Incorporation of (5-3H) uridine into ribonucleotide pools and RNA during thyroxine-induced metamorphosis of Xenopus laevis tadpoles.

Incorporation of [3H] uridine into the ribonucleoside triphosphates UTP and CTP, total RNA, and nuclear and cytoplasmic RNA was followed in Xenopus laevis tadpole liver during thyroxine (T4)-induced metamorphosis. Pool sizes of UTP and CTP were found to remain unchanged, although turnover the ribonucleoside triphosphates was found to be greatly stimulated after 4 days of hormone treatment. The time course of labeling of the 40-S pre-rRNA was very similar to that of UTP in both thyreostatic and T4-treated tadpoles, thus reflecting a direct relationship between turnover of the immediate precursors and labeling of RNA. Although a faster depletion of labeled UTP and pre-rRNA (precursor ribosomal RNA) was noted in T4-treated tadpoles, labeled cytoplasmic rRNA continued to accumulate almost linearly for 25 h. In thyreostatic larvae no further increase in labeled cytoplasmic rRNA occurred beyond 4 h of labeling. From these results we conclude that both enhanced transcription and more effective utilization of pre-rRNA are responsible for the net accumulation of rRNA observed on the 4th day of T4-induced metamorphosis.