genome, and 1 ORF (neab53) has a putative sequence homologue in the NeseNPV genome but not the NeleNPV genome. Specific repeat sequences are coincident with major genome rearrangements that distinguish NeabNPV and NeleNPV. Genes associated with these repeat regions encode a common amino acid motif, suggesting that they are a family of repeated contiguous gene clusters. Lepidopteran baculoviruses, similarly, have a family of repeated genes called the bro gene family. However, there is no significant sequence similarity between the NeabNPV and bro genes. Homologues of early-expressed genes such as ie-1 and lef-3 were absent in NeabNPV, as they are in the previously sequenced hymenopteran baculoviruses. Analyses of ORF upstream sequences identified potential temporally distinct genes on the basis of putative promoter elements.

Expression of Autographa californica multiple nucleopolyhedrovirus genes in mammalian cells and upregulation of the host beta-actin gene

The gene expression of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) was examined in two types of mammalian cells, human HeLa14 and hamster BHK cells. DNA microarray analysis followed by reverse transcription-PCR identified at least 12 viral genes transcribed in both HeLa14 cells and BHK cells inoculated with AcMNPV. 5' rapid amplification of cDNA ends was carried out to examine the transcriptional fidelity of these genes in HeLa14 cells. The transcription of ie-1, ie-0 and gp64 was initiated at a baculovirus early gene motif, CAGT, accompanied by a TATA motif. In addition, the same splicing observed for ie-0 mRNA in Sf9 cells occurred in HeLa14 cells. While the transcription initiation sites for pe38 and p6.9 were not located in the CAGT motif, most of them were in a typical eukaryotic RNA polymerase II promoter structure (a conventional TATA motif and/or an initiator). Interestingly, the expression of beta-actin was upregulated in the mammalian cells inoculated with AcMNPV. Subsequent experiments using UV-inactivated virus confirmed the upregulation, suggesting that de novo synthesis of viral products is not required for the event. These results indicated that the AcMNPV genome acts as a template for transcription in mammalian cells through the usual infection pathway, though there is no evidence for the functional expression of viral genes at present.

Analysis and prediction of baculovirus promoter sequences

Consensus patterns of baculovirus sequences upstream from the translational initiation sites have been analyzed and a web tool, Local Alignment Promoter Predictor (LAPP), for the prediction of baculovirus promoter sequences has also been developed. Potential consensus sequences, i.e., TCATTGT, TCTTGTA, CTCGTAA, TCCATTT and TCATT plus TCGT in approximately 30 bp spacing context, have been found in baculovirus promoter regions, in addition to well-characterized late and early promoter elements G/T/ATAAG and TATAA, which is accompanied about 30-bp downstream by a transcriptional initiation sequence CAGT or CATT. Promoter prediction is performed by a dynamic programming algorithm based on maximal segment pair measure with scores above some cutoff against each sequence in a refined promoter database. The algorithm was able to discriminate between promoter and non-promoter sequences in a test set of baculovirus sequences with prediction specificity and sensitivity superior to that using five other eukaryotic promoter recognition programs available on the Internet. A web server that implements the LAPP with continually updated promoter database is freely available at http://life.zsu.edu.cn/LAPP/.

Enhancement of cauliflower mosaic virus 35S promoter in insect cells infected with baculovirus

We happened to discover that the cauliflower mosaic virus (CaMV) 35S promoter inserted into a recombinant Autographa californica multicapsid nucleopolyhedrovirus (rAcMNPV) was strongly activated during the replication of the recombinant virus in Spodoptera frugiperda (Sf9) cells. The expression of the luciferase gene from the 35S promoter in rAcMNPV was remarkably increased late in infection and was resistant to alpha-amanitin treatment. Primer extension indicated that transcriptional initiation from the 35S promoter in Sf9 cells occurred within one of the two baculoviral late promoter TAAG motifs located in the vicinity of the transcription start site in plant cells. These observations suggested that the CaMV 35S promoter served as a transcription start site for AcMNPV-induced RNA polymerase.

Transcriptional activity of baculovirus very late factor 1

The product of the vlf-1 (very late factor 1) gene is required for expression of very late genes during the final phase of infection. To determine whether VLF-1 functions as a transcriptional activator, VLF-1 was overexpressed and purified by affinity and cation exchange chromatography. The addition of purified protein to transcription assays containing baculovirus RNA polymerase stimulated transcription of the very late polyhedrin promoter but not the late 39k promoter. Furthermore, construction and analysis of chimeric templates identified sequences within the polyhedrin promoter that were necessary for enhancement.

Identification and characterization of a major early-transcribed gene of Trichoplusia ni single nucleocapsid nucleopolyhedrovirus using the baculovirus expression system

An early transcribed gene (me-53) of a South Africa strain of Trichoplusia ni single nucleocapsid nucleopolyhedrovirus (TnSNPV) was sequenced and identified. It has an open reading frame of 1146 nucleotides that encodes a protein of 382 amino acids with a molecular mass of 45.2 kDa. The deduced protein sequence alignment of 13 baculovirus ME-53s indicated that the TnSNPV ME-53 shares the highest homologies with NPV subgroup II-A Spodoptera exigua multiple and Mamestra configurata (Maco) nucleopolyhedrovirus ME-53s. The zinc finger-like motifs at the C-termini of ME-53s are highly conserved with similar patterns of cysteine positions. Upon introduction of the gene and a green fluorescent protein reporter gene into the baculovirus expression vector system, the transcriptional analysis of me-53 in two cell lines infected with the Autographa californica nucleopolyhedrovirus (AcMNPV) recombinant revealed that an early TnSNPV me-53 transcript can be detected by 1 h postinfection (hpi) until 12 hpi and a late one from 18 hpi up to 48 hpi, while early and late transcripts of the AcMNPV me-53 of the recombinant can be detected at 3 and 24 hpi, respectively. This suggested that the early and late promoters of both AcMNPV and TnSNPV me-53s were recognized in recombinant virus-infected cells. The regulatory elements of the TnSNPV me-53 promoter were also analyzed.

Functional dissection of the baculovirus late expression factor-8 gene: sequence requirements for late gene promoter activation

The late expression factor-8 gene (lef-8) of Autographa californica M nucleopolyhedrovirus encodes the largest subunit of the virally encoded DNA-directed RNA polymerase specific for the transcription of late and very late viral genes. The sequence of lef-8 predicts a C-terminal motif of 13 amino acids that is conserved in other polymerases. Detailed mutagenesis throughout lef-8 was performed, including this C-terminal motif, to define sequences required for late promoter activation. It was found that the conserved C-terminal motif was critical for late gene expression. In addition, regions throughout the entire lef-8-encoding sequence were important for optimal function, suggesting complex protein-protein and protein-DNA interrelationships in the late gene-specific viral transcriptosome.

Baculovirus infection raises the level of TATA-binding protein that colocalizes with viral DNA replication sites

During the infection cycle of Autographa californica multicapsid nuclear polyhedrosis virus, the TATA-binding protein (TBP) of the insect host cell likely participates in early viral transcription, which is mediated by the host RNA polymerase II. However, the role of TBP in late and very late viral transcription, which is accomplished by an alpha-amanitin-resistant RNA polymerase, is unclear. We observed a dramatic increase of TBP protein during the late phases of infection. TBP mRNA levels, however, were not coordinately increased. Indirect-immunofluorescence studies revealed a nuclear redistribution of TBP during infection. After labeling of viral replication centers with bromodeoxyuridine (BrdU), costaining of TBP and BrdU showed that TBP localized to viral DNA replication centers. These results suggest a putative role of TBP during late viral transcription, which may occur in close proximity to viral DNA replication.

The baculovirus PE38 protein augments apoptosis induced by transactivator IE1

While studying apoptosis induced by baculovirus transactivator IE1 in SF-21 cells, we found that the levels of IE1-induced apoptosis were increased approximately twofold upon cotransfection with the baculovirus early pe38 gene. However, no apoptotic activity was observed in cells transfected with pe38 alone, even when placed under the control of a constitutive promoter. Thus, pe38 was able to augment IE1-induced apoptosis but was unable to induce apoptosis when expressed in SF-21 cells alone. PE38, the full-length product of pe38, is a nuclear protein with RING finger and leucine zipper motifs. Deletion of the amino-terminal region, which contains a putative nuclear localization motif, resulted in cytoplasmic localization of the PE38 mutants. These N-terminal deletion mutants were unable to enhance IE1-induced apoptosis. Mutation of a single conserved leucine (L242) of the leucine zipper motif also eliminated the ability of PE38 to augment apoptosis induced by IE1. In contrast, PE38 mutants with alanine substitutions for conserved cysteine residues (C109 or C138) of the RING finger motif were able to increase IE1-induced apoptosis to levels equivalent to those of wild-type PE38. We propose that PE38 is one of at least two viral factors which collectively evoke a cellular apoptotic response during baculovirus infection.

Mitochondrial DNA acts as potential promoter of the baculovirus RNA polymerase

We have examined whether mitochondrial DNA could act as target of the RNA polymerase encoded by the baculovirus Autographa californica multicapsid nuclear polyhedrosis virus, because the baculovirus late promoters and the control region of host mitochondrial DNA show a high degree of sequence similarity. In vitro transcription using mitochondrial DNA from Spodoptera frugiperda cells and nuclear extracts prepared from baculovirus infected cells demonstrates that mitochondrial DNA is recognized by the viral RNA polymerase. Transcriptional initiation occurs at TAAG sequences, although not all of the six TAAG motifs present in the mitochondrial DNA fragment are recognized. The TAAG motif in the control region served as weak transcriptional start site, but some of the TAAG motifs in the coding sequences of the adjacent tRNA and rRNA genes are recognized efficiently. The sequences flanking the TAAG motifs used as transcriptional start sites have a lower helix stability than the flanking sequences of the nonfunctional TAAG motifs. These results support the view that helix stability rather than sequence specificity is an important factor for recognition of TAAG motifs by the viral RNA polymerase.

Activation of baculovirus very late promoters by interaction with very late factor 1

Very late factor 1 (VLF-1) of Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV) activates the transcription of two genes, polyhedrin (polh) and p10, during the final, occlusion-specific phase of infection. Using transient expression assays responsive to VLF-1, we identified linker scan mutations in the polh and p10 promoters which abolished or weakened the ability of the promoters to respond to stimulation by VLF-1. These mutations were located between the transcriptional and translational initiation sites, a region previously shown to be essential for the burst of expression during the very late phase. Addition of partially purified, epitope-tagged VLF-1 to DNA encompassing this "burst sequence" resulted in a shift in the gel electrophoretic mobility of the DNA, indicating that VLF-1 forms a complex with DNA. Addition of an antibody specific for the epitope tag of VLF-1 decreased the mobility of the DNA further, confirming the presence of VLF-1 in the complex. DNase I footprint assays revealed that VLF-1 partially purified from either insect cells or bacterial cells interacted with the burst sequences of both the polh and p10 very-late promoters. Linker scan mutations within the burst sequences severely impaired interaction between VLF-1 and the promoters. We propose that VLF-1 transactivates the polh and p10 promoters by interacting with the burst sequences.

The LEF-4 subunit of baculovirus RNA polymerase has RNA 5'-triphosphatase and ATPase activities

The baculovirus Autographa californica nuclear polyhedrosis virus encodes a DNA-dependent RNA polymerase that is required for transcription of viral late genes. This polymerase is composed of four equimolar subunits, LEF-8, LEF-4, LEF-9, and p47. The LEF-4 subunit has guanylyltransferase activity, suggesting that baculoviruses may encode a full complement of capping enzymes. Here we show that LEF-4 is a bifunctional enzyme that hydrolyzes the gamma phosphates of triphosphate-terminated RNA and also hydrolyzes ATP and GTP to the respective diphosphate forms. Alanine substitution of five residues previously shown to be essential for vaccinia virus RNA triphosphatase activity inactivated the triphosphatase component of LEF-4 but not the guanylyltransferase domain. Conversely, mutation of the invariant lysine in the guanylyltransferase domain abolished the guanylyltransferase activity without affecting triphosphatase function. We also investigated the effects of substituting phenylalanine for leucine at position 105, a mutation that results in a virus that is temperature sensitive for late gene expression. We found that this mutation had no significant effect on the ATPase or guanylyltransferase activity of LEF-4 but resulted in a modest decrease in RNA triphosphatase activity.

Guanylyltransferase activity of the LEF-4 subunit of baculovirus RNA polymerase

The baculovirus Autographa californica nuclear polyhedrosis virus encodes a DNA-dependent RNA polymerase that transcribes viral late genes. This polymerase is composed of four equimolar subunits, LEF-4, LEF-8, LEF-9, and p47. Here we present data indicating that the LEF-4 subunit of RNA polymerase is a guanylyltransferase. Incubation of RNA polymerase in the presence of divalent cation and radiolabeled GTP resulted in the formation of a covalent enzyme-guanylate complex that comigrated with the LEF-4 subunit. The label transfer assay showed an absolute requirement for divalent cation which could be satisfied by either manganese or magnesium. The reaction was specific for guanine nucleotides, and GTP was more effective than dGTP in the formation of enzyme-guanylate complex. To demonstrate that LEF-4 was the guanylyltransferase, the single subunit was overexpressed in baculovirus-infected cells. The overexpressed protein was primarily cytosolic, indicating that other proteins in the RNA polymerase complex were responsible for nuclear targeting of LEF-4. LEF-4 alone was able to covalently bind GMP, although less efficiently than viral RNA polymerase.

A virus-encoded RNA polymerase purified from baculovirus-infected cells

A DNA-dependent RNA polymerase was purified to homogeneity, starting from insect cells infected with the baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV). The purified polymerase supported accurate and specific transcription from late and very late promoters but was not active on viral early promoters. Thus, promoter recognition is an integral function of the purified enzyme. The purified RNA polymerase was composed of only four equimolar subunits, which makes it the simplest DNA-directed RNA polymerase from a eukaryotic source described so far. Amino-terminal protein sequencing, peptide fingerprinting, and immunochemical analyses were used to identify the four subunits, all of which are virus encoded. Overexpression of the four viral proteins (LEF-8, LEF-4, LEF-9, and p47) in baculovirus-infected cells resulted in a significant increase in the levels of RNA polymerase produced in the infected cells. Thus, the overexpression data are consistent with our identification of the RNA polymerase subunits.