Multiple cis-acting signals, some weak by necessity, collectively direct robust transport of oskar mRNA to the oocyte

Abstract

Localization of mRNAs can involve multiple steps, each with its own cisacting localization signals and transport factors. How is the transition between different steps orchestrated? We show that the initial step in localization of Drosophila oskar mRNA − transport from nurse cells to the oocyte − relies on multiple cis-acting signals. Some of these are binding sites for the translational control factor Bruno, suggesting that Bruno plays an additional role in mRNA transport. Although transport of oskar mRNA is essential and robust, the localization activity of individual transport signals is weak. Notably, increasing the strength of individual transport signals, or adding a strong transport signal, disrupts the later stages of oskar mRNA localization. We propose that the oskar transport signals are weak by necessity; their weakness facilitates transfer of the oskar mRNA from the oocyte transport machinery to the machinery for posterior localization.

A putative stem-loop structure in Drosophila crumbs is required for mRNA localisation in epithelia and germline cells

“…Bruno recognises sequences in the 3'UTR of the oskar (osk) mRNA and prevents its translation before its arrival at the posterior pole of the oocyte (Kim-Ha, 1995). In addition, Bruno is required for the transport of osk mRNA, mediated by several weak signals in the 3'UTR (Ryu et al, 2017). In contrast, Me31B is required for translational repression only, and loss of Me31B causes the premature translation of osk and Bicaudal-D (BicD) mRNAs, but does not affect their transport (Nakamura, 2001).…”

Patterning the Drosophila embryo: A paradigm for RNA‐based developmental genetic regulation

“…A segment of the osk 3′ UTR containing the OES localizes in this assay, but localization is strongly impaired when the OES is absent (Jambor et al, 2014). However, the OES is only active in the context of larger regions of the 3′ UTR, and other partially redundant elements operate in concert with it, leading to the proposal that the presence of multiple weak 3′ UTR signals driving oocyte entry facilitates transfer of osk mRNA to the posterior localization pathway (Ryu, Kenny, Gim, Snee, & Macdonald, 2017).…”

Opposing roles for Egalitarian and Staufen in transport, anchoring and localization of oskar mRNA in the Drosophila oocyte

“…Transport of osk mRNA into the oocyte is mediated by multiple regulatory elements in its 3’ UTR. Those embedded within two stem-loop (SL) structures, SL2a and SL2b, are most critical and deletion of either SL greatly reduces transport [ 8 – 11 ]. The SL2a signal has been mapped at high resolution and corresponds to the most highly conserved portion of the stem structure [ 11 ].…”

“…Those embedded within two stem-loop (SL) structures, SL2a and SL2b, are most critical and deletion of either SL greatly reduces transport [ 8 – 11 ]. The SL2a signal has been mapped at high resolution and corresponds to the most highly conserved portion of the stem structure [ 11 ]. Neither SL2a nor SL2b has strong transport activity on its own; if either is added to a reporter mRNA, there is very limited transport to the oocyte [ 7 , 10 , 11 ].…”

“…The SL2a signal has been mapped at high resolution and corresponds to the most highly conserved portion of the stem structure [ 11 ]. Neither SL2a nor SL2b has strong transport activity on its own; if either is added to a reporter mRNA, there is very limited transport to the oocyte [ 7 , 10 , 11 ]. By contrast, another stem-loop signal—the TLS—that mediates transport of the fs(1)K10 mRNA from nurse cells to the oocyte is highly active in isolation: addition of the 44 nucleotide TLS to foreign mRNAs consistently confers highly efficient transport [ 12 ].…”