Characterization of deoxyribonucleic acid isolated from the granulosis viruses of the cabbage looper, Trichoplusia ni and the fall armyworm, Spodoptera frugiperda

Abstract

Large, covalently closed circular deoxyribonucleic acid molecules have been isolated from the granulosis viruses that infect the cabbage looper, Trichoplusia ni, and the fall armyworm, Spodoptera frugiperda. As extracted from either virus, the nucleic acid is a mixture of double-stranded covalently closed circular molecules (about 30%), circular molecules (about 60%) with one or few nicks, and linear molecules (less than 10%). Centrifugation to equilibrium in cesium chloride gradients containing ethidium bromide shows the presence of three bands of DNA: a light band of DNA containing linear and relaxed circular DNAs, a heavy band of covalently closed DNA, and a third band of DNA intermediate to the light and heavy bands of DNA. Significantly, lyophilization of the occluded virus preparation before purification of the DNA does not allow the recovery of covalently closed DNA as compared to that recovered from nonlyophilized virus. Data herein suggest that lyophilization introduces, for the most part, a single-strand nick in the double-stranded molecule. A comparison of the sedimentation rates of the intact linear forms of the insect virus DNAs with those of T4 and T7 phage DNA standards on both neutral and alkaline sucrose gradients was used to estimate the molecular weight of the insect virus genomes. The genome from Spodoptera frugiperda granulosis virus is about 95 × 10 6 daltons and that from Trichoplusia ni granulosis virus is about 100 × 10 6 daltons. The base compositions of the virus and host DNAs were determined by thermal denaturation measurements.