XENOPUS CHORDIN - A NOVEL DORSALIZING FACTOR-ACTIVATED BY ORGANIZER-SPECIFIC HOMEOBOX GENES

A Xenopus gene whose expression can be activated by the organizer-spec ific homeobox genes goosecoid and Xnot2 was isolated by differential s creening. The chordin gene encodes a novel protein of 941 amino acids that has a signal sequence and four Cys-rich domains. The expression o f chordin starts in Spemann's organizer subsequent to that of goosecoi d, and its induction by activin requires de novo protein synthesis. Mi croinjection of chordin mRNA induces twinned axes and can completely r escue axial development in ventralized embryos. This molecule is a pot ent dorsalizing factor that is expressed at the right time and in the right place to regulate cell-cell interactions in the organizing cente rs of head, trunk, and tail development.