Characterization of Bombyx mori nucleopolyhedrovirus with a deletion of bm118

Bombyx mori nucleopolyhedrovirus (BmNPV) ORF118 (bm118) is homologous to Autographa californica nucleopolyhedrovirus (AcMNPV) ORF142, one of the core genes existing in all baculovirus genomes sequenced to date, suggesting that Bm118 plays a critical role in viral infection. In this study, the primary role of Bm118 was investigated by using homologous recombination in Escherichia coli to generate a bm118 knockout bacmid containing the BmNPV genome. In addition, the bm118 rescue bacmid was constructed by transposing a bm118 gene cassette into the polh locus of the bm118 knockout bacmid. Transfection assays demonstrated that the bm118 knockout bacmid was incapable of producing budded virion (BV). Nevertheless, this defect could be partially recovered by a rescue bacmid. Electron microscopy analysis revealed that the bm118 knockout produced aberrant capsids characterized by translucent, elongated nucleocapsids present as bundles within the nuclei. This construct also produced polyhedra lacking virions. These results reveal that Bm118 is essential for BV production and nucelocapsid maturation.