The establishment of dorsoventral polarity in the Drosophila oocyte and future embryo is dependent on the efficient transport of K10 mRNA from nurse cells into the oocyte. To investigate the cis-requirements of K10 mRNA transport, we used a transgenic fly assay to analyze the expression patterns of a series of K10 deletion variants. Such studies identify a 44 nucleotide sequence within the K10 3′ untranslated region that is required and sufficient for K10 mRNA transport and subsequent localization to the oocyte's anterior cortex. An inspection of the 44 nucleotide transport/localization sequence (TLS) reveals a strong potential for the formation of a stem-loop secondary structure. Nucleotide substitutions that interfere with the predicted base-pairing of the TLS block mRNA transport and anterior localization. Conversely, mutations that alter the base composition of the TLS while maintaining predicted base-pairing do not block mRNA transport or anterior localization. We conclude that K10 mRNA transport and anterior localization is mediated by a 44 nucleotide stem-loop structure. A similar putative stem-loop structure is found in the 3′ untranslated region of the Drosophila orb mRNA, suggesting that the same factors mediate the transport and anterior localization of both K10 and orb mRNAs. Apart from orb, the K10 TLS is not found in any other localized mRNA, raising the possibility that the transport and localization of other mRNAs, e.g., bicoid, oskar and gurken, are mediated by novel sets of cis- and trans-acting factors. Moreover, we find that the K10 TLS overrides the activity of oskar cis-regulatory elements that mediate the late stage movement of the mRNA to the posterior pole. We propose the existence of a family of cis-regulatory elements that mediate mRNA transport into the oocyte, only some of which are compatible with the elements that mediate late stage movements.

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