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Loss of infectivity by tobacco mosaic virus exposed to ultraviolet radiation appeared to result entirely from changes in the RNA of the virus. Any changes that may have occurred in the virus protein did not seem to contribute to loss of infectivity of the virus, but there was an interaction between the protein and the RNA because free RNA was equally sensitive to inactivation by absorbed radiation energy of any wavelength (i.e. the extent of inactivation depended entirely on the number of absorbed quanta irrespective of the wavelength), while inside the virus the RNA was about 24 times more sensitive to inactivation at 230 mµ than at 280 mµ. Inside the virus the RNA seemed to be largely protected by the protein from damage by radiation of 280 mµ and 254 mµ, but not of 230 mµ to which it was about as sensitive as when free.
Ultraviolet irradiation at any wavelength caused at least two kinds of damage to free RNA, one of which was photoreversible, but the photoreversible damage did not occur in the RNA irradiated when inside the virus.