RNA turnover in cultured hamster embryo cells: Identification of modified nucleoside end products

Fourteen methylated nucleosides (N-2-dimethylGuo, N-2-methylGuo, N-1-methylGuo, N-5-methylUrd, N-3-methylUrd, N-1-methylAdo, N-3-methylCyd, N-5-methylCyd, N-1-methyllno, 2′-0methyl-Cyd, 2′-0-methylUrd, 2′-0-methylGuo, 2′-0-methyllno, and thymidine) and one methylated base (m⁷Gua) have been identified as normal excretion products of cultured hamster embryo cells. The methylated nucleosides are excreted in the culture media subsequent to RNA turnover. The excretion pattern of the base-methylated nucleosides was determined by continuous labeling of serum-stimulated quiescent hamster embryo cells with ³H-CH₃]methionine and measurement of radioactivity in the excreted nucleosides between 23 and 81^1/2 hours after the label was added. These nucleosides accumulate exponentially until a maximum level is reached after 60 hours. These maximum levels were maintained for at least an additional 20 hours.