Figure 36.1 
RNA polymerase. This large enzyme comprises many subunits, including β (blue) and β′ (red), which form a “claw” that holds the DNA to be transcribed. Notice that the active site includes a magnesium ion (green) at the center of the structure.
RNA polymerase. This large enzyme comprises many subunits, including β (blue) and β′ (red), which form a “claw” that holds the DNA to be transcribed. Notice that the active site includes a magnesium ion (green) at the center of the structure.