Figure 11.

 Sp1 and RBP target the same step in complex formation. The levels of pIX transcription obtained during preinitiation complex formation are shown as a function of time of addition of Sp1 and RBP as indicated at the top of the lanes. Conditions of single round transcription were used. (Vertical arrows) Preincubation times of 30 min each before addition of NTPs. (S) Sp1; (R) RBP. Later addition of Sp1 gives rise to reduced levels of activated pIX transcription. Sp1 is required during complex formation to inhibit repression by RBP.