Binding of thyroid hormone receptors to the rat thyrotropin-beta gene

Negative regulation of rat TSH beta gene expression by thyroid hormone is mediated largely by decreased transcription of the gene. This is apparently mediated by a cis-acting element which has been localized to a 57-basepair fragment spanning the second transcriptional start site of the rat TSH beta gene. We have investigated whether thyroid hormone receptors bind specifically to DNA sequences in this region of the gene. We compared binding of native T3 receptor to the TSH beta gene sequences and to the rat GH (rGH) gene T3 response element (TRE), and examined the ability of two different forms of in vitro synthesized T3 receptor to bind to the TSH beta gene. The avidin-biotin complex DNA binding assay was used to examine sequence-specific binding of the receptor. [125I]T3-labeled receptor in GH3 cell nuclear extracts bound to a site within the first exon of TSH beta and also to a region immediately upstream of the second transcriptional start site of the gene. In addition, the Hc-erbA beta and r-erbA alpha-1 forms of the T3 receptor each bound to TSH beta and rGH sequences, demonstrating that both alpha- and beta-forms of T3 receptor can bind to TREs exerting either positive or negative transcriptional regulation. Competition experiments showed that both native and in vitro synthesized T3 receptor bound to the first exon of TSH beta with an affinity slightly less than that for the rGH TRE. The two receptor-binding sites of the rTSH beta gene show sequence similarity to adjacent regions of the rGH TRE. These data indicate that negative regulation of rat TSH beta gene transcription may be effected by direct binding of the T3-receptor complex to one or both of the binding sites flanking the second transcriptional start site.