ETS-mediated cooperation between basic helix-loop-helix motifs of the immunoglobulin mu heavy-chain gene enhancer

The muE motifs of the immunoglobulin mu heavy-chain gene enhancer bind ubiquitously expressed proteins of the basic helix-loop-helix (bHLH) family. These elements work together with other, more tissue-restricted elements to produce B-cell-specific enhancer activity by presently undefined combinatorial mechanisms. We found that muE2 contributed to transcription activation in B cells only when the muE3 site was intact, providing the first evidence for functional interactions between bHLH proteins. In vitro assays showed that bHLH zipper proteins binding to muE3 enhanced Ets-1 binding to muA. One of the consequences of this protein-protein interaction was to facilitate binding of a second bHLH protein, E47, to the muE2 site, thereby generating a three-protein-DNA complex. Furthermore, transcriptional synergy between bHLH and bHLH zipper factors also required an intermediate ETS protein, which may bridge the transcription activation domains of the bHLH factors. Our observations define an unusual form of cooperation between bHLH and ETS proteins and suggest mechanisms by which tissue-restricted and ubiquitous factors combine to generate tissue-specific enhancer activity.