Ultraviolet Irradiation of Tobacco Leaves: Inhibition of Tobacco Mosaic Virus‐ribonucleic Acid Photoreactivation*

Short‐wave (254‐nm) ultraviolet irradiation of the leaves of Nicotiana tabacum var. Xanthi, n.c. inhibits their ability to photoreactivate ultraviolet‐inactivated tobacco mosaic virus ribonucleic acid (TMV‐RNA). The inhibition is stable in the dark; however, subsequent illumination of the leaves relaxes the inhibition. The spectral region most effective in relaxing the inhibition is the same one effective in photoreactivation. These results are consistent with the hypothesis that ultraviolet‐induced lesions in cellular nucleic acids in the dark form stable complexes with agents responsible for the photoreactivation of TMV‐RNA (effectively isolating the agents from the TMV‐RNA) and that upon illumination these complexes dissociate (presumably with repair of the lesions). We therefore suggest that the system which photoreactivates ultraviolet‐damaged TMV‐RNA also photorepairs ultraviolet damage in cellular nucleic acids.