Polyphasic changes in incorporation of precursors into ribonucleic acid of oestradiol-stimulated mammary gland

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Abstract
1. At 3 weeks after ovariectomy, mammary glands (5th pair) of adult Swiss mice show (i) no significant decrease in weight, (ii) 20% of the original rate of incorporation of [3H]-uridine into RNA (after a 30min pulse), and (iii) 90% of the original rate of incorporation of l-[3H]leucine into protein (after a 15min pulse). 2. A single injection of oestradiol-17β into these ovariectomized mice produces, during the next 17h, a series of discrete bursts of increased incorporation of [3H]uridine into mammary-gland RNA; the bursts, which are variable in height, reach peaks at approx. 1, 9, 12 and 16h after hormone administration; an increase is already detected at 15min, the earliest time-point investigated; each burst lasts for approx. 2h. There is no significant stimulation of [3H]uridine incorporation into RNA of liver and quadriceps femoris muscle. 3. Nuclear incorporation of [3H]UTP into RNA of mammary gland in vitro is linear with time for up to 20min at 15°C; it requires CTP, GTP and ATP and is inhibited by actinomycin D. Also, the incorporation is strongly inhibited by α-amanitin in high salt concentrations but only weakly in low salt concentrations, a result indicating that RNA polymerase II activity predominates in high salt, whereas RNA polymerase I activity predominates in low salt concentrations. Injection of oestradiol-17β in vivo followed by measurement of nuclear RNA synthesis in vitro shows a definite increase in both RNA polymerase activities 30min after oestradiol-17β injection, the earliest time-point investigated, a higher increase at 1h, a decline at 4h, and again a large increase at 12h. These results in general agree with the changes in precursor incorporation into RNA measured directly in the animal and suggest that changes in [3H]uridine uptake into RNA are not precursor-pool-dependent.