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

The μE motifs of the immunoglobulin μ 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 μE2 contributed to transcription activation in B cells only when the μE3 site was intact, providing the first evidence for functional interactions between bHLH proteins. In vitro assays showed that bHLH zipper proteins binding to μE3 enhanced Ets-1 binding to μA. One of the consequences of this protein-protein interaction was to facilitate binding of a second bHLH protein, EA7, to the μE2 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.