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one of the two main forms of the enzyme. A situation of this kind has been found during the development of Rana pipiens embryos (Kohl, Norman & Brooks, 1973); Polymerase I (the nucleolar form) and Polymerase II (nucleoplasmic) behave differently. The levels of Form II do not change during development despite changing patterns of DNA-like RNA synthesis, while Form I increases in the nuclei after gastrulation when ribosomal RNA synthesis begins, suggesting that the expression of the ribosomal cistrons may be controlled in this case by the availability of the corresponding polymerase. We have attempted, therefore, to discriminate between the levels of Polymerases I and II at various developmental stages of the mouse embryo by measuring at two different ionic strengths the enzymatic activity present in embryo homogenates. The activities of RNA polymerases are markedly influenced by the ionic strength. In the presence of excess native DNA, Polymerase I shows optimal activity at 25 to 50 mM-ammonium sulphate, Polymerase II at 100 to 120 min (Gissinger, Kedinger & Chambon, 1974). The technical procedures used were those previously reported (Siracusa, 1973), with the following main differences. Native DNA was used as template in the assay; tritiated uridine triphosphate ([3H]UTP) concentration was raised to 50 μ (NEN, sp. act. 19-8 Ci/mmol), incubation time was prolonged to 90 min. The final incubation mixture (40 μ ) contained 50 mM-tris-HCl, pH 7-9; 2-5 mM-MgCl2 ; 2 mM-MnCl2 ; 0-2 mM-ATP; 0-2 mM-GTP; 0-2 mM-CTP; 50 ^m-[3H]UTP; 0-2 mg native DNA/ml (calf thymus); 12-5% glycerol; 0-05 mM-EDTA; 0-5 mM-dithiothreitol; 2-5 mmNaF; 0-5 mg BSA/ml; 25 or 125 mM-(NH4)2S04. A total of 4140 embryos was used.

LOW-SALT AND HIGH-SALT RNA POLYMERASE ACTIVITY DURING PREIMPLANTATION DEVELOPMENT IN THE MOUSE