Serial passage of nuclear polyhedrosis viruses (NPVs) through cultured cell lines results in the appearance of mutants with a complex phenotype referred to as the ‘few polyhedra’ (FP) phenotype. The altered plaque morphology and reduced occlusion production associated with the FP phenotype have been observed in multinucleocapsid nuclear polyhedrosis virus (AcMNPV) bearing mutations in the gene encoding the 25 kDa protein (25K gene). In this study, we sequenced the 25K genes of four spontaneously occurring AcMNPV FP mutants. These mutants, together with an artificially generated FP mutant (AcFPβgal, in which the gene for β-galactosidase is fused in frame with the 25K ORF), were examined at the ultrastructural level to see if they exhibited the reduced virion occlusion and intranuclear envelopment which is associated with the FP phenotype. Observations on Sf9 cells infected with the FP mutants revealed that all five mutants were impaired in virion occlusion and intranuclear nucleocapsid envelopment. The 25K mutants were also found to release two- to fivefold more infectious virus (p.f.u.) into the media of infected Sf9 cells. Marker rescue of AcFPβgal restored wild-type virion occlusion, intranuclear envelopment and levels of infectious virus production.


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