Bombyx mori nucleopolyhedrovirus orf133 and orf134 are involved in the embedding of occlusion-derived viruses into polyhedra.

Bombyx mori nucleopolyhedrovirus (BmNPV) orf133 (bm133) and orf134 (bm134), the orthologues of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) ac4 and ac5, are two adjacent genes with opposite transcriptional orientations and are highly conserved in all sequenced group I nucleopolyhedroviruses (NPVs). A double bm133-bm134 knockout bacmid was generated to enable the functional study of each gene independently or together. Compared with wild-type and double-repair viruses, deletion of both bm133 and bm134 did not affect budded virus (BV) production or viral DNA replication in transfected BmN cells. Electron microscopy revealed that the double knockout did not affect nucleocapsid assembly, virus-induced intranuclear microvesicle formation or occlusion-derived virus (ODV) production, but the number of virions embedded in the polyhedra decreased significantly. Further investigations showed that disruption of either gene was unable to recover the defect of ODV occlusion, suggesting that Bm133 and Bm134 are indispensable to the embedding of ODVs into polyhedra. Confocal microscopy analysis showed that Bm133 and Bm134 distributed throughout the whole cell during viral infection and Bm134 concentrated on the mature polyhedra in lysed cells. These results suggest that although Bm133 and Bm134 are not essential for BV or ODV development, they play vital roles in polyhedra morphogenesis.