Two modes of action by which Xenopus hairy2b establishes tissue demarcation in the Spemann-Mangold organizer.

The Hairy and Enhancer-of-Split (HES) family of transcriptional repressors plays important roles in pattern formation during development throughout the animal kingdom. Generally, HES proteins repress the expression of genes specific for neighboring tissues to maintain the nature of cells expressing HES proteins, resulting in pattern formation. Xhairy2b, a Xenopus HES, establishes the prospective anterior prechordal mesoderm identity in the Spemann-Mangold organizer by both inducing specific genes and repressing the genes specific for neighboring tissues. Here we report that Xhairy2b has two modes of action, each of which corresponds to inductive and repressive functions. We show that the inductive function is independent of direct transcriptional regulation and is exhibited by the C-terminal WRPW tetrapeptide motif alone, although it induces the expression of a wide variety of the organizer genes that Xhairy2b represses. The transcriptional repression by Xhairy2b is responsible for only the repressive function. We propose that the activity of the WRPW motif intrinsically induces the expression of genes specific for the organizer in a rather non-specific manner to ensure the organizer environment. Then, the transcriptional repression selectively down-regulates the expression of some of these genes, resulting in the regionalization of the axial mesoderm. Our study provides new insight into how a region of the vertebrate embryo is demarcated by one dual-functional transcription factor in the early stages of development.