Cyclic AMP receptor protein (CRP) activates transcription of the lactose operon in the presence of cAMP. To clarify the mechanism of transcription activation, a DNase I footprinting experiment was done with mutant CRP, 91 CRP, which activated transcription of the lactose operon in the absence of cAMP (Harman et al., 1986). This study provides additional evidence that CRP-mediated transcription activation of the lactose promoter occurs through a mechanism that includes a role for protein-protein interaction between CRP and RNA polymerase.