An early developmental transcription factor complex that is more stable onnucleosome core particles than on free DNA

In vivo footprinting studies have shown that transcription factor binding s ites for HNF3 and GATA-4 are occupied on the albumin gene enhancer in embry onic endoderm, prior to the developmental activation of liver gene transcri ption. We have investigated how these factors can stably occupy silent chro matin. Remarkably, we find that HNF3, but not GATA-4 or a GAL4 control prot ein, binds far more stably to nucleosome core particles than to free DNA. I n the presence of HNF3, GATA-4 binds stably to an HNF3-positioned nucleosom e. Histone acetylation does not affect HNF3 binding. This is evidence for s table nucleosome binding by a transcription factor and shows that a winged helix protein is sufficient to initiate the assembly of an enhancer complex on nonacetylated nucleosomes.