A novel 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 (hdio1), an enzyme which catalyses the activation of thyroxine 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 spaced by 10 bp. The pyrimidines in the -2 position relative to the core hexamer are both essential to function. In vitro binding studies of TRE1 showed no heterodimer formation with retinoid X receptor (RXR) beta or JEG nuclear extracts (containing RXR alpha) and bacterially expressed chicken T3 receptor alpha 1 (TR alpha) can occupy both half-sites although the 3' half-site is dominant. T3 causes dissociation of TR alpha 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 cooperativity was noted for a functional mutant in which the half-sites are separated by 15 bp, implying that TRs bind as independent monomers. Nonetheless, T3 still causes TR dissociation from the DR+15, indicating 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 typical TR homodimer and TR-RXR heterodimer binding properties. The TRE1 of hdio1 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.
Toyoda
N
N
Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.
Zavacki
A M
AM
Maia
A L
AL
Harney
J W
JW
Larsen
P R
PR
D001483:Base Sequence; D001665:Binding Sites; D015971:Gene Expression Regulation, Enzymologic; D015698:Genomic Library; D006801:Humans; D007453:Iodide Peroxidase / Q000235:genetics*; D008024:Ligands; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics*; D011485:Protein Binding; D018168:Receptors, Retinoic Acid / Q000378:metabolism; D011988:Receptors, Thyroid Hormone / Q000378:metabolism*; D012091:Repetitive Sequences, Nucleic Acid; D047488:Retinoid X Receptors; D017422:Sequence Analysis, DNA; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic; D014284:Triiodothyronine / Q000378:metabolism*
D008024:Ligands; D009687:Nuclear Proteins; D018168:Receptors, Retinoic Acid; D011988:Receptors, Thyroid Hormone; D047488:Retinoid X Receptors; D014157:Transcription Factors; D014284:Triiodothyronine; D007453:Iodide Peroxidase