Thyroid hormone (triiodothyronine, T 3 ) is known to activate transcription by binding heterodimers of thyroid hormone receptors (TRs) and retinoid X receptors (RXRs). RXR-TRs bind to T 3 response elements (TREs) composed of direct repeats of the sequence AGGTCA spaced by four nucleotides (DR-4). In other TREs, however, the half-sites can be arranged as inverted palindromes and palindromes (Pal). Here we show that TR homodimers and monomers activate transcription from representative TREs with alternate half-site placements. TR␤ activates transcription more efficiently than TR␣ at an inverted palindrome (F2), and this correlates with preferential TR␤ homodimer formation at F2 in vitro. Furthermore, reconstruction of TR transcription complexes in yeast indicates that TR␤ homodimers are active at F2, whereas RXR-TRs are active at DR-4 and Pal. Finally, analysis of TR␤ mutations that block homodimer and/or heterodimer formation reveal TRE-selective requirements for these surfaces in mammalian cells, which suggest that TR␤ homodimers are active at F2, RXR-TRs at DR-4, and TR monomers at Pal. TR␤ requires higher levels of hormone for activation at F2 than other TREs, and this differential effect is abolished by a dimer surface mutation suggesting that it is related to composition of the TR⅐TRE complex. We propose that interactions of particular TR oligomers with different elements play unappreciated roles in TRE-selective actions of liganded TRs in vivo.

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Thyroid hormone receptors (TR) play critical roles in virtually all tissues. The TR ligand-binding domain (LBD) participates in important activities, such as transcriptional activation and repression, through conformational changes induced by hormone binding. Two crystal forms of isoform alpha1 of the human thyroid hormone receptor LBD (hTRalpha1) in complex with the thyroid hormones T3 and Triac were obtained. The hTRalpha1-T3 complex was crystallized in a previously unobserved crystal form (space group P2(1)2(1)2(1), a = 59.98, b = 80.80, c = 102.21 A), with diffraction patterns extending to 1.90 A resolution on a rotating-anode X-ray source, and in space group C2 (a = 117.54, b = 80.66, c = 62.55 A, beta = 121.04 degrees), with data extending to 2.32 A resolution. The hTRalpha1-Triac complex was also crystallized in the new space group P2(1)2(1)2(1), with unit-cell parameters a = 60.01, b = 80.82, c = 102.39 A; its resolution limit extended to 2.20 A on a home source. Phasing was carried out by the molecular-replacement method and structural refinement is currently in progress. The refined structures may provide insight into the design of new thyromimetics.

We studied the importance of genetic predisposition in the development of stress-induced hypertension in the spontaneously hypertensive rat (SHR), Wistar-Kyoto (WKY) rat, and borderline hypertensive rat (BHR; first-generation offspring of SHR and WKY). Rats were submitted to seven 72-hour sessions of rapid eye movement sleep deprivation (REM-sd) every other week during 13 weeks. Tail arterial pressure was determined throughout the experiment At the end of the study, mean arterial pressure (direct measurement), sympathetic activity (acute blockade with propranolol and phentolamine), and ventricular weight were determined. has proved difficult to show. In animal spe-L cies, acute stress almost always induces blood pressure (BP) elevations. Results showed that REM-sd induced sustained hypertension only in rats with a partial predisposition to developing hypertension (BHRs12 However, in the majority of studies, chronic stress has failed to induce sustained hypertension.3 -3 The models of stress used and the genetic susceptibility to the development of hypertension have been suggested to be responsible, at least in part, for the difficulty in causing this form of hypertension. Food-shock conflict can increase BP only in the Dahl strain of salt-sensitive rat.6 Immobilization, sensorial stimuli, or exposure to a cold environment causes chronic BP elevation in spontaneously hypertensive rats (SHRs) but not in geneti-

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