Recently developed target gene identification strategies based upon the chromatin immunoprecipitation assay provide a powerful method to determine the localization of transcription factor binding within mammalian genomes. However, in many cases, it is unclear if the binding capacity of a transcription factor correlates with an obligate role in gene regulation in diverse contexts. It is therefore important to carefully examine the relationship between transcription factor binding and its ability to functionally regulate gene expression. T-bet is a T-box transcription factor expressed in several hematopoietic cell types. By utilizing a chromatin immunoprecipitation assay coupled to genomic microarray technology approach, we identified numerous promoters, including CXCR3, IL2R␤, and CCL3, that are bound by T-bet in B cells. Most surprisingly, the ability of T-bet to associate with the target promoters is not dependent upon the cell type background. Several of the promoters appear to be functionally regulated by T-bet. However, we could not detect a functional consequence for T-bet association with many of the identified promoters in overexpression studies or an examination of wild type and T-bet Upon exposure to pathogenic stimuli, an intricately coordinated immune response composed of specialized cells is required for host defense. In order for these tightly regulated cellular responses to occur, individual hematopoietic cell types must carry out very precise and coordinated alterations in their gene expression patterns to up-regulate specific functional capabilities. A great deal of research has been undertaken to better address the molecular mechanisms by which critical, lineage-restricted transcription factors contribute to this process in the immune system, but much still remains unknown.T-box expressed in T cells (T-bet) is a member of the T-box transcription factor family and plays a critical role in the generation of CD4 (2). Taken together, the data strongly indicate that T-bet plays a critical role in Th1 development and cellmediated immunity.The importance of T-bet in generating a productive immune response is not limited to its role in Th1 development. Several studies have highlighted the role of T-bet in B, NK, NKT, DC, and CD8 ϩ T cell development and function (3-6). For example, immunoglobulin isotype class switching is defective in T-bet Ϫ/Ϫ mice, and this defect appears to be B cell intrinsic and not solely due to the deficient Th1 response (3). In addition, NK and NKT cells intrinsically require T-bet to develop into functionally mature cells (4). In contrast to CD4 ϩ T cells, the requirement for T-bet in CD8 ϩ T cells is less pronounced (2). Data using dominant negative constructs suggest that the less stringent requirement for T-bet in CD8 ϩ T cells is at least in part due to the expression of another closely related T-box family member, eomesodermin (Eomes), in these cells (7). At least in some cases, Eomes can act in a functionally redundant manner with T-bet activity. Taken together, the cur...

Appropriate cellular differentiation and specification rely upon the ability of key developmental transcription factors to precisely establish gene expression patterns. These transcription factors often regulate epigenetic events. However, it has been unclear whether this is the only role that they play in functionally regulating developmental gene expression pathways or whether they also participate in downstream transactivation events at the same promoter. The T-box transcription factor family is important in cellular specification events in many developmental systems, and determining the molecular mechanisms by which this family regulates gene expression networks warrants attention. Here, we examine the mechanism by which T-bet, a critical T-box protein in the immune system, influences transcription. T-bet is both necessary and sufficient to induce permissive histone H3-K4 dimethyl modifications at the CXCR3 and IFN-␥ promoters. A T-bet structurefunction analysis revealed that the conserved T-box domain, with a small C-terminal portion, is required for recruiting histone methyltransferase activity to promoters. Interestingly, this function is conserved in the T-box family and is necessary, but not sufficient, to induce transcription, with an independent transactivation activity also required. The requirement for two separable functional activities may ultimately contribute to the stringent role for T-box proteins in establishing specific developmental gene expression pathways.