Hypovirulence in is known to be associated with dsRNA. The characteristics of this cytoplasmically transmissible element suggest it might be viral in nature. Attempts to isolate viral particles indicate the presence of two particulate fractions in hypovirulent (HV) strains and a similar one in virulent (V) strains. Composition analyses of the fractions show these to be similar except that those from HV strains include dsRNA. Also present in these particles are carbohydrate, protein and chloroform-methanol-extractable substances. The protein composition is too low to indicate the presence of a capsid. The neutral sugars that are present (arabinose, mannose, galactose, glucose) in these particles are also present in the fungal cell wall. Silver- and ethidium bromide-stained polyacrylamide gels resolve several minor as well as the known major dsRNA components. These components are arbitrarily divided into four mol. wt. groups: 4.5 × 10 to 5.0 × 10, 1.2 × 10 to 1.5 × 10, 5.8 × 10 to 6.2 × 10 and < 4.6 × 10. Not all segments are present at all times, although the phenotype of the culture is not affected. Proteins are not detectable in vesicles of HV strains, but six proteins are detectable from V strains by gel electrophoresis. Assays for [P]UMP incorporation indicate the presence of a RNA polymerase closely associated with the dsRNA. Our results suggest that the transmissible element responsible for hypovirulence is a naked dsRNA genome packaged within vesicles formed by the host.


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