Function of basonuclin in increasing transcription of the ribosomal RNA genes during mouse oogenesis

Active protein synthesis during early oogenesis requires accelerated transc ription of ribosomal RNA genes (rDNAs), In response to this demand, rDNAs a re amplified more than 1000-fold early in Xenopus oogenesis, Here, we repor t evidence that rDNA is not amplified in mouse oocytes, but these cells may instead employ the zinc-finger protein basonuclin, a putative rDNA transcr iption factor, to enhance rRNA synthesis. This conclusion is based on obser vations that basonuclin is localized in the nucleolus in the mouse oocyte e arly in its growth phase, when rRNA transcription is highly active; and tha t the binding sites of basonuclin zinc fingers on the human and mouse rDNA promoters are homologous. In a co-transfection assay, basonuclin can elevat e transcription from an rDNA promoter, and its zinc-finger domain can inhib it RNA polymerase I transcription, as detected by a run-on assay, in growin g mouse oocytes.