The Xenopus homologue of Bicaudal-C is a localized maternal mRNA that can induce endoderm formation

In Xenopus, zygotic transcription starts 6 hours after fertilization at the midblastula transition and therefore the first steps in embryonic developm ent are regulated by maternally inherited proteins and mRNAs. While animal- vegetal polarity is already present in the oocyte, the dorsoventral axis is only established upon fertilization by the entry of the sperm and the subs equent rotation of the egg cortex. In a screen for maternal mRNAs whose sta bility is regulated by this cortical rotation, we isolated the Xenopus homo logue of the Drosophila gene Bicaudal-C (xBic-C), It encodes a putative RNA -binding molecule expressed maternally and localized predominantly to the v egetal half of the egg, Upon fertilization and cortical rotation, xBic-C mR NA is displaced together with the heavy yolk towards the future dorsal side of the embryo. In UV-ventralized embryos, xBic-C is polyadenylated less th an in untreated embryos that undergo cortical rotation, Overexpression of x Bic-C by injection of synthetic mRNA in whole embryos or in ectodermal expl ants leads to ectopic endoderm formation. This endoderm-inducing activity i s dependent on the presence of the RNA-binding domain of the protein, In co ntrast to the two other known maternally encoded endoderm inducers, Vg1 and VegT, xBic-C ectopic expression leads specifically to endoderm formation i n the absence of mesoderm induction.