Translational control of cyclin B1 mRNA during meiotic maturation: Coordinated repression and cytoplasmic polyadenylation

Translational control is prominent during meiotic maturation and early deve lopment. In this report, we investigate a mode of translational repression in Xenopus laevis oocytes, focusing on the mRNA encoding cyclin B1. Transla tion of cyclin B1 mRNA is relatively inactive in the oocyte and increases d ramatically during meiotic maturation. We show, by injection of synthetic m RNAs, that the cis-acting sequences responsible for repression of cyclin B1 mRNA reside within its 3'UTR. Repression can be saturated by increasing th e concentration of reporter mRNA injected, suggesting that the cyclin B1 3' UTR sequences provide a binding site for a trans-acting repressor. The sequ ences that direct repression overlap and include cytoplasmic polyadenylatio n elements (CPEs), sequences known to promote cytoplasmic polyadenylation. However, the presence of a CPE per se appears insufficient to cause repress ion, as other mRNAs that contain CPEs are not translationally repressed. We demonstrate that relief of repression and cytoplasmic polyadenylation are intimately linked. Repressing elements do not override the stimulatory effe ct of a long poly(A) tail, and polyadenylation of cyclin B1 mRNA is require d for its translational recruitment. Our results suggest that translational recruitment of endogenous cyclin B1 mRNA is a collaborative effect of dere pression and poly(A) addition. We discuss several molecular mechanisms that might underlie this collaboration. (C) 2000 Academic Press.