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

In a previous paper (Siracusa, 1973) we have shown that the variations in the level of total RNA polymerase activity per embryo and per blastomere in the preimplantation mouse embryo are not correlated with patterns of RNA synthesis at corresponding stages of development. We concluded that, in the mouse embryo, in contrast with the situation found in the sea urchin embryo(Roeder & Rutter, 1970), RNA polymerase apparently does not play any direct regulatory r\l=o^\lein RNA synthesis. The possibility remained, however, that a correlation could still exist for only 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