Characterization of a baculovirus with a deletion of vlf-1

A baculovirus alkaline nuclease knockout construct produces fragmented DNA and aberrant capsids

DNA replication of bacmid-derived constructs of the Autographa californica multiple nucleocapsid nucleopolyhedrovirus (AcMNPV) was analyzed by field inversion gel electrophoresis (FIGE) in combination with digestion at a unique Eco81I restriction enzyme site. Three constructs were characterized: a parental bacmid, a bacmid deleted for the alkaline nuclease gene, and a bacmid from which the gp64 gene had been deleted. The latter was employed as a control for comparison with the alkaline nuclease knockout because neither yields infectious virus and their replication is limited to the initially transfected cells. The major difference between DNA replicated by the different constructs was the presence in the alkaline nuclease knockout of high concentrations of relatively small, subgenome length DNA in preparations not treated with Eco81I. Furthermore, upon Eco81I digestion, the alkaline nuclease knockout bacmid also yielded substantially more subgenome size DNA than the other constructs. Electron microscopic examination of cells transfected with the alkaline nuclease knockout indicated that, in addition to a limited number of normal-appearing electron-dense nucleocapsids, numerous aberrant capsid-like structures were observed indicating a defect in nucleocapsid maturation or in a DNA processing step that is necessary for encapsidation. Because of the documented role of the baculovirus alkaline nuclease and its homologs from other viruses in homologous recombination, these data suggest that DNA recombination may play a major role in the production of baculovirus genomes.

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.

Conserved molecular systems of the Baculoviridae

Although the Baculoviridae are a large and diverse family of viruses, they are united by a number of shared features that form the basis for their unique life cycle. These include the mechanism of cell entry, genome replication and processing, and late and very late gene transcription. In this review, the molecular systems that are conserved within the Baculoviridae and that are responsible these processes are described.

Characterization of the role of very late expression factor 1 in baculovirus capsid structure and DNA processing

Very late expression factor 1 (VLF-1) of Autographa californica multiple nucleopolyhedrovirus is a putative tyrosine recombinase and is required for both very late gene expression and budded virus production. In this report, we show that a vlf-1 knockout bacmid was able to synthesize viral DNA at levels similar to that detected for a gp64 knockout bacmid that served as a noninfectious control virus. Additionally, analysis of replicated bacmid DNA by field-inversion gel electrophoresis indicated that VLF-1 is not required for synthesizing high-molecular-weight intermediates that could be resolved into unit-length genomes when cut at a unique restriction site. However, immunoelectron microscopic analysis revealed that in cells transfected with a vlf-1 knockout bacmid, aberrant tubular structures containing the capsid protein vp39 were observed, suggesting that this virus construct was defective in producing mature capsids. In contrast, rescuing the vlf-1 knockout bacmid construct with a copy of VLF-1 that carries a mutation of a highly conserved tyrosine (Y355F) was sufficient to restore the production of nucleocapsids with a normal appearance, but not infectious virus production. Furthermore, the results of a DNase I protection assay indicated that the DNA packaging efficiency of the VLF-1(Y355F) virus construct was similar to that of the gp64 knockout control. Finally, a recombinant virus containing a functional hemagglutinin epitope-tagged version of VLF-1 was constructed to investigate the association of VLF-1 with the nucleocapsid. Analysis by immunoelectron microscopy of Sf-9 cells infected with this virus showed that VLF-1 localized to an end region of the nucleocapsid. Collectively, these results indicate that VLF-1 is required for normal capsid assembly and serves an essential function during the final stages of the DNA packaging process.

vlf-1 deletion brought AcMNPV to defect in nucleocapsid formation

Recent studies have provided direct evidence that the baculovirus very late factor 1 (VLF-I) of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) was essential for BV production. To elucidate how vlf-1 deletion blocks BV production we generated a vlf-1 knockout bacmid by ET-recombination technology on AcMNPV bacmid propagated in Escherichia coli. Bacmid DNA transfection and supernatant passage assay revealed that the vlf-1 knockout bacmid was unable to replicate in cell culture, while vlf-1 repair bacmid, which was generated by transposition of the vlf-1 ORF under control of its native promoter into polyhedrin gene locus of vlf-1 knockout bacmid, resumed viral replication ability at wildtype levels. Results of these assays proved the correct construction of the vlf-1 knockout bacmid. Subsequent electron microscopy revealed that the vlf-1 knockout bacmid failed to form nueleocapsid in the nuclei of the transfected cells. Instead, intensely electron-dense virogenic stroma characteristic of viral DNA synthesis were observed. Thus, it is demonstrated for the first time that vlf-1 knockout blocked nucleocapsid formation and the defective nucleocapsid formation resulted in the abolishment of BV and ODV production. Possible roles of vlf-1 in genome processing are suggested and discussed.

The redox state of the baculovirus single-stranded DNA-binding protein LEF-3 regulates its DNA binding, unwinding, and annealing activities

The single-stranded (ss) DNA-binding protein LEF-3 of Autographa californica multinucleocapsid nucleopolyhedrovirus promoted Mg(2+)-independent unwinding of DNA duplexes and annealing of complementary DNA strands. The unwinding and annealing activities of LEF-3 appeared to act in a competitive manner and were determined by the ratio of protein to DNA. At subsaturating and saturating concentrations, LEF-3 promoted annealing, whereas it promoted unwinding at oversaturation of DNA substrates. The LEF-3 binding to ssDNA and unwinding activity were sensitive to redox agents and were inhibited by oxidation of thiol groups in LEF-3 with 1,1'-azobis(N,N-dimethylformamide) (diamide) or by modification with the thiol-conjugating agent N-ethylmaleimide. Both oxidation and alkylation increased the dissociation constant of the interaction with model oligonucleotides indicating a decrease in an intrinsic affinity of LEF-3 for ssDNA. These results proved that free thiol groups are essential both for LEF-3 interaction with ssDNA and for DNA unwinding. In contrast, oxidation or modification of thiol groups stimulated the annealing activity of LEF-3 partially due to suppression of its unwinding activity. Treatment of LEF-3 with the reducing agent dithiothreitol inhibited annealing, indicating association of this activity with the oxidized protein. Thus, the balance between annealing and unwinding activities of LEF-3 was determined by the redox state of protein with the oxidized state favoring annealing and the reduced state favoring unwinding. An LEF-3 mutant in which the conservative cysteine Cys(214) was replaced with serine showed both a decreased binding to DNA and a reduced unwinding activity, thus indicating that this residue might participate in the regulation of LEF-3 activities.

Characterization of the replication of a baculovirus mutant lacking the DNA polymerase gene

In a previous study, the DNA polymerase gene (dnapol) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) was identified as one of six genes required for plasmid replication in a transient replication assay (M. Kool, C. Ahrens, R.W. Goldbach, G.F. Rohrmann, J.M. Vlak, Identification of genes involved in DNA replication of the Autographa californica, Proc. Natl. Acad. Sci. U.S.A. 91, (1994) 11212-11216); however, another study based on a similar approach reported that the virally encoded polymerase was only stimulatory (A. Lu, L.K. Miller, The roles of 18 baculovirus late expression factor genes in transcription and DNA replication, J. Virol. 69, (1995) 975-982). To reconcile the conflicting data and determine if the AcMNPV DNA polymerase is required for viral DNA replication during the course of an infection, a dnapol-null virus was generated using bacmid technology. To detect viral DNA replication, a highly sensitive assay was designed based on real-time PCR and SYBR green chemistry. Our results indicate that a bacmid in which the dnapol ORF was deleted is unable to replicate its DNA when transfected into Spodoptera frugiperda (Sf-9) cells, although when the dnapol ORF was introduced into the polyhedrin (polh) locus, this repaired virus could propagate at levels similar to the control virus. These results confirm that the AcMNPV-encoded DNA polymerase is required for viral DNA replication and the host DNA polymerases cannot substitute for the viral enzyme in this process.

Characterization of a baculovirus lacking the alkaline nuclease gene

The Autographa californica multiple nucleocapsid nucleopolyhedrovirus (AcMNPV) alkaline nuclease (AN) associates with the baculovirus single-stranded DNA binding protein LEF-3 and possesses both a 5'-->3' exonuclease and an endonuclease activity. These activities are thought to be involved in DNA recombination and replication. To investigate the role of AN in AcMNPV replication, the lambda Red system was used to replace the an open reading frame with a chloramphenicol acetyltransferase gene (cat) and a bacmid containing the AcMNPV genome in Escherichia coli. The AcMNPV an knockout bacmid (vAcAN-KO/GUS) was unable to propagate in Sf9 cells, although an an-rescued bacmid (vAcAN-KO/GUS-Res) propagated normally. In addition, the mutant did not appear to produce budded virions. These data indicated that an is an essential baculovirus gene. Slot blot and DpnI assays of DNA replication in Sf9 cells transfected with vAcAN-KO/GUS, vAcAN-KO/GUS-Res, and a wild-type bacmid showed that the vAcAN-KO/GUS bacmid was able to replicate to levels similar to those seen with the vAcAN-KO/GUS-Res and wild-type bacmids at early stages posttransfection. However, at later time points DNA did not accumulate to the levels seen with the repaired or wild-type bacmids. Northern analysis of Sf9 cells transfected with bacmid vAcAN-KO/GUS showed that transcription of late and very late genes was lower at later times posttransfection relative to the results seen with wild-type and vAcAN-KO/GUS-Res bacmids. These data suggest that the an gene might be involved in the maturation of viral DNA or packaging of the DNA into virions.