Abstract
The effects of thyroid hormones are mediated through nuclear receptor proteins that modulate the transcription of specific genes in target cells. We previously isolated cDNAs encoding two different mammalian thyroid hormone receptors, one from human placenta (hTR.beta.) and the other from rat brain (rTR.alpha.), and showed that their in vitro translation products bind thyroid hormones with the characteristic affinities of the native thyroid hormone receptor. We now demonstrate that both of the cloned receptors activate transcription from a thyroid hormone-responsive promoter in a hormone-dependent manner, with rTR.alpha. eliciting a greater response than hTR.beta.. The putative functional domains of the thyroid hormone receptors were examined by creating chimeric thyroid hormone/glucocorticoid receptors, producing receptors with hybrid functional properties. These experiments support the proposal that the thyroid hormone receptors are composed of interchangeable functional domains, and indicate that the mechanism of hormone-inducible gene regulation has been conserved in steroid and thyroid hormone receptors.