Retinoid X receptors (RXRs) are transcription factors that traditionally have been thought to bind DNA as protein dimers. Recently, however, it has been recognized that RXRs can also bind to DNA as protein tetramers. Receptor tetramers form cooperatively on response elements containing suitably reiterated half-sites, and play an important role in determining the specificity of DNA recognition by different nuclear receptors. We report here that RXR tetramers exhibit significant functional plasticity, and form on response elements possessing diverse half-site orientations and spacings. This ability of RXRs to form tetramers and related oligomers appears to contribute to the synergistic transcriptional activation observed when multiple, spatially separated response elements are introduced into a single promoter. Oligomerization may therefore be a common paradigm for DNA recognition and combinatorial regulation by several different classes of transcription factors.
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