1989 Volume 55 Issue 5 Pages 635-642
Activity of membrane-bound RNA-dependent RNA polymerase (M-RDRP) from cucumber mosaic virus (CMV)-infected tobacco plants increased prior to virus multiplication and that of soluble enzyme (S-RDRP) increased rapidly after concentration of CMV has reached its maximum. In vitro products of M-RDRP was mainly doublestranded CMV RNAs as replicative forms. M-RDRP was suggested to be associated with endoplasmic reticulum (ER) by subcellular fractionation of the membrane-fraction using Percoll discontinuous gradient-centrifugation and electron microscopic observation. The ER-associated enzyme had viral RNAs as endogenous templates and was firmly integrated into membrane structure, even after treated with detergent or Mg++-depletion or partially purified by Sepharose 4B chromatography. The purification procedures could effectively remove S-RDRP which synthesized small heterogeneous RNA in vitro. Joined with our previous study using CMV-infected tobacco protoplasts, it was suggested that S-RDRP is a host-encoded enzyme induced in leaf tissues but not in protoplasts as a consequence of virus infection process like “Pathogenesis-related proteins.” We speculate that the CMV RNA replication complex is composed at least of translation products of viral RNAs, ER membranes and viral template RNAs.