INHIBITION BY a-AMANITIN OF THE OESTRADIOLINDUCED INCREASE IN a-AMANITIN INSENSITIVE RNA POLYMERASE IN IMMATURE RAT UTERUS

The injection of oestradiol to the prepuberal or castrated rat results, 2-3 hr later, in a large increase in RNA biosynthesis [l] and in an increased ability of uterine nuclei to incorporate nucleoside triphosphates into RNA [2]. Various reports have established that oestrogen-stimulated RNA is largely of ribosomal nature [3,4] . Recently, two different classes of DNAdependent RNA polymerases have been solubilized and separated from rat liver and calf thymus [5,6] : one is of nucleolar origin (enzymes A or I) and insensitive to cwamanitin [lo] , the other is mainly nucleoplasmic (enzymes B,, B,, or II) and inhibited by o-amanitin [7, IO]. It has been shown that 2 hr after injection of oestradiol to immature female rats, the activity of polymerase A, which is unaffected by cll-amanitin, increases. This enzyme catalyses the synthesis of ribosomal-type RNA and may be assayed in a medium of low ionic strength [ 111. The administration of inhibitors of protein synthesis, such as puromycin or cycloheximide, just before or even immediately after oestradiol injection suppresses the increased RNA synthesis by low salt RNA polymerase activity [12] . It thus appears that one of the earliest effects of oestradiol is to induce a “first-stage” protein synthesis, sufficiently slight to pass easily undetected by conventional techniques during the “lag period” of the first hour, but of crucial importance