A NAVEL RETINOID-X RECEPTOR-INDEPENDENT THYROID-HORMONE RESPONSE ELEMENT IS PRESENT IN THE HUMAN TYPE-1 DEIODINASE GENE

We identified two thyroid hormone response elements (TREs) in the 2.5- kb, 5'-flanking region of the human gene encoding type 1 iodothyronine deiodinase (hdiol), an enzyme which catalyses the activation of thyro xine to 3,5,3'-triiodothyronine (T3). Both TREs contribute equally to T3 induction of the homologous promoter in transient expression assays , The proximal TRE (TRE1), which is located at bp -100, has an unusual structure, a direct repeat of the octamer YYRGGTCA hexamer that is sp aced by 10 bp. The pyrimidines in the -2 position relative to the core hexamer are both essential to function. In vitro binding studies of T RE1 showed no heterodimer formation with retinoid X receptor (RXR) bet a or JEG nuclear extracts (containing RXR alpha) and bacterially expre ssed chicken T3 receptor alpha 1 (TR alpha) can occupy both half-sites although the 3' half-site is dominant. T3 causes dissociation of TR a lpha from the 5' half-site but increases binding to the 3' half-site. Binding of a second TR to TRE1 is minimally cooperative; however, no c ooperativity was noted for a functional mutant in which the half-sites are separated by 15 bp, implying that TRs bind as independent monomer s. Nonetheless, T3 still causes TR dissociation from the DR+15, indica ting that dissociation occurs independently of TR-TR contact and that rebinding of a T3-TR complex to the 3' half-site occurs because of its slightly higher affinity. A distal TRE (TRE2) is found at bp -700 and is a direct repeat of a PuGGTCA hexamer spaced by 4 bp. It has typica l TR homodimer and TR-RXR heterodimer binding properties. The TRE1 of hdiol is the first example of a naturally occurring TRE consisting of two relatively independent octamer sequences which do not require the RXR family of proteins for function.