Region 2b directs apical localization of transcripts in blastoderm embryos. (A–H) RNA transcripts were injected into blastoderm embyos and fixed 8–10 min later (i.e., 8 min after injection of the last embryo). Images are oriented with apical to the top and basal to the bottom. Bar, 30 μm. The arrows in A and B indicate the apical cytoplasm of the blastoderm embryo. Schematic representations of the reporter RNAs are shown above the images. Thin black line: UTR RNAs, thick black line: ORF, dashed line: deletions, stem–loop: oocyte entry signal (OES) region, green rectangle: spliced oskar localization element (SOLE). (A′–H′) Categorization of RNA enrichment in the apical cytoplasm. Percentage of embryos showing different efficiencies of transport is shown: (++) strong, (+) weak, (+/−) very weak, (−) no enrichment. (n) Number of embryos scored for each RNA. The following reporter mRNAs were injected: fs(1)K10 RNA, containing the entire 3′ UTR and flanking 3′ sequences (A–A′); a mutated version of the fs(1)K10 RNA, containing a randomized TLS sequence (Bullock et al. 2010) (B–B′); full-length oskar mRNA (C–C′); region 2 + 3 of oskar 3′ UTR (D–D′); region 2 + 3 of oskar 3′ UTR bearing a deletion of the region 2b stem (E–E′); region 2 + 3 of oskar 3′ UTR with a deletion of the distal stem 2b (F–F′); region 2 + 3 of oskar 3′ UTR with a mutated 3′ portion of the terminal stem (G–G′); region 3 of oskar 3′ UTR (H–H′).
