Cloning and Characterization of Three Xenopus Slug Promoters Reveal Direct Regulation by Lef/β-Catenin Signaling

In amphibians and birds, one of the first steps of neu- ral crest cell (NCC) determination is expression of the transcription factor Slug. This marker has been used to demonstrate that BMP and Wnt molecules play a major role in NCC induction. However, it is unknown whether Slug expression is directly or indirectly regulated by these signals. We report here the cloning and character- ization of three Xenopus Slug promoters: that of the Xenopus tropicalis slug gene and those of two Xenopus laevis Slug pseudoalleles. Although the three genes en- code proteins with almost identical amino acid se- quences and are expressed with similar spatiotemporal patterns, their 5 -flanking regions are quite different. A striking difference is a deletion in the X. tropicalis gene located precisely at the transcription initiation site that results in the X. tropicalis promoter being inefficient in X. laevis. Additionally, we identified two regions com- mon to the three promoters that are necessary and suf- ficient to drive specific expression in NCCs. Interest- ingly, one of the common regulatory regions presents a functional Lef/ -catenin-binding site necessary for spe- cific expression. As the Lef -catenin complex is a down- stream effector of Wnt signaling, these results suggest that Xenopus Slug is a direct target of NCC determina- tion signals.