DISTINCT MECHANISMS DIRECT SCL TAL-1 EXPRESSION IN ERYTHROID-CELLS AND CD34 POSITIVE PRIMITIVE MYELOID CELLS/

The SCL/tal-1 gene (hereafter designated SCL) encodes a basic helix-lo op-helix transcription factor which is pivotal for the normal developm ent of all hematopoietic lineages and which is expressed in committed erythroid, mast, and megakaryocytic cells as well as in hematopoietic stem cells. The molecular basis for expression of SCL in stem cells an d its subsequent modulation during lineage commitment is of fundamenta l importance for understanding how early ''decisions'' are made during hematopoiesis. We now compare the activity of SCL promoters 1a and 1b in erythroid cells and in CD34 positive primitive myeloid cells. SCL mRNA expression in CD34 positive myeloid cells did not require GATA-1. Promoter 1a activity was weak. or absent in CD34 positive myeloid cel ls and appeared to correlate with the presence or absence of low level s of GATA-1, However, promoter 1b, which was silent in committed eryth roid cells, was strongly active in transient assays using CD34 positiv e myeloid cells, and functioned in a GATA-independent manner. Interest ingly, RNase protection assays demonstrated that endogenous promoter 1 b was active in both erythroid and CD34 positive myeloid cells. These results demonstrate that fundamentally different mechanisms regulate t he SCL promoter region in committed erythroid cells and in CD34 positi ve myeloid cells. Moreover these observations suggest that in erythroi d, but not in CD34 positive myeloid cells, promoter 1b required integr ation in chromatin and/or additional sequences for its activity, Stabl e transfection experiments showed that both core promoters were silent following integration in erythroid or CD34 positive myeloid cells. Ou r data therefore indicate that additional regulatory elements were nec essary for both SCL promoters to overcome chromatin-mediated repressio n.