XENOPUS VEGT RNA IS LOCALIZED TO THE VEGETAL CORTEX DURING OOGENESIS AND ENCODES A NOVEL T-BOX TRANSCRIPTION FACTOR INVOLVED IN MESODERMAL PATTERNING

An RNA localized to the vegetal cortex of Xenopus oocytes encodes a no vel T-box protein (VegT) capable of inducing either dorsal or posterio r ventral mesoderm at different times in development, VegT is a nuclea r protein and its C-terminal domain can activate transcription in a ye ast reporter assay, observations consistent with VegT functioning as a transcription factor, Zygotic expression is dynamic along the dorsove ntral axis, with transcripts first expressed in the dorsal marginal zo ne, By the end of gastrulation, VegT is expressed exclusively in poste rior ventral and lateral mesoderm and is excluded from the notochord, Later expression is confined to a subset of Rohon-Beard cells, a type of primary sensory neuron, In animal cap assays, VegT is capable of co nverting prospective ectoderm into ventral lateral mesoderm. Such ecto pic expression of VegT induces its own expression as well as that of X wnt-8 in caps, suggesting that a Wnt pathway may be involved, Mis-expr ession of VegT in dorsal animal blastomeres fated to contribute to bra in suppresses head formation, Our results suggest that VegT is a local ized transcription factor, which operates sequentially in several deve lopmental pathways during embryogenesis, including dorsoventral and po sterior patterning of mesoderm.