Photoreversible Uv‐inactivation of Messenger Rna in an Insect Embryo (Smittia Spec., Chironomidae, Diptera)

Smittia embryos were UV‐irradiated during intravitelline cleavage. At this stage, nuclei are heavily shielded by yolk‐rich cytoplasm, and do not synthesize detectable amounts of RNA. Irradiation at 265, 285 and 295 nm wavelength caused biological inactivation, and pyrimidine dimer formation in maternal RNA as described earlier (Kalthoff, 1976; Jäckie and Kalthoff, 1978). In addition, we observed marked effects on protein synthesis: (1) The overall rate of [ 35 S]‐methionine incorporation in vivo was reduced to less than half of the normal rate. (2) Two‐dimensional gel electrophoresis revealed quantitative variations in the synthetic rate of some polypeptides, and the appearance of new ones in UV‐irradiated embryos. (3) Translation of polyadenylated RNA from Smittia embryos in a cell‐free system was inhibited by UV irradiation in vivo. (4) The apparent degradation, during early embryogenesis, of maternal polyadenylated RNA was retarded in UV‐irradiated embryos. Exposure to light (400 nm) after UV caused partial photoreversal of all UV effects observed. Both the photoreactivable sector of UV‐inactivation, and the photoreactivated portion of UV inhibition of protein synthesis, were correlated with the amounts of pyrimidine dimers generated in maternal RNA by UV irradiation at the three wavelengths used. These correlated effects were produced most efficiently by 295 nm radiation, indicating the involvement of photosensitizing components in the embryos. Our data show, for the first time to our knowledge, that animal mRNA, after UV irradiation, can be photoreactivated in vivo. Moreover, our results strongly suggest that the photorepairable lesions consist of pyrimidine dimers generated in a photosensitized reaction.