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A significant body of evidence assembled in recent years supports the hypothesis that hormonal augmentation of protein synthetic ability by ribosomal preparations from a number of tissues is reflected in an accelerated rate of RNA synthesis,'-4 including an RNA with rapid turnover, inferred to be messenger RNA.5-"2 An early enzymic manifestation of this temporal relationship between increased rates of incorporation of amino acids into ribosomal protein and RNA synthesis has been demonstrated for liver, namely, a rise in RNA polymerase activity. Within 30 min following injection of cortisol into rats, liver nuclear preparations from hormonetreated animals showed an elevation ofRNA polymerase activity significantly above that of preparations from untreated controls.'3-'5 Gorski'6 reported that estradiol injection in rats caused an increase in activity of uterine RNA polymerase within 2 hr. In contrast to the stimulation by cortisol of the ribosomal protein synthetic capacity of liver'7 18 is the inhibitory influence of this hormone on a cell-free amino acid incorporating system prepared from thymus."9 20 Indeed, liver and thymic tissue taken from rats 3 hr following injection of cortisol showed, respectively, an increase and a decrease in the activities of the ribosomal protein amino acid incorporating systems.'9' 20 Exploration of these opposite effects indicated that, for both tissues, the hormonal influence is reflected in alterations at the ribosomal level. The data also suggested that nuclear preparations from thymic tissue of fasted rats injected 3 hr prior to sacrifice with cortisol, as compared with similar preparations from control animals, had a diminished RNA polymerase activity.20 However, no direct evidence was obtained for a deficiency of messenger RNA on ribosomes prepared from cortisol-treated rats. The present paper is a report of studies designed to examine further the response of the RNA polymerase system of rat thymus to cortisol injection. Materials and Methods.-Male rats of the Sprague-Dawley strain, 6 weeks old, weighing approximately 150 gm, were purchased from Carworth Farms. The animals were kept at least 5 days before use and maintained on Purina chow and water ad libitum. The following chemicals were obtained from either Sigma Chemical Co., St. Louis, Mo., or Calbiochem, Los Angeles, Calif.: ATP, GTP, UTP, CTP, phosphoenolpyruvate, and pyruvate kinase (suspension in 2.1 M ammonium sulfate (pH 6.0) containing 10 mg/ml). H3-UTP (0.8 c/mmole) and H3-GTP (0.8 c/mmole) were purchased from Schwarz BioResearch, Inc., Orangeburg, N. Y., and cortisol from Steraloids, Inc., New York, N. Y. Other reagents were analytical grade and obtained from commercial sources. Experimental animals, either fed or fasted overnight with access to water, received intraperitoneally a single dose of cortisol (5 mg per 100 gm body weight). The steroid was administered as a suspension of 15 mg per ml in 0.85% NaCl. Paired treated and control animals were injected with the same volume of saline. At the desired time after steroid administration, the rats were sacrificed by decapitation. The thymi from two to three control or injected animals were rapidly excised, pooled, and placed in cold homogenization medium (0.25 M sucrose, 0.075 M KC1, 0.01 M MgCl2, 0.035 M Tris-Cl,

Acute decrease in RNA polymerase activity of rat thymus in response to cortisol injection.