Thyroid hormone receptors (TRs) are ligand-activated transcription factors that mediate the biological effects of thyroid hormone (T3) by binding to thyroid hormone response elements (TREs), typically located in the promoter regions of T3-responsive genes. It is generally held that T3-induced gene activation is mediated by retinoid X receptor (RXR)-TR heterodimers. Although TR homodimers can bind to some TREs, T3 destabilizes this interaction, calling into question the ability of TR to activate transcription in the absence of RXR. TR-DNA binding has been difficult to study in vitro because mammalian TRs are notoriously difficult to produce in Escherichia coli. We considered that this may be due to codon bias. Therefore, we produced TR beta 1 in E. coli Rosetta 2(DE3) which contains a plasmid that overexpresses the tRNAs corresponding to the seven rarest E. coli codons. This resulted in an improved yield of full-length TR beta 1, which we then used in electrophoretic mobility shift assays. We found the coactivator TIF2 greatly enhances binding of T3-occupied TRs to a subset of TREs, suggesting TRs may activate transcription from these TREs in an RXR-independent manner. (c) 2005 Elsevier Inc. All rights reserved.