THE SPEMANN ORGANIZER-EXPRESSED ZINC-FINGER GENE XEGR-1 RESPONDS TO THE MAP KINASE ETS-SRF SIGNAL-TRANSDUCTION PATHWAY/

The transcriptional activity of a set of genes, which are all expresse d in overlapping spatial and temporal patterns within the Spemann orga nizer of Xenopus embryos, can be modulated by peptide growth factors. We identify Xegr-1, a zinc finger protein-encoding gene, as a novel me mber of this group of genes. The spatial expression characteristics of Xegr-1 during gastrulation are most similar to those of Xbra, Making use of animal cap explants, analysis of the regulatory events that gov ern induction of Xegr-1 gene activity reveals that, in sharp contrast to transcriptional regulation of Xbra, activation of Ets-serum respons e factor (SRF) transcription factor complexes is required and sufficie nt for Xegr-1 gene expression. This finding provides the first indicat ion for Ets-SRF complexes bound to serum response elements to be activ ated during gastrulation, MAP kinase signalling cascades can induce an d sustain expression of both Xegr-1 and Xbra, Ectopic Xbra can induce Xegr-1 transcription by an indirect mechanism that appears to operate via primary activation of fibroblast growth factor secretion. These fi ndings define a cascade of events that links Xbra activity to the sign al-regulated control of Xegr-1 transcription in the context of early m esoderm induction in Xenopus laevis.