Inactivation of a Single Eucaryotic Gene Irradiated in Vitro in Transcriptionally Active Chromatin Form

The transcriptional activity of endogenous RNA polymerases has been used to study the inactivation of a single eucaryotic gene irradiated in chromatin form in aqueous solution. The gene codes for the ribosomal RNA precursor in Tetrahymena where it exists in multiple copies as free extrachromosomal molecules. The gene chromatin is isolated with intact RNA polymerases on the coding strand. Irradiation with 10-MeV electrons reduces the in vitro RNA synthesis on the isolated chromatin in a dose-dependent manner which allows the amount of DNA damage to be determined. Irradiation of the gene chromatin in dilute solution under various gas atmospheres demonstrates that OH radicals account for most of the inactivation. Under these conditions, $e_{{\rm aq}}^{-}$ is unimportant, while the results suggest that H may contribute to inactivation of gene transcription.