T-bet's ability to regulate individual target genes requires the conserved T-box domain to recruit histone methyltransferase activity and a separate family member-specific transactivation domain.

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-gamma promoters. A T-bet structure-function 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.

T-bet binding to newly identified target gene promoters is cell type-independent but results in variable context-dependent functional effects.

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, IL2Rbeta, 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-/- primary B, CD4+, and CD8+ T cells. Thus, there is a high variability in the functional consequences, if any, that result from the association of T-bet with individual target promoters.