ID GENE ACTIVITY DURING XENOPUS EMBRYOGENESIS

The activity of bHLH transcription factors that are involved in cell d etermination and differentiation is inhibited by Ids, HLH proteins lac king the basic amino acid sequence element. In order to determine the role of Id during development, we have isolated and characterized the Id genes expressed in Xenopus embryos. Three cDNAs were characterized: XIdIa and XIdIb, which are transcribed from one gene but differential ly spliced in the 3' untranslated part, and XIdII which is transcribed from a second copy of the gene. One of the two forms of the different ially spliced mRNAs exhibits, 30 nucleotides upstream from the AATAAA site, a sequence box homologous to the cytoplasmic polyadenylation ele ment (CPE) which is present also in Id2 and Id3 mRNAs from higher vert ebrates. This raises the question of whether this CPE-like element may link Id mRNA polyadenylation and translation to the cell cycle metabo lism. The Xenopus Id gene is transcribed at low level in oocytes and a t high level in embryos, after midblastula transition, in a large numb er of tissues, including the notochord, neural tube, eye, ear, neural crest cells, presomitic mesoderm, myotomes, tailbud and dorsal fin. In myotomes, expression is high in the areas of proliferating myoblasts and decreases as terminal differentiation proceeds, consistent with a function in cell determination and differentiation and possibly also i n cell cycle regulation.