The transcription factor Bright (or B cell regulator of IgH transcription) is a B cell-specific protein that was first discovered in a mature mouse B cell line, BCg3R1-d, as a mobilityshifted protein complex that caused 3-6-fold increases in heavy chain mRNA levels in response to stimulation with a T-dependent antigen and interleukin-5 (1, 2). Bright binds to AϩT-rich regions of the intronic heavy chain enhancer that have been identified as matrix association regions and regions 5Ј of some V H promoters, including the V1 S107 family gene (3, 4). The cDNA for Bright was isolated in 1995, and the protein was shown to interact with DNA as a multimeric complex that includes multiple copies of Bright (4).Recent studies suggest that Bruton's tyrosine kinase (〉tk), 1 the defective enzyme in xid (X-linked immunodeficiency disease) mice, is a component of the Bright DNA-binding complex (5). xid mice exhibit deficiencies in B cell development and abnormalities in immunoglobulin production (6, 7). Initial evidence indicated that the Bright DNA-binding complex was supershifted by antibodies that reacted with several different domains of Btk (5). Furthermore, Bright co-precipitated with Btk from wild type, activated splenic B cells, but Btk and Bright did not co-precipitate from xid B cells despite both proteins being present. These data suggested that Bright and Btk are functionally linked. However, the role of Bright in normal B cell development is unknown. Expression of Bright in both mouse and human tissues is tightly regulated at the level of transcription (8, 9). In the mouse, Bright mRNA was detected in all embryonic tissues, but became B-lymphocyte restricted in the adult. Bright mRNA is apparent at the pre-B and activated germinal center stages of B cell differentiation (8). The human Bright homologue (10) shows a similar expression pattern in the adult where mRNA expression is primarily limited to B lymphocytes of the pro-B cell, germinal center centroblasts, and mature recirculating subpopulations (9). Thus, in both the adult mouse and human, Bright expression is predominantly B cell-specific.Bright belongs to a growing family of proteins that bind DNA through an AϩT-rich interacting domain (ARID) (11). ARIDcontaining proteins are highly conserved throughout evolution where many of them play important roles as developmental regulators (11). ARID-containing proteins occur in all eukaryotic organisms and include 13 sequences in the human data base (11,12). Bright is the only member of the ARID family for which gene target sequences have been identified. One of the best characterized members of this family is the Dri (Drosophila homologue Dead Ringer) gene, first identified in 1996 (13). The ARID region of Dri is essential for anterior-posterior patterning and for muscle development in the fly embryo (14 -16). The crystal structure of the ARID region of Dri has been solved and revealed a unit of eight ␣-helices with a short two-stranded anti-parallel -sheet (17). NMR analysis predicted that helices 5 and 6 together ...