Roles of Auxiliary Transcription Factors
A strong promoter must have both a high affinity for polymerase and also a high isomerization rate. The reverse is true of the weakest promoters. They have low affinity and slow isomerization rates. Me-dium-activity promoters are weak binders or slow at isomerization (Fig. 4.22). An auxiliary factor that stimulates RNA polymerase can change either the binding constant or the isomerization rate. The lac operon
Figure 4.22 Requirements onKdand k2 for strong, moderate, and weak promoter activity.
promoter is stimulated by the cyclic AMP receptor protein called CAP or CRP. Characterization of the lac promoter with and without CAP shows that the protein primarily accelerates the isomerization step.
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