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Methylation of Nuclear and Cytoplasmic RNA of Mouse Liver with Dimethylnitrosamine-³H¹

Department of Chemistry [M. M.] and Experimental Pathology [Y. A., N. N., S. T.], Cancer Institute, Toshimaku, Tokyo, Japan

The labeling patterns of mouse nuclear and cytoplasmic RNA were examined at various times after i.p. administration of tritiated dimethylnitrosamine (DMN-³H). Sedimentation analysis of nuclear and cytoplasmic RNA showed that most RNA species of the cell, including cycloplasmic ribosomal 28 S and 18 S RNA, 4 S² RNA, nuclear ribosomal and low-molecular-weight (4 to 7 S) RNA, and possibly rapidly labeled nRNA, were methylated rapidly by DMN. The average specific activity of nRNA was almost the same as that of cytoplasmic RNA at 10 and 30 min after injection, indicating the rapid penetration and reaction of DMN with nRNA's. The degree of methylation, however, was different with RNA species; e.g., the ratio of the number of methyl groups that were introduced per unit nucleotide length was 1.3:1.0:1.6 for 28 S, 18 S, and 4 S RNA, respectively, at 5 different times studied. Although most of the methyl group was found on the N⁷ position of guanine moiety, this ratio could not be accounted for by the guanine content of these RNA species. Methylation of the position 2' O of ribose moiety was not detected.

Methylated RNA was found to become unstable in vivo. A fraction of rRNA that had been methylated by DMN degraded at a half-life of approximately 1.5 day, the remainder at approximately 3 to 4 days, while normal rRNA has a half-life of 5 days.

¹ Supported in part by grants from Ministry of Education of Japan.

Received 4/20/71. Accepted 11/17/71.