Abstract
Abstract The complex nature of most promoters and enhancers makes it difficult to identify key determinants of tissue-specific gene expression. Furthermore, most tissue-specific genes are regulated by transcription factors that have expression profiles more widespread than the genes they control. NFAT is an example of a widely expressed transcription factor that contributes to several distinct patterns of cytokine gene expression within the immune system and where its role in directing specificity remains undefined. To investigate distinct combinatorial mechanisms employed by NFAT to regulate tissue-specific transcription, we examined a composite NFAT/AP-1 element from the widely active GM-CSF enhancer and a composite NFAT/Oct element from the T cell-specific IL-3 enhancer. The NFAT/AP-1 element was active in the numerous cell types that express NFAT, but NFAT/Oct enhancer activity was T cell specific even though Oct-1 is ubiquitous. Conversion of the single Oct site in the IL-3 enhancer to an AP-1 enabled activation outside of the T cell lineage. By reconstituting the activities of both the IL-3 enhancer and its NFAT/Oct element in a variety of cell types, we demonstrated that their T cell-specific activation required the lymphoid cofactors NIP45 and OCA-B in addition to NFAT and Oct family proteins. Furthermore, the Oct family protein Brn-2, which cannot recruit OCA-B, repressed NFAT/Oct enhancer activity. Significantly, the two patterns of combinatorial regulation identified in this study mirror the cell-type specificities of the cytokine genes that they govern. We have thus established that simple composite transcription factor binding sites can indeed establish highly specific patterns of gene expression.