Mechanism of Action of Gonadotropin II. COSTROL OF OVARIAN NUCLEAR RIBONUCLEIC ACID POLYMERASE ACTIVITY AND CHROMATIK

The role of RNA synthesis was investigated in the action of chorionic gonadotropin on ovaries of immature rats. Chorionic gonadotropin accelerates the ovarian incorporation of [3H]uridine in uivo into rapidly labeled nuclear RNA within 10 min after hormone injection. Analysis of the RNA base composition, expressed as base ratio (A + U):(G + C), shows that hormonal stimulation of the rate of [3H]uridine incorporation is accompanied by a relative increase of the A + U content of the rapidly labeled nuclear RNA. Hormonal stimulation of nuclear RNA synthesis in vivo is found to be specific for chorionic gonadotropin and is inhibited by actinomycin D. The extent of ovarian chromatin template available for transcription in the presence of added Escherichia coli RNA polymerase is increased 1 hour to 5 days after injection of a single dose of chorionic gonadotropin. No modification of the ovarian template capacity is observed 10 to 30 min after hormone administration. The effect of chorionic gonadotropin on RNA polymerase (EC 2.7.7.6) activity in intact nuclei from ovaries of immature rats was studied in the presence of Mg++ ions at low ionic strength, or in the presence of Mn++ ions and 0.4 M ammonium sulfate. Administration of 200 i.u. of chorionic gonadotropin to rats is followed by a 400 to 500% increase over control values of the Mn++-dependent RNA polymerase activity 2, 3, and 18 hours following hormone injection, but it has no effect when studied during shorter time intervals. Under similar conditions of hormone treatment, the Mg++dependent RNA polymerase activity in isolated rat ovary nuclei is significantly increased over controls 10 and 15 min and 3 hours following gonadotropin injection. It is concluded that an early stimulation of the Mg++-dependent RNA polymerase activity in ovarian nuclei is an essential feature of the early action of chorionic gonadotropin in the ovary of the immature rat.