The strict temporal control of maternal mRNA translational activation is critical for vertebrate oocyte maturation and meiotic cell cycle progression. However, the molecular mechanisms regulating the temporal order of mRNA translation have not been established. The frog, Xenopus laevis, has proven to be a useful model system to study the process of mRNA translation. In the frog, the translational induction of the mRNA encoding the Mos proto-oncogene has been shown to occur soon after exposure to the maturation stimulus, progesterone. This early translation of the Mos mRNA is essential for subsequent progression of progesterone-stimulated maturation. We have recently identified an element within the 3' UTR of the Mos mRNA, distinct from the previously described cytoplasmic polyadenylation element (CPE), which mediates this early translational induction. This polyadenylation response element (PRE) is responsive to mitogen-activated protein kinase (MAP kinase) signaling and differs from CPE sequences which direct late mRNA translation in response to maturation promoting factor (MPF) signaling. Our recent data suggest that functional PRE sequences may be present in the 3' UTRs of other Xenopus maternal mRNAs. In this study we will test the hypothesis that PRE-mediated mRNA translational control is the critical regulatory step required for the progesterone-triggered all-or-none cell fate transition from an immature to a mature oocyte. We will characterize the molecular basis for PRE-mediated translational activation and elucidate the combinatorial effects of the PRE and CPE regulatory processes on the temporal control of maternal mRNA translation. Lastly, we will characterize the signal transduction pathways which mediate induction of PRE-dependent mRNA translation in response to the initial, Mos-independent, progesterone stimulus. These studies will further our understanding of the coordinated action of distinct signaling pathways in the temporal control of maternal mRNA translation. ? ?
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