Early estrogen action. Stimulation of the synthesis of methylated ribosomal and transfer RNAs.

The effects of estrogen on the rates of incorporation in vivo of radioactive uridine and Me-methionine, administered together, into RNA in the uterus of the ovariectomized adult rat have been measured. The ratio of incorporation of methionine to uridine during a 45-min labeling period was increased several-fold by hormone treatment. The increased rate of methylation was apparent in the uterus taken from the rat administered estrogen for 1 h, and the effect was more striking following 2 and 3 h of hormone treatment. This stimulation of methylation of RNA occurred in association with an increase in the whole-organ concentration of RNA. Analysis of the doubly-labeled uterine RNA on sucrose gradients revealed that the methylated species were mainly ribosomal and transfer RNA. These results show that very little methylation of RNA occurs in the atrophied uterus of the ovariectomized rat. During the first 3 h following estrogen administration to the ovariectomized animal, an increasing percentage of the newly synthesized RNA formed by the uterus is methylated ribosomal and transfer RNA. This result is discussed in light of recent studies of the efficiency of processing of ribosomal precursor RNA, as well as the synthesis of high-molecular-weight heterogeneous RNA in the early action of estrogen.