Viral RNA-dependent RNA polymerase preparations were isolated from cowpea mesophyll protoplasts 26 h after inoculation with the complete genome (RNAs 1, 2 and 3) and the incomplete genome (RNAs 1 and 2) of alfalfa mosaic virus. Although in incompletely inoculated protoplasts the ratio minus-/plus-strand RNA synthesis is much higher than in completely inoculated ones [(1983) Virology 124, 75–85)], both enzyme preparations produced largely plus-strand RNA on their endogenous templates. When made template-dependent both were strongly inhibited by small amounts of coat protein. The experiments show that among the two proteins encoded by RNA 3 only the (parental) coat protein can be responsible for the programming of the viral RNA polymerase to plus-strand synthesis.
