Chromatin and transcriptional activity in early Xenopus development.

Experiments with Xenopus oocytes and embryos have determined a direct biochemical relationship between chromatin structure and transcription. Nucleosomes within specific nucleoprotein architectures can either activate or repress transcription. Developmentally regulated changes in chromosomal composition direct the dominant repression of specific genes. Reconstruction of chromatin templates in vivo establishes that replication-coupled chromatin assembly both represses basal transcription and facilitates a full range of inducible gene activity. Chromatin structure emerges as a major contributory factor to the regulation of genes

Determinants of vitellogenin B1 promoter architecture. HNF3 and estrogen responsive transcription within chromatin.

The liver-specific vitellogenin B1 promoter is efficiently activated by estrogen within a nucleosomal environment after microinjection into Xenopus laevis oocytes, consistent with the hypothesis that significant nucleosome remodeling over this promoter is not a prerequisite for the activation by the estrogen receptor (ERalpha). This observation lead us to investigate determinants other than ERalpha of chromatin structure and transcriptional activation of the vitellogenin B1 promoter in this system and in vitro. We find that the liver-enriched transcription factor HNF3 has an important organizational role for chromatin structure as demonstrated by DNase I-hypersensitive site mapping. Both HNF3 and the estrogen receptor activate transcription synergistically and are able to interact with chromatin reconstituted in vitro with three positioned nucleosomes. We propose that HNF3 is the cellular determinant which establishes a promoter environment favorable to a rapid transcriptional activation by the estrogen receptor