Figure 2.

 Mesoderm-specific expression of reporter gene constructs containing 2.1 kb of Xbra2 5′-flanking sequence. (A) Xbra2.pGL2 or the promoterless plasmid pGL2Basic was coinjected with pSV.β-galactosidase into a single blastomere of either tier A (prospective ectoderm) or tier C (prospective mesoderm) of 32-cell stage embryos (see inset). Embryos were collected for luciferase and β-galactosidase activity assays at stage 12.5. (B) Luciferase activity in extracts of embryos at stage 8, 11, or 14 injected with Xbra2.pGL2 into a single blastomere of either tier A or tier C at the 32-cell stage. (Inset) RNase protection assay for expression of Xbra2.βg in embryos injected into a single blastomere of tier A or tier C and collected for assay at stage 8 or stage 13. EF-1α was used as a loading control. This experiment has been performed four times, with similar results each time. (C,D) Xbra2.GFP was injected equatorially into all four blastomeres of four-cell stage embryos, and distribution of GFP transcripts at stage 10.5 was determined by whole-mount in situ hybridization. Arrows point to the blastopore lip. (E,F) Xbra2.GFP was used to generate transgenic embryos that were analyzed as in C and D at gastrula (E) or tailbud (F; anterior to the left) stage.