Specificity of polyhedrin in the generation of baculovirus occlusion bodies

The role of polyhedrin in the occlusion of virions was studied by substitut ing two heterologous polyhedrin-coding sequences, one from a multiple-nucle ocapsid (M) nucleopolyhedrovirus (NPV) of Spodoptera exigua (Se) and one fr om a single-nucleocapsid (S) NPV of Buzura suppressaria (BusuNPV), into the genome of Autographa californica (Ac) MNPV. Both heterologous polyhedrin g enes were highly expressed and polyhedra were produced in the nuclei of cel ls infected with the respective recombinant AcMNPVs, Polyhedra produced by the recombinant with BusuNPV polyhedrin showed normal occlusion of multiple -nucleocapsid virions and were equally as infectious to S. exigua larvae as were wild-type AcMNPV polyhedra. This indicates that virion occlusion is n ot specific with respect to whether the virions or polyhedrin are from an S NPV or MNPV. Polyhedra produced by the recombinant containing the SeMNPV po lyhedrin had an altered morphology, being pyramidal rather than polyhedral in shape, and many fewer virions were occluded. These occlusion bodies were less infectious to S. exigua larvae than were those of wild-type AcMNPV. T hese results indicate that virion occlusion is a finely controlled process that is to some extent specific to the polyhedrin involved and may also req uire other viral or host factors for optimal morphogenesis.