REDUNDANCY OF RIBOSOMAL AND TRANSFER RNA GENES IN SACCHAROMYCES CEREVISIAE

DNA-RNA hybridization studies have been performed to determine the number of 4, 18 and 26 s RNA cistrons present in purified nuclear and mitochondrial DNA of Saccharomyces cerevisiae. It is found that 0.8% of the nuclear DNA hybridizes with 18 s RNA and 1.6% with 26 s RNA. Between 0.064 and 0.08% of the nuclear DNA is complementary to transfer RNA. No hybridization is observed between mitochondrial DNA and the three classes of cytoplasmic RNA. Assuming a genome size of yeast nuclear DNA of 1.25 × 1010 daltons, the hybridization data correspond to 140 cistrons for ribosomal RNA and to 320 to 400 cistrons for transfer RNA. With the possible exception of a minor homology between 18 and 26 s ribosomal RNA, the competition experiments indicate that there are separate cistrons for all three classes of RNA. The significance of these findings to the continuous synthesis of ribosomal RNA during the cell cycle of yeast is discussed. © 1969.