MUTATIONS IN THE AUTOGRAPHA-CALIFORNICA MULTINUCLEOCAPSID NUCLEAR POLYHEDROSIS-VIRUS 25 KDA PROTEIN GENE RESULT IN REDUCED VIRION OCCLUSION, ALTERED INTRANUCLEAR ENVELOPMENT AND ENHANCED VIRUS PRODUCTION

Serial passage of nuclear polyhedrosis viruses (NPVs) through cultured cell lines results in the appearance of mutants with a complex phenot ype referred to as the 'few polyhedra' (FP) phenotype. The altered pla que morphology and reduced occlusion production associated with the FP phenotype have been observed in Autographa californica multinucleocap sid nuclear polyhedrosis virus (AcMNPV) bearing mutations in the gene encoding the 25 kDa protein (25K gene). In this study, we sequenced th e 25K genes of four spontaneously occurring AcMNPV FP mutants. These m utants, together with an artificially generated FP mutant (AcFP beta g al, in which the gene for beta-galactosidase is fused in frame with th e 25K ORF), were examined at the ultrastructural level to see if they exhibited the reduced virion occlusion and intranuclear envelopment wh ich is associated with the FP phenotype. Observations on Spodoptera fr ugiperda SfP cells infected with the FP mutants revealed that all five mutants were impaired in virion occlusion and intranuclear nucleocaps id envelopment. The 25K mutants were also found to release two- to fiv efold more infectious virus (p.f.u.) into the media of infected Sf9 ce lls. Marker rescue of AcFP beta gal restored wild-type virion occlusio n, intranuclear envelopment and levels of infectious virus production.