Thyroid transcription factor-1 activates the promoter activity of rat thyroid Na+/I- symporter gene.

We have cloned 15 kbp of rat thyroid Na+/I− symporter gene from liver genomic library, which contains 6 kbp upstream sequence from the translation initiation site. Southern blot analysis of the genomic DNA from the liver has revealed that thyroid Na+/I− symporter gene is the single gene in the rat. To study the tissue-selective expression mechanism of the gene, we at first determined the transcriptional start site of the gene. Results of a rapid amplification of cDNA end procedure as well as that of primer extension analysis indicated that the transcriptional start sites clustered between −96, −95, and −93 bp of the gene (A in ATG is designated as +1). Chimeras containing 1.9 kbp (−1967 to −46 bp) of the 5′-flanking sequence of the Na+/I− symporter gene and luciferase gene expressed significant enzyme activity when transfected into a rat thyroid cell line, FRTL-5, but little activity was observed in BRL-3A rat liver cells. Deletion analysis of the constructs indicated that a minimal region, exhibiting pro...