A 10-AMINO-ACID SEQUENCE IN THE N-TERMINAL A B DOMAIN OF THYROID-HORMONE RECEPTOR-ALPHA IS ESSENTIAL FOR TRANSCRIPTIONAL ACTIVATION AND INTERACTION WITH THE GENERAL TRANSCRIPTION FACTOR TFIIB/

The effects of the thyroid hormone (3,5,3'-triiodo-L-thyronine [T3]) o n gene transcription are mediated by nuclear T3 receptors (T3Rs). alph a- and beta-isoform T3Rs (T3R alpha and -beta) are expressed from diff erent genes and are members of a superfamily of ligand-dependent trans cription factors that also includes the receptors for steroid hormones , vitamin D, and retinoids. Although T3 activates transcription by med iating a conformational change in the C-terminal similar to 220-amino- acid ligand-binding domain (LED), the fundamental mechanisms of T3R-me diated transcriptional activation remain to be determined. We found th at deletion of the 50-amino-acid N-terminal A/B domain of chicken T3R alpha(cT3R alpha) decreases T3-dependent stimulation of genes regulate d by native thyroid hormone response elements about 10- to 20-fold. Th e requirement of the A/B region for transcriptional activation was map ped to amino acids 21 to 30, which contain a cluster of five basic ami no acids. The A/B region of cT3R alpha is not required for T3 binding or for DNA binding of the receptor as a heterodimer with retinoid X re ceptor. In vitro binding studies indicate that the N-terminal region o f cT3R alpha interacts efficiently with TFIIB and that this interactio n requires amino acids 21 to 30 of the A/B region. In contrast, the LE D interacts poorly with TFIIB. The region of TFIIB primarily involved in the binding of cT3R alpha includes an amphipathic alpha helix conta ined within residues 178 to 201. Analysis using a fusion protein conta ining the DNA-binding domain of GALA and the entire A/B region of cT3R alpha suggests that this region does not contain an intrinsic activat ion domain. These and other studies indicate that cT3R alpha mediates at least some of its effects through TFIIB in vivo and that the N-term inal region of PNA-bound cT3R alpha acts to recruit and/or stabilize t he binding of TFIIB to the transcription complex. T3 stimulation could then result from ligand-mediated changes in the LED which may lead to the interaction of other factors with cT3R alpha, TFIIB, and/or other components involved in the initiation of transcription.