It has been demonstrated that sex steroid hormones, namely estrogen and progesterone can regulate gene activity. Glucocorticoids, another set of steroids, found endogenously within the cells and used for treatment of leukaemia, asthma and arthritis also regulate gene expression. These glucocorticoids have a multitude of effects in various tissues. Classical example of a sex steroid inducible protein is chick ovalbumin and those of glucocorticoid-inducible proteins are the enzymes tyrosine aminotransferase (TAT) and tryptophan oxydase. The glucocorticoids induce synthesis of these enzymes through receptor molecules. The steroid-receptor complex interacts each with a specific DNA sequence, celled steroid receptor element (SRE) or glucocorticoid receptor element (GRE) or hormone receptor element (HRE). Through recombinant DNA technology, gene constructs were prepared by fusing different lengths of DNA sequences flanking the 5' end of TAT gene with the marker gene for CAT (chloramphenicol acetyltransferase). These cloned gene constructs were introduced into animal cells and though glucocorticoid induced expression of CAT, steroid receptor elements (SRE) were identified (Fig. 37.24)

Steroid inducible synthesis of protein, which involves the following six steps, is illustrated in Figure 37.25. (i) Uptake of steroid hormone (S) and binding to receptor molecule in the cytoplasm (Rc) to form S-Rc complex. (Receptors for glucocorticoids are predominantly found in the cytoplasm, but those for sex steroids are found in the nucleus), (ii) Transport of S-Rc complex to nucleus (wherever cytosolic receptor is involved), where it is now called S-Rn to specify its occurrence in the nucleus, (iii) Bindings of S-Rn complex to specific acceptor site SRE on genome, (iv) Activation of transcriptional apparatus in presence of specific non-histone proteins to synthesize specific mRNA. (v) Transport of specific RNA to cytoplasm, (vi) Synthesis of protein (like ovalbumin) and occurrence of specific steroid mediated functional response.