Two copies of a subelement from the Vg1 RNA localization sequence are sufficient to direct vegetal localization in Xenopus oocytes

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Abstract
Localization of mRNA has emerged as a fundamental mechanism for generating polarity during development. In vertebrates, one example of this phenomenon is Vg1 RNA, which is localized to the vegetal cortex of Xenopus oocytes. Vegetal localization of Vg1 RNA is directed by a 340-nt sequence element contained within its 3′ untranslated region. To investigate how such cis-acting elements function in the localization process, we have undertaken a detailed analysis of the precise sequence requirements for vegetal localization within the 340-nt localization element. We present evidence for considerable redundancy within the localization element and demonstrate that critical sequences lie at the ends of the element. Importantly, we show that a subelement from the 5′ end of the Vg1 localization element is, when duplicated, sufficient to direct vegetal localization. We suggest that the Vg1 localization element is composed of smaller, redundant sequence motifs and identify one such 6-nt motif as essential for localization. These results allow insight into what constitutes an RNA localization signal and how RNA sequence elements may act in the localization process.