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A Comparison of ³²P Distribution in Oligonucleotides of Ribosomal 28 S RNA from Normal Liver and Novikoff Hepatoma Ascites Cells¹

Department of Pharmacology, Baylor College of Medicine, Houston, Texas 77025

Differences between the nucleolar RNA's of tumors and other tissues were found in earlier studies on the ³²P nucleotide compositions and the isotope content of oligonucleotides produced by complete digestion with pancreatic RNase. The present studies were carried out on digestion products produced by T₁ RNase from 28 S rRNA of the Novikoff hepatoma and normal rat liver. In particular, the smaller fragments produced were examined, inasmuch as earlier studies have shown that one large fragment, the B₃ fragment, is very similar in the 28 S rRNA of both Novikoff hepatoma and normal liver. In the di- to decanucleotide region analyzed, the (AMP + UMP)/(GMP + CMP) ratio was approximately 1.5, as compared to 0.53 in whole 28 S rRNA and 0.23 in the B₃ fraction.

Marked differences were found in the nucleotide compositions of the isopleths produced and separated by chromatography on diethylaminoethyl-Sephadex. The dinucleotides of these 28 S rRNA's were separated by chromatography on Dowex 1 and 2-dimensional ionophoresis; they differed primarily in the numbers of UG and CG residues. The liver 28 S rRNA digest contained 5 UG and 15 CG residues, and the Novikoff hepatoma digest contained 12 UG and 8 CG residues. Since earlier studies have shown a higher GU content of nucleolar RNA and rRNA of the tumors, it seems likely that the tetranucleotide GUGU is present in higher concentration in the rRNA of the Novikoff hepatoma than in normal liver. The differences found by these and earlier studies on oligonucleotides are supported by hybridization studies on 18 S and 28 S rRNA.

¹ This work was supported in part by USPHS Cancer Research Center Grant 10893-02, Cancer Training Grant CA 05154-07, American Cancer Society Grant P-339, and a grant from the M.D. Anderson Foundation.

² Postdoctoral trainee of the National Cancer Institute.

Received 5/19/70. Accepted 7/14/70.