Bmo-miR-6498-5p suppresses Bombyx mori nucleopolyhedrovirus infection by down-regulating BmPLPP2 to modulate pyridoxal phosphate content in B. mori

The RNA interference pathway mediated by microRNAs (miRNAs) is one of the methods to defend against viruses in insects. Recent studies showed that miRNAs participate in viral infection by binding to target genes to regulate their expression. Here, we found that the Bombyx mori miRNA, miR-6498-5p was down-regulated, whereas its predicted target gene pyridoxal phosphate phosphatase PHOSPHO2 (BmPLPP2) was up-regulated upon Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Both in vivo and in vitro experiments showed that miR-6498-5p targets BmPLPP2 and suppresses its expression. Furthermore, we found miR-6498-5p inhibits BmNPV genomic DNA (gDNA) replication, whereas BmPLPP2 promotes BmNPV gDNA replication. As a pyridoxal phosphate (PLP) phosphatase (PLPP), the overexpression of BmPLPP2 results in a reduction of PLP content, whereas the knockdown of BmPLPP2 leads to an increase in PLP content. In addition, exogenous PLP suppresses the replication of BmNPV gDNA; in contrast, the PLP inhibitor 4-deoxypyridoxine facilitates BmNPV gDNA replication. Taken together, we concluded that miR-6498-5p has a potential anti-BmNPV role by down-regulating BmPLPP2 to modulate PLP content, but BmNPV induces miR-6498-5p down-regulation to promote its proliferation. Our findings provide valuable insights into the role of host miRNA in B. mori-BmNPV interaction. Furthermore, the identification of the antiviral molecule PLP offers a novel perspective on strategies for preventing and managing viral infection in sericulture.

Susceptibility of Autographa californica multiple nucleopolyhedrovirus to inhibitors of DNA replication

The objectives of this study were to develop methods to evaluate the susceptibility of the type baculovirus AcMNPV to various antiviral compounds and to select potential inhibitors for investigating baculovirus DNA replication. In concert with the classical cytopathic effects (CPE) and cytotoxicity inhibition assays, two approaches, which could be amenable for high throughput application for evaluating several classes of known antiviral compounds were developed. (i) An indirect approach based on spectrofluorimetric analysis of EGFP expression in Sf21 cells infected with a recombinant AcMNPV (AcEGFP) and (ii) a direct DNA quantitative assay based on quantitative real time PCR (qPCR). Initial CPE results suggested that of 21 compounds tested, aphidicolin, abacavir, camptothecin, (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), l-mimosine, hydroxyurea and phosphonoacetic acid (PAA) were selective inhibitors of AcMNPV replication. Consistent with the CPE results, the EGFP fluorescence and the qPCR of viral DNA accumulation exhibited a dose dependent depression of EGFP expression and DNA accumulation, respectively, in infected cells exposed to them. The inhibitory effects of aphidicolin, abacavir, l-mimosine and hydroxyurea on AcMNPV DNA replication were reversible. Taken together, both spectrofluorimetric and qPCR assays are suitable and rapid quantitative approaches to investigate inhibitors of baculovirus DNA replication in infected cells.

Association of Sf9 cell proliferating cell nuclear antigen with the DNA replication site of Autographa californica multicapsid nucleopolyhedrovirus

The accumulation of cellular proliferating cell nuclear antigen (PCNA) in the nucleus of Sf9 cells has been shown to increase upon infection with Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). Here, analysis by DNase I treatment and chromatin immunoprecipitation revealed that cellular PCNA in the nucleus of Sf9 cells bound AcMNPV DNA. Immunocytochemical analysis showed colocalization of Sf9 cell PCNA and AcMNPV DNA replication sites. Similar colocalization was also observed in BmN-4 cells infected with Bombyx mori NPV, which is inherently missing the pcna gene. The amount of cellular PCNA associated with viral DNA replication sites was greater in cells infected with pcna-defective AcMNPV mutants than in cells infected with wild-type AcMNPV. These results suggest that both cellular and viral PCNAs are involved in AcMNPV DNA replication and that pcna-defective AcMNPV mutants are able to substitute cellular PCNA for viral PCNA.

Purification of Autographa californica nucleopolyhedrovirus DNA polymerase from infected insect cells

Autographa californica nucleopolyhedrovirus (AcMNPV) DNA polymerase was purified from virus-infected cells using conventional chromatographic methods. The enzymatic activity of fractions eluting from single-stranded agarose gels was found to exactly coincide with a single polypeptide with an apparent molecular mass of approximately 110,000 Da on denaturing polyacrylamide gels stained with Coomassie blue. This purification scheme resulted in a 228-fold purification of AcMNPV DNA polymerase with recovery of 3.5% of the initial activity. The specific activity of the most purified fraction of DNA polymerase was 5000 units/mg, which is sufficiently high to eliminate the possibility that contaminants significantly contribute to the polymerase activity. Preparations of purified DNA polymerase had 3'-5' exonuclease activity, but no 5'-3' exonuclease activity. The proofreading activity was apparently an intrinsic property of the enzyme as the ratio of nuclease activity to polymerase activity was constant throughout purification. Using a singly-primed M13 DNA template, RF-II DNA was detected within 3 min, indicating a polymerization rate of 40 nt/s. The effects of several DNA polymerase inhibitors on the enzymatic activity of purified DNA polymerase were also determined.

Bombyx mori nucleopolyhedrovirus encodes a DNA-binding protein capable of destabilizing duplex DNA

A DNA-binding protein (designated DBP) with an apparent molecular mass of 38 kDa was purified to homogeneity from BmN cells (derived from Bombyx mori) infected with the B. mori nucleopolyhedrovirus (BmNPV). Six peptides obtained after digestion of the isolated protein with Achromobacter protease I were partially or completely sequenced. The determined amino acid sequences indicated that DBP was encoded by an open reading frame (ORF16) located at nucleotides (nt) 16189 to 17139 in the BmNPV genome (GenBank accession no. L33180). This ORF (designated dbp) is a homolog of Autographa californica multicapsid NPV ORF25, whose product has not been identified. BmNPV DBP is predicted to contain 317 amino acids (calculated molecular mass of 36.7 kDa) and to have an isoelectric point of 7.8. DBP showed a tendency to multimerization in the course of purification and was found to bind preferentially to single-stranded DNA. When bound to oligonucleotides, DBP protected them from hydrolysis by phage T4 DNA polymerase-associated 3'-->5' exonuclease. The sizes of the protected fragments indicated that a binding site size for DBP is about 30 nt per protein monomer. DBP, but not BmNPV LEF-3, was capable of unwinding partial DNA duplexes in an in vitro system. This helix-destabilizing ability is consistent with the prediction that DBP functions as a single-stranded DNA binding protein in virus replication.

Herpes simplex virus type 1 DNA polymerase. Mutational analysis of the 3'-5'-exonuclease domain

Like true DNA replicases, herpes simplex virus type 1 DNA polymerase is equipped with a proofreading 3'-5'-exonuclease. In order to assess the functional significance of conserved residues in the putative exonuclease domain, we introduced point mutations as well as deletions within and near the conserved motifs' exonuclease (Exo) I, II, and III of the DNA polymerase gene from a phosphonoacetic acid-resistant derivative of herpes simplex virus-1 strain ANG. We examined the catalytic activities of the partially purified enzymes after overexpression by recombinant baculovirus. Mutations of the motifs' Exo I (D368A, E370A) and Exo III (Y577F, D581A) yielded enzymes without detectable and severely impaired 3'-5'-exonuclease activities, respectively. Except for the Exo I mutations, all other Exo mutations examined affected both exonuclease and polymerization activities. Mutant enzymes D368A, E370A, Y557S, and D581A showed a significant ability to extend mispaired primer termini. Mutation Y557S resulted in a strong reduction of the 3'-5'-exonuclease activity and in a polymerase activity that was hyperresistant to phosphonoacetic acid. The results of the mutational analysis provide evidence for a tight linkage of polymerase and 3'-5'-exonuclease activity in the herpesviral enzyme.

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

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

Stimulation of thymidine kinase activity in baculovirus infected cells is not due to a virus-coded enzyme

A polyhedrin-positive recombinant autographa californica nuclear polyhedrosis virus (AcNPV) carrying a herpes simplex virus thymidine kinase gene under the control of the SynXIV promoter, a fusion of synthetic and linker-modified polyhedrin promoters, has been constructed. When this recombinant baculovirus was used to infect a variant of Spodoptera frugiperda cells deficient in thymidine kinase (TK-), a high level of TK activity was detected. These results, in conjunction with the demonstration that AcNPV could replicate in TK- S. frugiperda cells and no TK activity was found throughout infection, imply that the wild type virus-stimulated TK activity observed in S. frugiperda (TK+) cells is not contributed by a virus-coded enzyme.

Bioreactor development for production of viral pesticides or heterologous proteins in insect cell cultures

The insect cell-baculovirus expression system has significant potential for producing proteins requiring some degree of posttranslational modification. T. ni cells appear to be as good a host as S. frugiperda cells for heterologous protein production as demonstrated by production of beta-galactosidase. Attachment-dependent cells of T. ni can be effectively cultured in a packed-bed reactor using glass beads. When cell in such a reactor were infected, they produced 35% of the total protein as beta-galactosidase. No cell detachment was observed even 70 h postinfection. A model of viral entry has been proposed and tested.

Baculovirus transcription in the presence of inhibitors and in nonpermissive Drosophila cells

Regulation of transcription within four well-defined regions of the DNA genome of the baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV) was studied in a permissive lepidopteran cell line. Spodoptera frugiperda IPLB-SF-21 and a nonpermissive dipteran cell line, Drosophila melanogaster DL-1. Cycloheximide, an inhibitor of protein synthesis, was used to identify immediate early transcripts and aphidicolin, an effective inhibitor of DNA replication, was used to distinguish early and late transcripts in S. frugiperda cells. Immediate early transcripts were identified in the HindIII-K (85 to 87.5%), the HindIII-I/EcoRI-F (I/F from 35 to 37%) and the HindIII-P/EcoRI-P (p10 from 88.5 to 89.5%) regions of the genome. Late RNAs, defined by their sensitivity to aphidicolin as well as cycloheximide, overlapped the immediate early RNAs. Specific viral transcripts were synthesized in nonpermissive D. melanogaster cells infected with AcNPV but late viral transcripts were not observed.