cAMP-dependent binding of a trans-acting factor to the thyroglobulin promoter

We have investigated the interaction of a nuclear factor(s) with the promoter region of the thyroglobulin (Tg) gene, which is only expressed in differentiated thyroid cells under the positive control of the pituitary hormone thyrotropin (TSH) via a cAMP-dependent pathway. Using the mobility shift assay, we first demonstrated that a thyroid nuclear factor interacts with a short segment of 60 bp (-136 - -77) which is conserved among species in the regulatory region of the Tg gene. A specific binding site was then localized in a subfragment of 20 bp located between -126 bp and -107 bp relative to the transcription initiation site. The corresponding nuclear factor is absent in a tissue which does not express the Tg gene. This factor differs from previously identified factors shown to mediate a direct cAMP response since the observed binding is neither competed out by the cAMP responsive element (CRE) nor by the activator protein 2 (AP2) binding site. This trans-acting factor represents a new candidate intermediate in the regulation of transcription by a cAMP dependent mechanism.