GATA-BINDING AND SP1-BINDING SITES ARE REQUIRED FOR THE FULL ACTIVITYOF THE TISSUE-SPECIFIC PROMOTER OF THE TAL-1 GENE

The tal-1 gene, which is frequently activated in human T cell acute le ukemias (T-ALLs), codes for a protein of the basic helix-loop-helix fa mily (b-HLH) and potentially a transcription factor. In human and muri ne hematopoiesis tal-1 is expressed during the differentiation of the erythroid, megakaryocytic and mastocytic cell lineages. The expression of tal-1 appears to be comodulated with that of the transcription fac tor GATA-1 gene, suggesting that the GATA-1 protein may regulate the t al-1 gene activity in these hematopoietic lineages. To get further ins ights into the molecular mechanisms that control tal-1 expression, we have isolated 5' sequences of the murine gene and compared them to the ir human counterparts. The 5' flanking sequences from the two genes sh ow several regions of high homology. The alignment of both sequences e nabled us to predict that similarly, to the human, the mouse gene cont ains two alternative first exons (Ia and Ib). Remarkably, in both spec ies, the proximal region of the tissue-specific exon Ia (i.e. gene seg ment -122 to +1) contains two GATA-motifs (at -65 and -33) and one SP- 1 consensus binding site (-59). Mobility shift assays demonstrate that GATA proteins are able to interact with both GATA-motifs in a sequenc e specific fashion, but with different efficiencies. Moreover transfec tion studies show that the GATA-1 protein directly mediates tal-1 tran scription by interacting with the -122/+1 fragment, defined as a minim al promoter in erythroid cells. Mutagenesis of the promoter establishe s that the -33 GATA-binding site present in this fragment is critical for tal-1 expression in erythroid cells, but by itself does not lead t o full promoter activity. Indeed, further mutations show that the seco nd -65 GATA-binding site and the binding motif for SP1 (-59) significa ntly contribute to the overall activity of the proximal tal-1 promoter . Altogether, our data provide evidence that GATA-1 cooperates with th e transcription factor SP1 to mediate the erythroid-specific expressio n of the tal-1 gene.