Final answer:
Mediators and co-activators are critical in gene expression regulation by enhancing RNA polymerase-promoter interactions and participating in signaling cascades that affect downstream gene expression.
Step-by-step explanation:
Mediators and co-activators play crucial roles in the regulation of gene expression. Activator proteins enhance the interaction between RNA polymerase and a particular promoter, enabling transcription to occur more efficiently. These activators often bind to specific DNA sequences known as enhancers. Upon binding to their enhancer, they then attract protein cofactors and mediate the formation of a transcription initiation complex. This process often involves a bend in the DNA, which allows RNA polymerase II to access the DNA more easily and initiate transcription.
In addition to these mechanisms, mediators and co-activators can also participate in signaling pathways, which may affect gene expression indirectly. For instance, when a hormone binds to a cell surface receptor, it can initiate a signaling cascade involving the activation of second messengers and protein kinases, which ultimately leads to phosphorylation of transcription factors that bind to DNA and affect gene expression. Such signaling pathways play a significant role in relaying signals from the cell surface to the nucleus, influencing downstream gene expression.