The absence of photoreversible pyrimidine dimers in the RNA of ultraviolet-irradiated tobacco mosaic virus

Pyrimidine dimers of cyclobutane type were found in the RNA of tobacco mosaic virus when irradiated free (about 1.0 per photoreactivable lesion), but not in the RNA irradiated inside the virus. As the photoreactivable kind of lethal damage is known to occur only in the free RNA, the occurrence of this kind of damage coincides with the formation of the dimers.