Base analysis of RNA by 3H postlabeling — A study of ribothymidine content and degree of base methylation of 4 S RNA

Abstract 1. Ribothymidine (5-methyluridine) and methylated base content of 4 S RNA from various sources have been determined. 2. Whereas bacterial 4-S RNAs contain one mole of ribothymidine per mole of RNA and have a low degree of base methylation, mammalian cytoplasmic 4-S RNAs, e.g. from rat liver, muscle, and brain, were found to contain only 0.45 to 0.5 mole of ribothymidine per mole of RNA and to have a methylated base content about three times higher than that of bacterial 4S RNA. 3. Cytoplasmic 4-S RNAs from Morris Hepatomas 7777 and 5123D are slightly undermethylated when compared with liver 4 S RNA, the ribothymidine content being slightly lower (Hepatoma 7777) or equal (Hepatoma 5123D). Pellet (15 000 × g ) 4-S RNAs contain less ribothymidine and methylated bases than do the respective cytoplasmic 4-S RNAs. These differences are more pronounced for the tumor 4-S RNAs. 4. Mitochondrial 4-S RNAs from liver and Hepatomas 5123D and 7777 contain about 3 to 5 times less ribothymidine and about 2 times less methylated bases than does liver cytoplasmic 4 S RNA, ribothymidine and methylated base content of mitochondrial 4 S RNA being lower for the tumors than for liver.