The CDK9/Cyclin T1 subunits of P-TEFb in mouse oocytes and preimplantation embryos: A possible role in embryonic genome activation

Here, we show that the CDK9 and cyclin T1 subunits of P-TEFb are present in mouse oocytes and preimplantation embryos. Both proteins translocate to pronuclei at the late one-cell stage and are predominantly localized in nuclei at the two-cell stage. We additionally examine the effects of the CDK9-specific inhibitor, flavopiridol, on mouse preimplantation development. Our data show that treatment with the drug results in mislocalization of CDK9, cyclin T1, and phosphorylated Pol II, as well as developmental arrest at the two-cell stage.A change in CDK9 localization from the cytoplasm to the pronucleus occurs at the time of minor embryonic genome activation, and CDK9 accumulation at the two-cell stage is evident, concomitant with major transcriptional activation of the embryonic genome. Moreover, CDK9 inhibition triggers a developmental block at the two-cell stage. Our findings clearly indicate that CDK9 is essential for embryonic genome activation in the mouse.The maternal-zygotic transition is a critical event in early mouse embryogenesis. This transition transforms the highly differentiated oocyte into a totipotent blastomere, and is complete by the two-cell stage. During this transition, maternal mRNAs are degraded and the embryonic genome is activated [1]. Genome activation results in the replacement of transcripts common to both the oocyte and the embryo and the generation of new transcripts necessary for further development. Development of mouse embryos unable to accomplish genome activation is terminated at the two-cell stage.In the mouse, two transcriptional stages have been identified: a minor transcriptional wave at the one-cell stage, and a second major wave at the two-cell stage [2]. These findings are supported by the results of experiments showing that the one-cell stage features significant RNA polymerase II (Pol II)-dependent incorporation of bromouridine triphosphate (BrUTP) into RNA, and RNA synthesis is accompanied by an obvious increase in BrUTP i