THE MOS PATHWAY REGULATES CYTOPLASMIC POLYADENYLATION IN XENOPUS OOCYTES

Cytoplasmic polyadenylation controls the translation of several matern al mRNAs during Xenopus oocyte maturation and requires two sequences i n the 3' untranslated region (UTR), the U-rich cytoplasmic polyadenyla tion element (CPE), and the hexanucleotide AAUAAA. c-mos mRNA is polya denylated and translated soon after the induction of maturation, and t his protein kinase is necessary for a kinase cascade culminating in cd c2 kinase (MPF) activation. Other mRNAs are polyadenylated later aroun d the time of cdc2 kinase activation. To determine whether there is a hierarchy in the cytoplasmic polyadenylation of maternal mRNAs, we abl ated c-mos mRNA with an antisense oligonucleotide. This prevented hist one B4 and cyclin A1 and B1 mRNA polyadenylation, indicating that the polyadenylation of these mRNAs is Mos dependent. To investigate a poss ible role of cdc2 kinase in this process, cyclin B was injected into o ocytes lacking c-mos mRNA. cdc2 kinase was activated, but mitogen-acti vated protein kinase was not. However, polyadenylation of cyclin B1 an d histone B4 mRNA was still observed. This demonstrates that cdc2 kina se can induce cytoplasmic polyadenylation in the absence of Mos. Our d ata further indicate that although phosphorylation of the CPE binding protein may be involved in the induction of Mos-dependent polyadenylat ion, it is not required for Mos-independent polyadenylation. We charac terized the elements conferring Mos dependence (Mos response elements) in the histone B4 and cyclin B1 mRNAs by mutational analysis. For his tone B4 mRNA, the Mos response elements were in the coding region or 5 ' UTR. For cyclin B1 mRNA, the main Mos response element was a CPE tha t overlaps with the AAUAAA hexanucleotide. This indicates that the pos ition of the CPE can have a profound influence on the timing of cytopl asmic polyadenylation.