Characterization of yeast ribosomal DNA

High-molecular-weight yeast ribosomal DNA, purified by preparative Hg2+/Cs2SO4 gradient centrifugation, was characterized using RNA-DNA hybridization, CsC1 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 X 10(6). 3. The overall G + C content fo yeast rDNA is 42%. On the 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 precurosr cistrons which show very little or no base sequence divergence these intervening (A + T)-rich sequences display a considerable heterogeneity in base sequence.