We have extensively characterized the sequences of the rat growth hormone (rGH) promoter required for induction by T3 (thyroid hormone, 3,5,3′-L-triiodothyronine) in a transient transfection system. Oligonucleotides containing portions of the rGH promoter sequence with various deletions and point mutations were placed upstream of the first 137 base pairs of the rGH promoter or the heterologous herpes virus thymidine kinase promoter in chloramphenicol acetyltransferase expression vectors. The rGH137 and thymidine kinase promoters show no or minimal response to T3 in the basal state. The constructs were tested in GH₄C₁ rat pituitary cells and COS cells (functionally deficient in thyroid hormone receptor) with and without a co-transfected plasmid expressing a β type c-erbA gene coding for a functional T3 receptor.

Oligonucleotides containing the T3 receptor binding site confer hormone-dependent induction in a manner that is independent of either orientation or variation in position on the helix relative to the promoter. Point mutations in the sequence −189 to −173 result in loss of T3 induction, and bases between −173 and −167 were also required for a full T3 response. The minimal length to confer T3 induction to the rGH promoter was 23 base pairs (−190 to −167). Point mutations creating a perfect duplication of 7 base pairs within the receptor binding site conferred 12-fold T3 response to the rGH137 promoter, 3-fold greater than the wild type rGH237 construct. T3 inductibility was also transferred to the thymidine kinase promoter by an oligonucleotide containing the sequence −200 to −157, demonstrating that cell type specific elements located 3′ to 157 of the rGH promoter are not required for thyroid hormone responsiveness.