As described in Chapter 15 (Figures 15.7 and 15.8), certain regulatory transcription factors bind to DNA and activate RNA polymerase II. When glucocorticoid binds to the glucocorticoid receptor (a regulatory transcription factor), this changes the con-formation of the receptor and allows it ultimately to bind to the DNA. The glucocorticoid receptor binds to a DNA sequence called a glucocorticoid response element (GRE). In contrast, other regulatory transcription factors, such as the CREB protein, do not require hormone binding in order to bind to DNA. The CREB protein can hind to the DNA in the absence of any hormone, but it does not activate RNA polymerase II unless the CREB protein is phosphory-lated. (Phosphorylation is stimulated by certain hormcyies.) The CREB protein binds to a DNA sequence called a cAMP response element (CRE). With these ideas in mind, draw the expected results of a gel retardation assay conducted on the following samples:
Lane 1. A 600-bp fragment containing a GRE, plus the gluco-corticoid receptor
Lane 2. A 600-bp fragment containing a GRE, plus the gluco-corticoid receptor, plus glucocorticoid hormone Lane 3. A 600-bp fragment containing a GRE, plus the CREB protein
Lane 4. A 700-bp fragment containing a CRE, plus the CREB protein
Lane 5. A 700-bp fragment containing a CRE, plus the CREB protein, plus a hormone (such as epinephrine) that causes the phosphorylation of the CREB protein
Lane 6. A 700-bp fragment containing a CRE, plus the gluco¬corticoid receptor, plus glucocorticoid hormone