The human β-globin locus control region confers an early embryonic erythroid-specific expression pattern to a basic promoter driving the bacterial lacZ gene

The β-globin locus control region (LCR) is contained on a 20 kb DNA fragment and is characterized by the presence of five DNaseI hypersensitive sites in erythroid cells, termed 5′HS1-5. A fully active 6.5 kb version of the LCR, called the μLCR, has been described. Expression of the β-like globin genes is absolutely dependent on the presence of the LCR. The developmental expression pattern of the genes in the cluster is achieved through competition of the promoters for the activating function of the LCR. Transgenic mice experiments suggest that subtle changes in the transcription factor environment lead to the successive silencing of the embryonic ε-globin and fetal γ-globin promoters, resulting in the almost exclusive transcription of the β-globin gene in adult erythropoiesis. In this paper, we have asked the question whether the LCR and its individual hypersensitive sites 5′HS1-4 can activate a basic promoter in the absence of any other globin sequences. We have employed a minimal promoter derived from the mouse Hsp68 gene driving the bacterial β-galactosidase (lacZ) gene. The results show that the μLCR and 5′HS3 direct erythroid-specific, embryonic expression of this construct, while 5′HS1, 5′HS2 and 5′HS4 are inactive at any stage of development. Expression of the μLCR and 5′HS3 transgenes is repressed during fetal stages of development. The transgenes are in an inactive chromatin conformation and the lacZ gene is not transcribed, as shown by in situ hybridization. These data are compatible with the hypothesis that the LCR requires the presence of an active promoter to adopt an open chromatin conformation and with models proposing progressive heterochromatization during embryogenesis. The results suggest that the presence of a β-globin gene is required for LCR function as conditions become more stringent during development.