The effect of various activating regions on the targeting of Srb10/11 to an attached CTD3 substrate. The activating regions used were AH (PGIELQELQELQALLQQQ), Gal4 (840–874), Gcn4 (107–144), p53 (1–97), and Vp16 (420–490). Each reaction contained Srb10/11 complex and equimolar amounts of each substrate, as demonstrated in the Coomassie-blue-stained gel.