Cloning and characterization of three Xenopus Slug promoters reveal directregulation by Lef/beta-catenin signaling

In amphibians and birds, one of the first steps of neural crest cell (NCC) determination is expression of the transcription factor Slug. This marker h as been used to demonstrate that BMP and Writ molecules play a major role i n NCC induction. However, it is unknown whether Slug expression is directly or indirectly regulated by these signals. We report here the cloning and c haracterization of three Xenopus Slug promoters: that of the Xenopus tropic alis slug gene and those of two Xenopus laevis Slug pseudoalleles. Although the three genes encode proteins with almost identical amino acid sequences 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. Addit ionally, we identified two regions common to the three promoters that are n ecessary and sufficient to drive specific expression in NCCs. Interestingly , one of the common regulatory regions presents a functional Lef/beta -cate nin-binding site necessary for specific expression. As the Lef.beta -cateni n complex is a downstream effector of Wnt signaling, these results suggest that Xenopus Slug is a direct, target of NGC determination signals.