Expression, purification, and enzymatic characterization of Bombyx mori nucleopolyhedrovirus DNA polymerase

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.

Codon optimization enhances protein expression of Bombyx mori nucleopolyhedrovirus DNA polymerase in E. coli

Bombyx mori nucleopolyhedrovirus (BmNPV) is a major viral agent that causes deadly grasserie disease in silkworms, while BmNPV DNA polymerase (BmNPV-pol), encoded by ORF53 gene, plays a central role in viral DNA replication. Efficacy studies of BmNPV-POL are limited because of poor heterologous protein expression in E. coli. Here, we redesigned the BmNPV-pol to preferentially match codon frequencies of E. coli without altering the amino acid sequence. Following de novo synthesis, codon-optimized BmNPV-pol (co-BmNPV-pol) gene was cloned into pET32a and pGEX-4T-2 vector. The expression of co-BmNPV-POL in E. coli was significantly increased when BmNPV-POL was fused with GST protein rather than a His-tag. The co-BmNPV-POL fusion proteins were isolated using GST affinity chromatography and Mono Q iron exchange chromatography. Protein purity and identity were confirmed by western blot and MALDI-TOF analyses. The biological activity of purified proteins was measured on a poly(dA)/oligo(dT) primer/template. The specific polymerasing activity of the recombinant BmNPV-POL was 6,329 units/mg at optimal conditions. Thus, a large amount of purified protein as a soluble form with high activity would provide many benefits for the functional research and application of BmNPV-POL.