CHARACTERIZATION AND DEVELOPMENTAL EXPRESSION OF XENOPUS C/EBP GENE

The Xenopus homolog of the transcription factor C/EBP (CCAAT/enhancer core binding protein), cloned from an adult Xenopus liver cDNA library, encodes a protein whose sequence is 67% homologous to that of rat C/EBP at the amino acid level, with virtually identical sequence of the basic-zipper region at the carboxyl terminus. As determined by gel electrophoretic mobility shift assays, the protein synthesized from xC/EBP cDNA bound specifically to the consensus binding site for C/EBP-like proteins. Northern blotting and RNase protection revealed a single species of xC/EBP mRNA of 2.7 kb which was most abundant in adult Xenopus liver, with smaller amounts in spleen, kidney, oviduct and brain and undetectable in heart and skeletal muscle. Although a small amount of this transcript could be detected in unfertilized eggs and early embryos, its accumulation rose sharply at the onset of metamorphosis (stage 55156), and continued to increase through metamorphic climax to reach its highest level in stage 66 froglet liver, but thereafter declining in adult liver. In situ hybridization revealed a uniform pattern of distribution of xC/EBP mRNA in the liver and fat body throughout metamorphosis. Towards the end of metamorphosis, high levels of xC/EBP mRNA were detected in epithelial cells of the digestive tract. However, the spatial pattern of cells expressing the transcript changed markedly in the developing kidney. Our results suggest that xC/EBP may be involved as a transcription factor in the establishment of the adult phenotype during post-embryonic development of Xenopus.