The process of mRNA localization, often used for regulation of gene expression in polarized cells, requires recognition of cis-acting signals by components of the localization machinery. Many known RNA signals are active in the contexts of both the Drosophila ovary and the blastoderm embryo, suggesting a conserved recognition mechanism. We used variants of the bicoid mRNA localization signal to explore recognition requirements in the embryo. We found that bicoid stem-loop IV/V, which is sufficient for ovarian localization, was necessary but not sufficient for full embryonic localization. RNAs containing bicoid stem-loops III/IV/V did localize within the embryo, demonstrating a requirement for dimerization and other activities supplied by stem-loop III. Protein complexes that bound specifically to III/IV/V and fushitarazu localization signals copurified through multiple fractionation steps, suggesting that they are related. Binding to these two signals was competitive but not equivalent. Thus, the binding complexes are not identical but appear to have some components in common. We have proposed a model for a conserved mechanism of localization signal recognition in multiple contexts.
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