STRUCTURAL FEATURES OF THYROID-HORMONE RESPONSE ELEMENTS THAT INCREASE SUSCEPTIBILITY TO INHIBITION BY AN RTH MUTANT THYROID-HORMONE RECEPTOR

The chicken lysozyme silencer F2 (F2) thyroid hormone response element (TRE) contains an unusual everted palindromic arrangement, has a high affinity for thyroid hormone receptor (TR) homodimers, and is especia lly sensitive to dominant negative inhibition by the T-3 resistance (R TH) mutant TR beta P453H. We used various TREs and TR mutations to det ermine the mechanisms for this sensitivity. Changing the F2 orientatio n from an everted palindrome to a direct repeat with a 4-bp gap (DR+4) (F2-DR) decreased the sensitivity to inhibition at high T-3 concentra tions, while a loss of this sensitivity occurred with a palindromic ar rangement of these same half-sites. F2 contains the dinucleotide TG 5' to each consensus half-site conforming to the optimal TR-binding octa mer, YRRGGTCA. A T to A change in position 1 of both F2 half-sites mar kedly reduced T-3-induction, yet only slightly reduced TR homodimer or TR-retinoid X receptor (RXR) heterodimer binding. The TR beta ninth h eptad mutation, L428R, prevents TR heterodimerization with RXR and eli minates the inhibitory effect of the P453H mutant TR on the FB-DR, but not the F2 element. Structural features of a TRE that favor strong TR binding of both TR homodimers and TR-RXR heterodimers containing the mutant TR, such as the everted palindromic conformation or the optimal TR-binding consensus octamer, enhance the sensitivity of a TRE to inh ibition by the mutant TR. Thus, both half-site orientation and sequenc e contribute to the sensitivity of a given TRE to dominant negative in hibition by a mutant TR.