Erythroid Kruppel-like factor (EKLF) is active in primitive and definitive erythroid cells and is required for the function of 5′HS3 of the β-globin locus control region

Disruption of the gene for transcription factor EKLF (erythroid Kruppel-like factor) results in fatal anaemia caused by severely reduced expression of the adult beta-globin gene, while other erythroid-specific genes, including the embryonic epsilon- and fetal gamma-globin genes, are expressed normally. Thus, EKLF is thought to be a stage-specific factor acting through the CACC box in the beta-gene promoter, even though it is already present in embryonic red cells. Here, we show that a beta-globin gene linked directly to the locus control region (LCR) is expressed at embryonic stages, and that this is only modestly reduced in EKLF-/- embryos. Thus, embryonic beta-globin expression is not intrinsically dependent on EKLF. To investigate whether EKLF functions in the locus control region, we analysed the expression of LCR-driven lacZ reporters. This shows that EKLF is not required for reporter activation by the complete LCR. However, embryonic expression of reporters driven by 5'HS3 of the LCR requires EKLF. This suggests that EKLF interacts directly with the CACC motifs in 5'HS3 and demonstrates that EKLF is also a transcriptional activator in embryonic erythropoiesis. Finally, we show that overexpression of EKLF results in an earlier switch from gamma- to beta-globin expression. Adult mice with the EKLF transgene have reduced platelet counts, suggesting that EKLF levels affect the balance between the megakaryocytic and erythroid lineages, Interestingly, the EKLF transgene rescues the lethal phenotype of EKLF null mice, setting the stage for future studies aimed at the analysis of the EKLF protein and its role in beta-globin gene activation.