Transcription of the mouse oocyte genome

Transcription of the oocyte genome has been measured during follicle growth in the mouse. Ovaries were sectioned on a cryostat and assayed for endogenous DNA-dependent RNA polymer-ase activity. The presence of the enzyme was detected by autoradiography as incorporation of ‘H-UMP into an acid insoluble product. Polymerases I and II were distinguished by their localization and sensitivity to 0-amanitin. The activities of these polymerases were relatively low in oocytes of the resting pool of small follicles. Radioactive incorporation increased as growth commenced, reaching a peak as the oocytes approached maximum size. Thereafter RNA polymerase activities declined rapidly and were very low in the oocytes of large, antral follicles. Similar changes were detected in the activity of a nucleoplasmic, a-amanitin-resistant enzyme, tentatively identified as form III polymerase. These data provide some insights into the way transcription is regulated during oocyte growth in a mammal. There appears to be a close relationship between the synthesis of the major RNA species and their storage in the cytoplasm