Configuration of the alpha-fetoprotein regulatory domain during development

The transcription of the mouse alpha-fetoprotein gene in the fetal liver, gut, and yolk sac is under the control of at least four regulatory sequences, three 5' distal enhancers, and a proximal promoter region. In transgenic mice, the three enhancers exhibited distinct tissue preferences, with all three active in the visceral endoderm of the yolk sac, two in the fetal liver, and one in the fetal gut. To ask whether the enhancers are differentially utilized by the endogenous gene in the three tissues in vivo, we examined their differential sensitivity to the endonuclease DNase I. The experiments indicated that two of the three enhancers exhibited sensitivity to DNase I in all three fetal tissues, as well as in the adult liver, where the gene is transcriptionally repressed. The third enhancer, which exhibited the weakest activity in transgenic mice, was the least sensitive to DNase I in fetal liver and yolk sac and insensitive in the fetal gut. The major changes in the chromatin structure of the gene during postnatal development occurred within the proximal promoter region. The major DNase I cleavage site shifted from a position at 120 nucleotides upstream of the transcriptional start site to the start site itself. Two new sites, at 300 nucleotides upstream of the start site and 1.5 kb within the structural gene, were observed. These results suggest that the distal regulatory regions have the capability to retain biological activity throughout development, and that the primary repression of transcription of the gene proceeds through elements proximal to the promoter of the gene.