Delineation of three different thyroid hormone-response elements in promoter of rat sarcoplasmic reticulum Ca2+ATPase gene. Demonstration that retinoid X receptor binds 5' to thyroid hormone receptor in response element 1.

Thyroid hormone (3,5,3'-triiodothyronine) positively regulates transcription of the sarcoplasmic reticulum Ca2+ATPase gene in rat heart, and sequences within 559 nucleotides upstream from the transcription start site confer thyroid hormone responsiveness upon a reporter gene. In the present study, three thyroid hormone-response elements (TREs) are identified between nucleotides -485 and -190. Each TRE is active in transient transfection assays and specifically binds 3,5,3'-triiodothyronine receptors (TRs) alpha 1 and beta 1 alone and in combination with retinoid X receptors (RXRs) alpha and beta. TRE 1 is a direct repeat of two half-sites separated by four nucleotides; TREs 2 and 3 are inverted palindromes of two half-sites separated by four and six nucleotides, respectively. Methylation interference analysis of TRE 1 showed binding of a TR alpha 1 monomer to the 3' half-site, whereas the heterodimer contacts both half-sites. Subsequent studies employed TR beta and RXR alpha mutants in which their P-boxes were replaced with the P-box of the glucocorticoid receptor. Bandshifts of wild type and mutant proteins with either wild type TRE 1 or a mutant version, in which the 5' half-site was converted to a glucocorticoid response element half-site, demonstrated preferential binding of RXR to the 5' half-site and of TR to the 3' half-site of TRE 1.