PROGRAMMING OF A REPRESSED BUT COMMITTED CHROMATIN STRUCTURE DURING EARLY DEVELOPMENT

The determination of chromatin for transcription during early developm ent as well as the requirement for trans-acting factors during this pe riod has been analysed in Xenopus. Basal transcription is repressed bo th during oogenesis and after the mid-blastula transition (MBT), and t ransactivators are required to relieve this repression. In contrast, t ransactivators cannot overcome the generalized transcriptional repress ion which occurs in embryos before MBT. However, they do bind to promo ters leading to a repressed but preset chromatin structure. Experiment s involving the pre-binding of TATA binding protein (TBP) or of the st rong transactivator GAL4-VP16 further show that there is no limiting f actor before the MET, and that it is the recruitment and stabilization of the basal transcription machinery and not of transactivators which is repressed during early development. This multi-step process in gen e activation, with activation of promoters temporally uncoupled from t heir commitment, may be of importance in the regulation of early embry onic events by providing molecular signposts for future determinations .