Developmental Silencing of the Embryonic ζ-Globin Gene: Concerted Action of the Promoter and the 3′-Flanking Region Combined with Stage-Specific Silencing by the Transcribed Segment

Abstract

Globin gene switching is a well-described model of eucaryotic developmental control. In the case of the human α-globin gene cluster, migration of erythropoietic activity from the embryonic yolk sac to the fetal liver is paralleled by ζ-globin gene silencing and enhanced expression of the α-globin genes. To map critical cis determinants of this switch, the human ζ-globin gene, the α-globin gene, and chimeric recombinants were introduced into the mouse genome. Consistent with previous studies, expression of the individual α- and ζ-globin transgenes was found to be developmentally appropriate. Contrary to current models, however, the α-and ζ-globin gene promoters were not sufficient to establish this control. Instead, full silencing of the ζ-globin gene required the combined activities of its promoter, transcribed region, and 3′-flanking sequences. Individually, the silencing activities of the ζ-globin gene promoter and 3′-flanking region were minimal but increased markedly when both regions were present. The ζ-globin transcribed region appeared to contribute to gene silencing by a mechanism specifically activated in definitive erythroblasts in the fetal liver. These data demonstrate that a complex set of controls, requiring at least three determinants and involving at least two independent mechanisms, is necessary for full developmental silencing of the human ζ-globin gene.