RNA polymerase activity and protein synthesis in brome mosaic virus-infected protoplasts

RNA-dependent RNA polymerase activity and protein synthesis in brome mosaic virus (BMV)-infected and uninfected protoplasts were investigated during the course of viral replication. In protoplast homogenates, the membrane-bound RNA polymerase activity resistant to actinomycin D was enhanced by BMV infection up to 30-fold that found in mock-inoculated protoplasts. The activity was first detected 7 to 8 hr postinfection. It reached a maximum at around 30 hr postinfection, then decreased gradually while virus and the viral antigen in the protoplasts, as measured by infectivity and fluorescent antibody staining, respectively, continued to increase. The bound enzyme activity was not enhanced by inoculation with BMV RNA 3 and/or RNA 4. Four proteins with molecular weights of 120, 110, 36, and 19.5 x 103 were observed in BMV-infected protoplasts from three plant species (a systemic host, a local lesion host, and a nonhost of BMV).