Fig. 5. CPEB is required for cytoplasmic polyadenylation. GV stage oocytes were injected with a mixture of radiolabeled cyclin B1 3' UTR and mRNA encoding either wild-type CPEB or a mutant CPEB (N) that lacks the N-terminal 146 amino acids. Both proteins contain the two LDSR motifs, as well as the RNA recognition motifs (RRM) and zinc finger (ZF) that are necessary for RNA binding (Hake et al., 1998). The RNA was then extracted from maturing oocytes and the polyadenylation of the cyclin B1 3' UTR was analyzed by denaturing PAGE and phosphorimaging.