CHROMATIN IMMUNOSELECTION DEFINES A TAL-1 TARGET GENE

Despite the major functions of the basic helix-loop-helix transcriptio n factor TAL-1 in hematopoiesis and T-cell leukemogenesis, no TAL-1 ta rget gene has been identified. Using immunoprecipitation of genomic fr agments bound to TAL-1 in the chromatin of murine erythro-leukemia (ME L) cells, we found that 10% of the immunoselected fragments contained a CAGATG or a CAGGTG E-box, followed by a GATA site. We studied one of these fragments containing two E-boxes, CAGATG and CAGGTC, followed b y a GATA motif, and showed that TAL-1 binds to the CAGGTG E-box with a n affinity modulated by the CAGATG or the GATA site, and that the CAGG TG-GATA moth exhibits positive transcriptional activity in MEL but not in HeLa cells. This immunoselected sequence is located within an intr on of a new gene co-expressed with TAL-1 in endothelial and erythroid cells, but not expressed in fibroblasts or adult liver where no TAL-1 mRNA was detected. Finally, in vitro differentiation of embryonic stem cells towards the erythro/megakaryocytic pathways showed that the TAL -1 target gene expression followed TAL-1 and GATA-1 expression. These results establish that TAL-1 is likely to activate its target genes th rough a complex that binds an E-box-GATA motif and define the first ge ne regulated by TAL-1.