Steroid Receptors Modulate the Rate of Transcription

The receptors for all classes of steroid hormones, including the receptors for 1,25-dihydroxy vitamin D3, and thyroid hormones are intracellular proteins that are not very abundant, usually only 102-105 molecules per cell. 

All of these classes of receptors have a similar structure composed of several functional domains, including regions that bind the hormone, bind to DNA, activate transcription and allow dimerization. In the absence of hormone these receptors are usually sequestered by other proteins located either in the cytoplasm or nucleus. For example, 

Thyroid hormone receptors differ from steroid hormone receptors in that they are always bound to DNA. In the absence of thyroid hormones, these receptors inhibit the expression of genes to which they bind the addition of hormone converts them into transcriptional activators. 

---

Multicomponent Hormonal Systems Facilitate a Great Variety of Responses The Guanylate Cyclase Pathway Calcium and the Inositol Trisphosphate Pathway Steroid Receptors Modulate the Rate of Transcription... 

---