Oestrogen and the Initiation of Transcription

Oestrogen is a steroid hormone that switches on genes by activating a transcription factor. It shows how a signal from outside a cell can control transcription.1. Oestrogen is lipid-soluble, so it diffuses through the phospholipid bilayer of the cell-surface membrane.
2. In the cytoplasm, oestrogen binds to a site on a receptor molecule, the oestrogen receptor, which is a transcription factor. The shape of the binding site is complementary to oestrogen.
3. Binding changes the shape of the receptor, releasing it from an inhibitor molecule that was holding it inactive. The transcription factor is now active.
4. The oestrogen-receptor complex moves from the cytoplasm into the nucleus through a nuclear pore.
5. It binds to a specific base sequence in the promoter region of the target gene.
6. This stimulates RNA polymerase to bind and begin transcription, so mRNA is made and later translated into protein.When oestrogen concentration falls, less binds to the receptors and transcription of the target genes slows down.Only cells that have oestrogen receptors respond to oestrogen. This is why the hormone affects some tissues, such as the uterus and the breast, and not others.Compare this with hormones such as adrenaline, which cannot cross the cell-surface membrane. They bind to receptors on the outside of the cell and act through a second messenger instead.