Characterization of Yeast Ribosomal DNA

High‐molecular‐weight yeast ribosomal DNA, purified by preparative Hg2/Cs2SO4 gradient centrifugation, was characterized using RNA‐DNA hybridization, CsCl gradient analysis and denaturation‐renaturation experiments. The following conclusions are drawn from the results of these studies. 1 55–60% of the total yeast rDNA is made up of sequences coding for 42‐S ribosomal precursor RNA. 2 The mean length of the 42‐S precursor cistrons corresponds to a molecular weight of 6.2 × 106. 3 The overall G + C content of yeast rDNA is 42%. On the other hand, both the segments coding for the nonribosomal excess RNA of the 42‐S precursor and those specifying 26‐S and 17‐S rRNA have a higher G + C content of 46.9–47.5%. 4 The 42‐S precursor cistrons are closely bound up with sequences rather low in G + C content (34–35%), which account for 40–45% of the total yeast rDNA. In contrast to the multiple 42‐S precursor cistrons which show very little or no base sequence divergence these intervening (A + T)‐rich sequences display a considerable heterogeneity in base sequence.