Ultraviolet-induced inhibition of ribosomal RNA synthesis in yeast strains differing in radiation sensitivities.

The ultraviolet-induced inhibition of rRNA synthesis has been measured during the first hour after irradiation for stationary yeast cells differing in radiation sensitivity. rRNA was isolated and separated on an agarose-polyacrylamide gel. The wild type and a mutant which is possibly defective in recombinational repair show a sigmoidal inhibition curve, an excision-deficient mutant shows an exponential one. From these curves it is deduced that a pyrimidine dimer acts as a transcription terminating lesion as was shown for bacteria. During the first hour after irradiation the excision repair system decreases the number of transcription terminating lesions by 22% in the wild type and 25% in the mutant defective in recombinational repair. An approximation of the repair efficiency gives a value of 7500-10 000 transcription terminating lesions per cell being removed during the first hour after irradiation by excision. Ultraviolet-induced lesions of this kind can partially be removed by photoreactivation. The inhibition coefficients are the same for 26 S and 18 S rRNA in stationary cells, whereas exponentially growing cells show different inhibition coefficients for 26 S and 18 S rRNA leading to the suggestion that the processing of the ribosomal precursor RNA is different in stationary and exponentially growing cells.