Xenopus Enhancer of Zeste (XEZ); an anteriorly restricted polycombgene with a role in neural patterning

We have identified the Xenopushomologue of Drosophila Enhancer of Zesteusing a differential display strategy designed to identify genes involved in early anterior neural differentiation. XEZcodes for a protein of 748 amino acids that is very highly conserved in evolution and is 96% identical to both human and mouse EZ(H)2. In common with most other Xenopus Pc-Ggenes and unlike mammalian Pc-Ggenes, XEZis anteriorly restricted. Zygotic expression of XEZcommences during gastrulation, much earlier than other anteriorly localized Pc-Ggenes; expression is restricted to the anterior neural plate and is confined later to the forebrain, eyes and branchial arches. XEZis induced in animal caps overexpressing noggin;up-regulation of XEZtherefore represents a response to inhibition of BMP signalling in ectodermal cells. We show that the midbrain/hindbrain junction marker En-2,and hindbrain marker Krox-20,are target genes of XEZand that XEZfunctions to repress these anteroposterior marker genes. Conversely, XEZdoes not repress the forebrain marker Otx-2. XEZoverexpression results in a greatly thickened floor of the forebrain. These results implicate an important role for XEZin the patterning of the nervous system.