In vitro evidence that the coat protein is the programming factor in alfalfa mosaic virus‐induced RNA synthesis

Abstract
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.