Inhibition of ribosomal RNA synthesis in yeast by ionizing radiations

The effects of ionizing radiations on transcriptional activity were studied in the diploid yeast Saccharomyces cerevisiae. As experimental indicator the synthesis of ribosomal RNA (r-RNA) was measured for 1 h after exposure of cells to gamma-rays, X-rays or alpha-particles. gamma- or X-ray induced transcription inhibition was always found to decrease exponentially with dose. D0 values of 2150 or 1950 Gy were determined in wild-type cells, corresponding to a mean energy of about 60 eV per r-RNA gene. The finding of differential sensitivities of the two high molecular-weight r-RNA species which are cotranscribed from r-DNA is compatible with the existence of a transcription terminating mechanism. Cells from a mutant strain (rad-9), radiation sensitive with respect to colony forming ability, show an approximately equal sensitivity for transcription inhibition compared to the wild-type (D0 (2095) = 2400 Gy). Inactivation of r-RNA synthesis in cells exposed to alpha-particles at room-temperature showed a decreased sensitivity with higher particle fluences ('resistant tail'). This phenomenon, however, was drastically reduced if the temperature during irradiation was lowered to 4 degrees C, and was completely abolished when dried cells were used. The results may be explained by the presence of recovery processes in vegetative cells during prolonged exposures to alpha-particles at room temperature. An inactivation cross-section for alpha-particle induced transcription inhibition of about 0.02 micron 2 can be derived from the experimental data. When comparing this value with the estimated target volume, it appears that no single genes but the whole cluster of about 120 r-RNA genes on each of the chromosomes XII in the diploid cell may be inactivated by a single alpha-particle.