GATA-1 controls hematopoietic development by activating and repressing gene transcription, yet the in vivo mechanisms that specify these opposite activities are unknown. By examining the composition of GATA-1 associated protein complexes in a conditional erythroid rescue system, we detected the SCL/TAL1, LMO2, Ldb1, E2A complex in vivo at all positively acting GATA-1-bound elements examined. ChIP-on-chip tiling arrays for SCL and Ldb1 revealed that chromatin occupancy of SCL and Ldb1 are extremely tightly correlated. Importantly, they showed a strong tendency for co-occupancy with GATA-1. In deed, all regions occupied by GATA-1 that showed enhancer activity were occupied by SCL and Ldb1, independent of the proximity of E-box sequences. Similarly, the SCL complex is present at all activating GATA elements in megakaryocytes and mast cells. In striking contrast, at sites where GATA-1 functions as a repressor, the SCL complex is depleted. A DNA-binding defective form of SCL maintains association with a subset of active GATA elements indicating that GATA-1 is a key determinant for SCL recruitment. Knockdown of LMO2 selectively impairs activation but not repression by GATA-1. ETO-2, an SCL-associated protein with the potential for transcription repression is also absent from GATA-1-repressed genes but, unlike SCL, fails to accumulate at GATA-1 activated genes. Together, these studies identify the SCL complex as a critical and consistent determinant of positive GATA-1 activity in multiple GATA-1-regulated hematopoietic cell lineages.

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