Functional comparison of Irx ultraconserved non-coding regions (UCRs). (A) Schematic representation of the vertebrate Irx clusters showing the position of the UCRs (purple boxes) between the different Irx genes. (B–E) Lateral views of late neurula Xenopus embryos showing enhanced green fluorescent protein (EGFP) mRNA localization promoted by the different Xenopus UCRs. Insets show EGFP fluorescence in stage 45 embryos transgenic for the Xenopus UCRs. Note similar expression in the brain. (mb) Midbrain; (ey) eye; (p) pronephros. (F, G) Sequence alignment of the four Xenopus UCRs (F) or Xenopus, zebrafish, and mouse UCRB1s (G). Asterisks mark identical bases. (H, I) Lateral views of stage 35 Xenopus embryos showing EGFP expression directed by zebrafish (H) or mouse (I) UCRB1s. Inset in H is a higher magnification of a living embryo transgenic for zUCRB1 showing EGFP fluorescence in the eye (arrowhead) and in the pronephros (arrow). (J) Xenopus embryo transgenic for a construct in which EGFP is directed by a 3-kb Irx3b promoter that harbors a duplicated UCRB1b. (K–O) EGFP fluorescence in Xenopus promoted by the core domain conserved between different vertebrate Irx UCRs.
