THE HUMAN BETA-GLOBIN LOCUS-CONTROL REGION CONFERS AN EARLY EMBRYONICERYTHROID-SPECIFIC EXPRESSION PATTERN TO A BASIC PROMOTER DRIVING THEBACTERIAL LACZ GENE

The beta-globin locus control region (LCR) is contained on a 20 kb DNA fragment and is characterized by the presence of five DNaseI hypersen sitive sites in erythroid cells, termed 5'HS1-5, A fully active 6.5 kb version of the LCR, called the mu LCR, has been described. Expression of the beta-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 activ ating function of the LCR. Transgenic mice experiments suggest that su btle changes in the transcription factor environment lead to the succe ssive silencing of the embryonic epsilon-globin and fetal gamma-globin promoters, resulting in the almost exclusive transcription of the bet a-globin gene in adult erythropoiesis. In this paper, we have asked th e question whether the LCR and its individual hypersensitive sites 5'H S1-4 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 beta-galactosidase (lacZ) gene. The r esults show that the mu LCR and 5'HS3 direct erythroid-specific, embry onic expression of this construct, while 5'HS1, 5'HS2 and 5'HS4 are in active at any stage of development, Expression of the mu LCR and 5'HS3 transgenes is repressed during fetal stages of development. The trans genes are in an inactive chromatin conformation and the lacZ gene is n ot transcribed, as shown by in situ hybridization. These data are comp atible with the hypothesis that the LCR requires the presence of an ac tive promoter to adopt an open chromatin conformation and with models proposing progressive heterochromatization during embryogenesis. The r esults suggest that the presence of a beta-globin gene is required for LCR function as conditions become more stringent during development.