| Maternal Vg1 mRNA is localized to the vegetal cortex of Xenopus oocytes during stage III of oogenesis. This process is mediated by protein factors that bind to a 340-nt localization element (VLE) within the 3′-untranslated region of the message. During localization, Vg1 mRNA is not translated due to a repression element (VTE) that is positioned downstream of the VLE. The Xenopus protein VgRBP71, which has an ATP-ndependent RNA strand-separation activity, binds with high affinity to a site within the 3′-half of the VLE in close proximity to a AAUAAA polyadenylation element and stimulates cleavage of Vg1 mRNA immediately downstream of this signal in vitro. There is a strong correlation between the temporal expression of VgRBP71, cleavage at this polyadenylation signal, and appearance of Vg1 protein, supporting a model in which the cleavage event accompanying polyadenylation activates translation of Vg1 mRNA by removing the cis-acting repressor element.; The predicted secondary structure of the RNA segment preceding the cleavage site contains two imperfect hairpin structures, each with sequence and structural homology with an aptamer RNA, selected for binding m7GTP. Cleavage assays show that the m7Gppp cap structurestimulates VLE-RNA cleavage at the polyadenylation signal. A protein factor, Prrp, with domain organization similar to cleavage factors, is able, in the absence of any additional factors, to stimulate cleavage at the polyadenylation signal. RNA footprinting of the protein binding site suggests that the role of Prrp is to promote the formation of an RNA structure that can undergo autocatalytic cleavage. |
