EDEN-BP is a Xenopus RNA-binding protein that triggers deadenylation [poly(A) tail shortening], and thereby translational repression and degradation, of a subset of maternal mRNAs soon after fertilization. We show here that this factor is expressed in the presomitic mesoderm of older embryos, the site where somitic segmentation takes place. Inhibiting EDEN-BP function using either antisense morpholino oligonucleotides or neutralizing antibodies leads to severe defects in somitic segmentation, but not myotomal differentiation. This is associated with defects in the expression of segmentation markers belonging to the Notch signalling pathway in the presomitic mesoderm. We show by a combination of approaches that the mRNA encoding XSu(H), a protein that plays a central role in Notch signalling, is regulated by the EDEN-BP pathway. Accordingly, XSu(H) is overexpressed in EDEN-BP knockdown embryos, and overexpressing XSu(H) causes segmentation defects. We finally give data indicating that, in addition to XSu(H), other segmentation RNAs are a target for EDEN-BP. These results show that EDEN-BP-dependent post-transcriptional regulation of gene expression is required for the process of somitic segmentation.

During Xenopus early development, the length of the poly(A) tail of maternal mRNAs is a key element of translational control. Several sequence elements (cytoplasmic polyadenylation elements) localized in 3' untranslated regions have been shown to be responsible for the cytoplasmic polyadenylation of certain maternal mRNAs. Here, we demonstrate that the mRNA encoding the catalytic subunit of phosphatase 2A is polyadenylated after fertilization of Xenopus eggs. This polyadenylation is mediated by the additive effects of two cis elements, one being similar to already described cytoplasmic polyadenylation elements and the other consisting of a polycytosine motif. Finally, a candidate specificity factor for polycytosine-mediated cytoplasmic polyadenylation has been purified and identified as the Xenopus homologue of human alpha-CP2.

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