ERYTHROID KRUPPEL-LIKE FACTOR (EKLF) IS ACTIVE IN PRIMITIVE AND DEFINITIVE ERYTHROID-CELLS AND IS REQUIRED FOR THE FUNCTION OF 5'HS3 OF THEBETA-GLOBIN LOCUS-CONTROL REGION

Disruption of the gene for transcription factor EKLF (erythroid Kruppe l-like factor) results in fatal anaemia caused by severely reduced exp ression 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 f actor acting through the CACC box in the beta-gene promoter, even thou gh 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 intr insically dependent on EKLF. To investigate whether EKLF functions in the locus control region, we analysed the expression of LCR-driven lac Z reporters. This shows that EKLF is not required for reporter activat ion by the complete LCR. However, embryonic expression of reporters dr iven by 5'HS3 of the LCR requires EKLF. This suggests that EKLF intera cts directly with the CACC motifs in 5'HS3 and demonstrates that EKLF is also a transcriptional activator in embryonic erythropoiesis. Final ly, we show that overexpression of EKLF results in an earlier switch f rom gamma- to beta-globin expression. Adult mice with the EKLF transge ne have reduced platelet counts, suggesting that EKLF levels affect th e balance between the megakaryocytic and erythroid lineages, Interesti ngly, the EKLF transgene rescues the lethal phenotype of EKLF null mic e, setting the stage for future studies aimed at the analysis of the E KLF protein and its role in beta-globin gene activation.