ABSTRACT
This chapter focuses on brome mosaic virus (BMV) and turnip yellow mosaic virus and reviews the status for related viruses. Efforts to ascribe viral replicase function to host activities have certainly obscured the study of true replicase, an enzyme characterized by its specificity for copying cognate viral RNA. Only recently has it become generally accepted that viral replication is indeed dependent on a unique and specific RNA-dependent RNA polymerase (replicase) present only in infected tissues. Extracts of TYMV-infected chinese cabbage leaves carry a membrane-bound, RNA-dependent RNA polymerase activity capable of synthesizing double-stranded, ribonuclease-resistant products. Study of the subunits present in the purified TYMV replicase has provided evidence that the replicase complex may contain a virally encoded protein. The BMV and TYMV replicase systems described above provide good examples of techniques and approaches that can be applied to studies on the replication of related plant viruses.
