B lymphocyte-restricted transcription of immunoglobulin heavy-chain (IgH) genes is specified by elements within the variable region (VH) promoter and the intronic enhancer (E mu). The gene encoding a protein that binds a VH promoter proximal site necessary for induced mu-heavy-chain transcription has been cloned. This B-cell specific protein, termed Bright (B cell regulator of IgH transcription), is found in both soluble and matrix insoluble nuclear fractions. Bright binds the minor groove of a restricted ATC sequence that is sufficient for nuclear matrix association. This sequence motif is present in previously described matrix-associating regions (MARs) proximal to the promoter and flanking E mu. Bright can activate E mu-driven transcription by binding these sites, but only when they occur in their natural context and in cell lines permissive for E mu activity. To bind DNA, Bright requires a novel tetramerization domain and a previously undescribed domain that shares identity with several proteins, including SWI1, a component of the SWI/SNF complex.