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Localization of Deoxyribonucleic Acid Complementary to Ribosomal Ribonucleic Acid and Preribosomal Ribonucleic Acid in the Nucleolus of Rat Liver

From the ¹ From the Department of Pharmacology, Baylor University College of Medicine, Houston, Texas 77025

The fraction and distribution of DNA complementary to ribosomal RNA and preribosomal RNA was investigated by the technique of molecular hybridization with the DNA of isolated nucleoli and nuclei of rat liver. Nuclear DNA complementary to ribosomal 18 S RNA and 28 S RNA was 0.014% and 0.032%, respectively. Nucleolar DNA complementary to ribosomal 18 S RNA and 28 S RNA was 0.10% and 0.22%, respectively. About 75% of the ribosomal RNA cistrons were contained in the DNA of isolated nucleoli. Nucleolar DNA complementary to nucleolar 28 S RNA, 35 S RNA, 45 S RNA, and 55 S RNA was 0.44%, 0.44%, 0.52%, and 0.53%, respectively.

The absence of competition between ribosomal 18 S RNA and 28 S RNA for the DNA of isolated nucleoli and nuclei indicated that there were distinct sequences of DNA complementary to the two ribosomal RNAs. The existence of competition between ribosomal RNAs and preribosomal RNAs for the DNA of isolated nucleoli indicated that the two ribosomal RNAs shared base sequences in common with nucleolar 28 S RNA, 35 S RNA, and 45 S RNA, and suggested that the latter may contain base sequences similar to ribosomal 18 S RNA that are not converted to either of the two ribosomal RNAs. The analyses for the base composition of preribosomal RNAs indicated that the conversion of nucleolar 45 S RNA to nucleolar 28 S RNA was accompanied by a progressive increase in the content of adenine and a decrease in the content of uracil.

The nucleolar DNA fraction contained a major band of buoyant density in CsCl of 1.700 g per cm³, a light satellite component of 1.692 g per cm³, and a heavy satellite component of 1.708 g per cm³. The heavy satellite, which comprised 8% of the total nucleolar DNA, contained a major part of the DNA complementary to ribosomal 18 S RNA and nucleolar 28 S RNA, 35 S RNA, and 45 S RNA, and a smaller percentage of the DNA complementary to ribosomal 28 S RNA. The base composition of nucleolar DNA differed from nuclear DNA in a disparity between the content of adenine (30%) and the content of thymine (27%).

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