IDENTIFICATION OF A POTENTIAL REGULATOR OF EARLY TRANSCRIPTIONAL RESPONSES TO MESODERM INDUCERS IN THE FROG EMBRYO

The activin/transforming growth factor-beta (TGF-beta) family of pepti de growth factors plays a central role in the induction of mesoderm du ring early Xenopus embryogenesis. Immediate transcriptional responses to mesoderm-inducing signals have been described, but the signal trans duction steps leading to these early responses are unknown. We describ e here the first pretranscriptional response to activin/TGF-beta mesod erm inducers in the early embryo. We have identified a cellular factor which binds to a 50 bp portion of the promoter for an activin/TGF-bet a early response gene. This factor is activated within 4 min of treatm ent of embryonic prospective ectoderm with mesoderm-inducing factors, making it the earliest response to these factors described in early em bryos, This factor can be activated throughout early cleavage and blas tula stages, is activated by mesoderm inducers of the activin/TGF-beta superfamily but not the fibroblast growth factor (FGF) family, and do es not appear to require an endogenous FGF signal for activation. Char acterization of this factor provides a powerful tool for studying the early steps in the induction of mesoderm by members of the activin/TGF -beta superfamily.