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Genetic Complementation between Middle and Bottom Components of Two Strains of Radish Mosaic Virus

Rothamsted Experimental Station, Harpenden, Hertfordshire, England

Kale virus (KV), turnip virus (HZ) and radish mosaic virus (RMV) are slightly related serologically to cowpea mosaic virus. Purified preparations comprize, top, middle and bottom components, of which only the last two contain nucleic acid. The nucleoprotein components of each virus were separated by centrifuging twice through a sucrose gradient in an MSE BXIV zonal rotor. Sometimes this was followed by centrifuging to equilibrium in a density gradient of rubidium bromide. Neither nucleoprotein component was infective alone, but infectivity was restored when each was mixed with the other component.

Mixtures in which the two components came from different viruses were infective only when they were made with KV and HZ. Serologically these two viruses are more closely related than either is to RMV. KV differed from HZ by two characters related to the coat protein. KV particles regularly formed aggregates of 12 particles and contained antigens that were not present in HZ. Single lesion isolates from mixtures of bottom component from one strain and middle component from the other did not form aggregates and lacked the antigens present in KV. Like cowpea mosaic virus, KV and HZ each have two coat proteins, and our results can be explained if each of the four virus components codes for a different coat protein. Therefore, the coat protein of the virus is coded by the nucleic acid of both middle and bottom components.

Received 6 March 1973; accepted 6 April 1973.