Specification of hematopoietic and vascular development by the bHLH transcription factor SCL without direct DNA binding

Transcription factors, such as those of the basie-helix-loop-helix (bHLH) a nd homeodomain classes, are primary regulators of cell fate decisions and d ifferentiation. It is considered axiomatic that they control their respecti ve developmental programs via direct binding to cognate DNA sequences in cr itical targets genes. Here we test this widely held paradigm by in vivo fun ctional assay of the leukemia oncoprotein SCL, a bHLH factor that resembles myogenic and neurogenic proteins and is essential for both hematopoietic a nd vascular development in vertebrates. Contrary to all expectation, we fin d that SCL variants unable to bind DNA rescue hematopoiesis from gene-targe ted SCL-/- embryonic stem cells and complement hematopoietic and vascular d eficits in the zebrafish mutant cloche. Our findings establish DNA-binding- independent functions of SCL critical for transcriptional specification, an d should encourage reassessment of presumed requirements for direct DNA bin ding by other transcription factors during initiation of developmental prog rams.