Higher-Order Chromatin Organization In Hematopoietic Transcription

Coordinated transcriptional networks underlie complex developmental processes. Transcription factors play central roles in such networks by binding to core promoters and regulatory elements and thereby controlling transcription activities and chromatin states in the genome. GATA1 is a hematopoietic transcription factor that controls multiple hematopoietic lineages by activating and repressing gene expression, yet the in vivo mechanisms that specify these opposing activities are unknown. By examining the composition of GATA1 associated protein complexes in a genetic complementary erythroid cell system as well as through the use of tiling arrays, we found that a multi-protein complex containing SCL/TAL1, LMO2, Ldb1, and E2A (the SCL complex thereafter) is present at most sites where GATA1 functions as an activator but depleted at most repressive GATA1 sites. Functional interference of the SCL complex selectively impairs activation but not repression by GATA1. These results identify the SCL complex as a critical and consistent determinant of positive GATA1 activity. The SCL complex and GATA1 co-occupy the active &beta-globin promoter and the distant locus control region (LCR), which are juxtaposed into close proximity by chromatin looping. The physical interaction of genes with their cis-regulatory elements are commonly observed in the metazoan genome, however, the molecular architecture of loop formation and the cause-effect relationship betwee